,transcrib]'s ,note3 ,an und]score "*   
is us$ ar\nd text to signify italics 9 !
 orig9al text1 z illu/rat$4 ,x also is  
us$ to signify a subscript1 us$         
frequ5tly 9 te*nical descriptions4 ,=   
example ,e,c w\ld h be5 orig9ally       
typeset z a capital ,e foll[$ by a      
small] ,c subscript1 & bo? w\ld h be5 9 
an italic typeface4     ,illu/ration3   
,pl4 ,i4--,"o ( ! ,l9es ( ,t[]s at      
,radio ,c5tral 7,c\rtesy ( ,radio       
,corporation ( ,am]ica74     ,,lett]s   
,,( ,a ,,radio-,,eng9e] ,,to ,,8 ,,son  
,,by  ,,john ,,mills  ,eng9e]+          
,dep>tm5t1 ,we/]n ,electric ,company1   
,inc41  ,au?or ( ,radio-,communication1 
,! ,realities ( ,mod]n ,sci5ce1 & ,)9 ! 
,atom  ,,new ,,york  ,,h>c\rt1 ,,brace  
,,& ,,company     ,,copyright1 #aibb1   
,,by  ,,h>c\rt1 ,,brace ,,& ,,company1  
,,inc4  ,,pr9t$ ,,9 ,,! ,u4 ,s4 ,a4 ,,by
  ,,! ,,qu9n  ,,bod5 ,,company          
,,rahway1 ,n4 ,j4     ,,to  ,j4 ,m41    
,jr4     ,,cont5ts          #a          
,electricity & ,matt]                   

#c          #b  ,:y a ,copp] ,wire ,w   
,conduct            ,electricity        
#i          #c  ,h[ a ,batt]y ,works    
#af          #d  ,! ,batt]ies 9 ,y\r    
,radio ,set              #bg          #e
  ,gett+ ,electrons f a ,heat$ ,wire    
#cd          #f  ,! ,audion             
#dj          #g  ,h[ to ,measure an     
,electron ,/ream            #dh         
#h  ,electron-,mov+-,=ces               
#eg          #i  ,! ,audion-,*>act]i/ic 
#ff         #aj  ,cond5s]s & ,coils     
#gg         #aa  ,a ,c-,w ,transmitt]   
#hf         #ab  ,inductance & ,capacity
                      #if         #ac   
,tun+                                   
#aab         #ad  ,:y & ,h[ to ,use a   
,detector               #abd         #ae
  ,radio-,telephony                     
#adj         #af  ,! ,human ,voice      
#aeb         #ag  ,grid ,batt]ies &     
,grid ,cond5s]s =            ,detectors 
#afe         #ah  ,amplifi]s & !        
,reg5]ative ,circuit     #agf           
#ai  ,! ,audion ,amplifi] & ,its        

,connections    #ahg         #bj        
,telephone ,receiv]s & ,o!r             
,electromagnetic ,devices               
#aii         #ba  ,y\r ,receiv+ ,set &  
,h[ to ,exp]im5t    #baa         #bb    
,high-,p[]$ ,radio-,telephone           
,transmitt]s                            
#bcj         #bc  ,amplification at     
,int]m$iate            ,frequ5cies      
#bdb         #bd  ,by ,wire & by ,radio 
#bea             ,index                 
#bfc     ,,li/ ,,( ,,plates          ,i 
,"o ( ! ,l9es ( ,t[]s at ,radio ,c5tral 
,frontispiece         ,,ii  ,bird's-,eye
 ,view ( ,radio ,c5tral             #aj 
,,iii  ,dry ,batt]y = ,use 9 ,audion    
,circuits1            & also ,/orage    
,batt]y                     #bg         
,,iv  ,radiotron                        
#db          ,v  ,v>iomet] & ,v>iable   
,cond5s] (            ! ,g5]al ,radio   
,company4 ,voltmet]            & ,ammet]
 ( ! ,we/on ,in/rum5t                   
,company                                
#ia         ,,vi  ,l[-,p[] ,transmitt+  

,tube1 ,u ,v #bjb        #ajf           
,,vii  ,photographs ( ,vibrat+ ,/r+s    
#aee       ,,viii  ,to ,illu/rate !     
,me*anism = !            ,production ( !
 ,human ,voice               #agj       
,,ix  ,we/]n ,electric ,l\d ,speak+     
,receiv]4 ,cry/al ,detector ,set ( !    
,g5]al ,electric ,co4 ,audibility ,met] 
( ,g5]al ,radio ,co4                    
#bjc          ,x  ,audio-,frequ5cy      
,trans=m] &            ,bank$-,w\nd     
,coil                           #bah    
,,xi  ,broadca/+ ,equipm5t1 ,develop$ by
            ! ,am]ican ,telephone &     
,telegraph            ,company & !      
,we/]n ,electric ,company    #bce       
,,xii  ,broadca/+ ,/ation ( ! ,am]ican  
,telephone & ,telegraph ,company on !   
,ro( ( ! ,walk]-,lisp5>d ,bldg4 9       
,new ,york ,city :]e ! ,long-di/ance    
,telephone ,l9es ,t]m9ate               
#bej     ,,lett]s ,,( ,a                
,,radio-,,eng9e] ,,to ,,8 ,,son  ,,lett]
 #a  ,,electricity ,,& ,,matt]   ,,my   
,,de> ,,son3  ,y are int]e/$ 9          

radio-telephony & want me to expla9 x to
 y4 ,i'll d s 9 ! %orte/ & easie/ way   
:i* ,i c devise4 ,! explanation w be !  
simple/ :i* ,i c give & /ill make x     
possible = y to build & op]ate y\r own  
set & to und]/& ! op]ation ( ! l>ge     
comm]cial sets to :i* y w li/54  ,i'll  
write y a s]ies ( lett]s :i* w conta9   
only :at is important 9 ! radio ( to-day
 & ^? ideas :i* seem necess>y if y are  
to foll[ ! rapid advances :i* radio is  
mak+4 ,"s ( ! lett]s y w f9d to require 
a second read+ & /udy4 ,9 ! case ( a few
 y might po/pone a second read+ until y 
h f9i%$ ^? :i* int]e/ y mo/4 ,i'll m>k !
 lett]s to omit 9 ?is way4  ,all !      
lett]s w be writt5 j z ,i w\ld talk to  
y1 = ,i %all draw little sket*es z ,i g 
along4 ,"o ( !m w tell y h[ to exp]im5t 
= y\rself4 ,?is w be ! mo/ int]e/+ (    
all4 ,y c f9d pl5ty ( books to tell y h[
 radio sets op]ate & :at to d1 b v few  
except "s = advanc$ /ud5ts tell y h[ to 
exp]im5t = y\rself4 ,n to wa/e "t 9 y\r 
own exp]im5ts1 h[ev]1 y w ne$ to be q   

famili> ) ! ideas ( ! o!r lett]s4  ,:at 
is a radio set8 ,copp] wires1 t9foil1   
glass plates1 %eets ( mica1 metal1 &    
wood4 ,:]e does x get its ability to    
"w--t is1 :]e does ! en]gy come f :i*   
runs ! set8 ,f batt]ies or f dynamos4 ,t
 mu* y "k already1 b :at is ! real      
reason t we c use copp] wires1 metal    
plates1 audions1 cry/als1 & batt]ies to 
s5d messages & to receive !m8  ,! reason
 is t all ^! ?+s are made ( little      
specks1 too t9y ev] to see1 :i* we might
 call specks ( electricity4 ,!re are    
only two k9ds ( specks & we _h bett]    
give !m _! "r names at once to save "t4 
,"o k9d ( speck is call$ electron & !   
o!r k9d proton4 ,h[ d !y diff]8 ,!y     
probably diff] 9 size b we don't yet "k 
s v mu* ab\t _! sizes4 ,!y diff] 9      
laz9ess a great deal4 ,"o is ab\t #ahde 
times z lazy z ! o!r4 ,t is1 x has      
eighte5 hundr$ & =ty-five times z mu*   
in]tia z ! o!r4 ,x is h>d] to get x />t$
 b x is j z mu* h>d] to get x to /op    
aft] x is once />t$ or to *ange its     

direction & g a diff]5t direction4 ,!   
proton has ! l>g] in]tia4 ,x is !       
electron :i* is ! easi] to />t or /op4  
,h[ else d !y diff]8 ,!y diff] 9 _!     
actions4 ,protons don't l to associate )
 o!r protons b take q ke5ly to          
electrons4 ,& electrons--!y g ) protons 
b !y won't associate ) ea* o!r4 ,an     
electron always likes to be close to a  
proton4 ,two is company :5 "o is an     
electron & ! o!r a proton b ?ree is a   
cr[d always4  ,x doesn't make any       
diff]5ce to a proton :at electron x is  
keep+ company ) provid$ only x is an    
electron & n ano!r proton4 ,all         
electrons are alike z f> z we c tell & s
 are all protons4 ,t means t all ! /uff1
 or matt]1 ( our _w is made up ( two    
k9ds ( build+ blocks1 & all ! blocks (  
ea* k9d are j alike4 ,( c\rse y mu/n't  
?9k ( ^! blocks z l bricks1 = we don't  
"k _! %apes4  ,!n !re is ano!r reason :y
 y mu/ n ?9k ( !m z bricks & t is       
because :5 y build a h\se out ( bricks  
ea* brick mu/ re/ on ano!r4 ,betwe5 an  

electron & any o!r electron or betwe5   
two protons or betwe5 an electron & a   
proton !re is usually a relatively      
enorm\s di/ance4 ,!re is en\gh space s t
 lots ( o!r electrons or protons c\ld be
 fitt$ 9 betwe5 if only !y 7 will+ to   
get t close toge!r4  ,sometimes !y d get
 v close toge!r4 ,i c tell y h[ if y w  
imag9e f\r small boys play+ tag4        
,suppose ,tom & ,dick don't l to play ) 
ea* o!r & run away f ea* o!r if !y c4   
,n[ suppose t ,bill & ,sam won't play ) 
ea* o!r if !y c help x b t ei!r ( !m w  
play ) ,tom or ,dick :5ev] !re is a     
*ance4 ,n[ suppose ,tom & ,bill see ea* 
o!r2 !y />t runn+ t[>d ea* o!r to get up
 "s sort ( a game4 ,b ,sam sees ,tom at 
! same "t1 s he />ts runn+ to jo9 him   
ev5 ?\gh ,bill is go+ to be !re too4    
,mean:ile ,dick sees ,bill & ,sam runn+ 
along & s9ce !y are 8 natural playmates 
he foll[s !m4 ,9 a m9ute !y are all     
toge!r1 & play+ a great game2 al?\gh "s 
( ! boys don't l to play toge!r4  ,:5ev]
 !re is a gr\p ( protons & electrons    

play+ toge!r we h :at we call an atom4  
,!re are ab\t n9ety diff]5t games :i*   
electrons & protons c play1 t is n9ety  
diff]5t k9ds ( atoms4 ,^! games diff] 9 
! numb] ( electrons & protons :o play & 
9 ! way !y arrange !mselves4 ,l>g] games
 c be =m$ if a numb] ( atoms jo9 toge!r4
 ,!n !re is a molecule4 ,( molecules !re
 are z _m k9ds z !re are diff]5t        
sub/ances 9 ! _w4 ,x takes a lot (      
molecules toge!r to =m some?+ big en\gh 
to see1 = ev5 ! l>ge/ molecule1 t ( />*1
 is mu* too small to be se5 by itself ) 
! be/ possible microscope4  ,:at sort ( 
a molecule is =m$ w dep5d ^u h[ _m & :at
 k9ds ( atoms gr\p toge!r to play ! l>g]
 game4 ,:5ev] !re is a big game x       
doesn't mean t ! little atomic gr\ps :i*
 ent] into x are all *ang$ ar\nd4 ,!y   
keep toge!r l a troop ( boy sc\ts 9 a   
gr& picnic 9 :i* lots ( troops are      
pres5t4 ,at any rate !y keep toge!r     
en\gh s t we c /ill call !m a gr\p1 t is
 an atom1 ev5 ?\gh !y d adapt _! game   
some:at s z to fit 9 ) o!r gr\ps--t is )

 o!r atoms4  ,:at w ! k9d ( atom dep5d  
^u8 ,x w dep5d ^u h[ _m electrons &     
protons are gr\p$ toge!r 9 x to play _! 
little game4 ,h[ any atom behaves s f> z
 associat+ ) o!r gr\ps or atoms w dep5d 
^u :at sort ( a game its own electrons &
 protons are play+4  ,n[ ! simple/ k9d (
 a game t c be play$1 & ! "o ) ! smalle/
 numb] ( electrons & protons1 is t play$
 by a s+le proton & a s+le electron4 ,i 
don't "k j h[ x is play$ b ,i %\ld guess
 t !y sort ( *ase ea* o!r ar\nd 9       
circles4 ,at any rate ,i d "k t ! atom  
call$ hydrog5 is =m$ by j "o proton & "o
 electron4 ,suppose !y 7 magnifi$ until 
!y 7 z l>ge z ! moon & ! e>?4 ,!n !y    
w\ld be j ab\t z f> ap>t b ! small] "o  
w\ld be ! proton4  ,t hydrog5 atom is   
responsible = lots ( int]e/+ ?+s = x is 
a great "o to jo9 ) o!r atoms4 ,we don't
 (t5 f9d x by itself al?\gh we c make x 
*ange its p>tn]s & g f "o molecule to   
ano!r v easily4 ,t is :at happ5s e "t y 
/a9 any?+ ) acid4 ,a hydrog5 atom leaves
 a molecule ( ! acid & !n x isn't acid  

any m4 ,:at rema9s isn't a happy gr\p   
ei!r = x has lo/ "s ( its playfell[s4 ,!
 hydrog5 goes & jo9s ) ! /uff :i* gets  
/a9$4 ,b x doesn't jo9 ) ! :ole         
molecule2 x picks out "p ( x to         
associate ) & t leaves ! o!r "p to take 
! place ( ! hydrog5 9 ! orig9al molecule
 ( acid f :i* x came4 ,_m ( ! actions   
:i* we call *emi/ry are m]ely ! result (
 su* *anges ( atoms f "o molecule to    
ano!r4  ,n only does ! hydrog5 atom l to
 associate 9 a l>g] game ) o!r k9ds (   
atoms b x likes to d s ) "o ( its own   
k9d4 ,:5 x does we h a molecule (       
hydrog5 gas1 ! same gas z is us$ 9      
balloons4  ,we hav5't seem$ to get v f> 
yet t[>d radio b y c see h[ we %all :5  
,i tell y t next "t ,i %all write ( m   
complicat$ games su* z are play$ 9 !    
atoms ( copp] :i* =m ! wires ( radio    
sets & ( h[ ^! wires c d :at we call    
c>ry+ an electric curr5t4     ,,lett] #b
  ,,:y ,a ,,copp] ,,wire ,,w ,,conduct  
,,electricity   ,,my ,,de> ,,"y         
,,atomi/3  ,y h le>n$ t ! simple/ gr\p  

:i* c be =m$ by protons & electrons is  
"o proton & "o electron *as+ ea* o!r    
ar\nd 9 a fa/ game4 ,?is gr\p is call$  
an atom ( hydrog54 ,a molecule ( hydrog5
 is two ( ^! gr\ps toge!r4  ,all ! o!r  
possible k9ds ( gr\ps are m complicat$4 
,! next simple/ is t ( ! atom ( helium4 
,helium is a gas ( :i* small quantities 
are obta9$ f c]ta9 oil wells & !re isn't
 v mu* ( x to be obta9$4 ,x is an in]t  
gas1 z we call x1 because x won't burn  
or comb9e ) any?+ else4 ,x doesn't c>e  
to ent] into ! l>g] games ( molecul>    
gr\ps4 ,x is satisfi$ to be z x is1 s t 
x isn't mu* use 9 *emi/ry because y c't 
make any?+ else out ( x4 ,t's ! reason  
:y x is s highly recomm5d$ = fill+      
balloons or air%ips1 because x _c burn  
or explode4 ,x is n z light z hydrog5 b 
x s]ves q well = mak+ balloons buoyant 9
 air4  ,?is helium atom is made up ( f\r
 electrons & f\r protons4 ,"r at ! c5t] 
!re is a small closely cr[d$ gr\p :i*   
conta9s all ! protons & two ( !         
electrons4 ,! o!r two electrons play    

ar\nd q a little way f ?is inn] gr\p4 ,x
 w make our explanations easi] if we    
le>n to call ?is inn] gr\p ! nucleus ( !
 atom4 ,x is ! c5t] ( ! atom & ! o!r two
 electrons play ar\nd ab\t x j z ! e>? &
 ,m>s & ! o!r planets play or revolve   
ab\t ! sun z a c5t]4 ,t is :y we %all   
call ^! two electrons planet>y          
electrons4  ,!re are ab\t n9ety diff]5t 
k9ds ( atoms & !y all h names4 ,"s ( !m 
are m famili> ?an hydrog5 & helium4 ,=  
example1 !re is ! iron atom1 ! copp]    
atom1 ! sulphur atom & s on4 ,"s ( ^!   
atoms y ought to "k & s1 be=e tell+ y m 
( h[ atoms are =m$ by protons &         
electrons1 ,i am go+ to write d[n !     
names ( "s ( ! atoms :i* we h 9 ! e>? & 
rocks ( our _w1 9 ! wat] ( ! oceans1 & 9
 ! air above4  ,/>t fir/ ) air4 ,x is a 
mixture ( sev]al k9ds ( gases4 ,ea* gas 
is a diff]5t k9d ( atom4 ,!re is j a    
slight trace ( hydrog5 & a v small am\nt
 ( helium & ( "s o!r gases :i* ,i won't 
bo!r y ) le>n+4 ,mo/ ( ! air1 h[ev]1 is 
nitrog51 ab\t #gh p]c5t 9 fact & almo/  

all ! re/ is oxyg54 ,ab\t #bj4h p]c5t is
 oxyg5 s t all ! gases o!r ?an ^! two   
make up only ab\t #a4b p]c5t ( !        
atmosph]e 9 :i* we live4  ,illu/ration3 
,pl4 ,,ii4--,bird's-eye ,view ( ,radio  
,c5tral 7,c\rtesy ( ,radio ,corporation 
( ,am]ica74  ,! e>? & rocks also conta9 
a great deal ( oxyg52 ab\t #dg4c p]c5t (
 ! atoms :i* =m e>? & rocks are oxyg5   
atoms4 ,ab\t half ( ! re/ ( ! atoms are 
( a k9d call$ silicon4 ,s& is made up ( 
atoms ( silicon & oxyg5 & y "k h[ mu* s&
 !re is4 ,ab\t #bg4g p]c5t ( ! e>? & its
 rocks is silicon4 ,! next mo/ important
 k9d ( atom 9 ! e>? is alum9um & aft] t 
iron & !n calcium4 ,"h is ! way !y run 9
 p]c5tages3 ,alum9um #g4h p]c5t2 iron   
#d4e p]c5t2 calcium #c4e p]c5t2 sodium  
#b4d p]c5t2 potassium #b4d p]c5t2       
magnesium #b4b p]c5t4 ,besides ^! :i*   
are mo/ important !re is ab\t #j4b p]c5t
 ( hydrog5 & ! same am\nt ( c>bon4 ,!n  
!re is a little phosphorus1 a little    
sulphur1 a little fluor9e1 & small      
am\nts ( all ( ! re/ ( ! diff]5t k9ds ( 

atoms4  ,sea wat] is mo/ly oxyg5 &      
hydrog51 ab\t #he4h p]c5t ( oxyg5 &     
#aj4g p]c5t ( hydrog54 ,t is :at y w\ld 
expect = wat] is made up ( molecules :i*
 9 turn are =m$ by two atoms ( hydrog5 &
 "o atom ( oxyg54 ,! oxyg5 atom is ab\t 
sixte5 times z heavy z ! hydrog5 atom4  
,h[ev]1 = e oxyg5 atom !re are two      
hydrog5 atoms s t = e p\nd ( hydrog5 9  
wat] !re are ab\t eight p\nds ( oxyg54  
,t is :y !re is ab\t eight times z high 
a p]c5tage ( oxyg5 9 sea wat] z !re is (
 hydrog54  ,mo/ ( sea wat]1 !re=e1 is j 
wat]1 t is1 pure wat]4 ,b x conta9s "s  
o!r sub/ances z well & ! be/ kn[n ( ^!  
is salt4 ,salt is a sub/ance ! molecules
 ( :i* conta9 atoms ( sodium & ( *lor9e4
 ,t is :y sea wat] is ab\t #a4a p]c5t   
sodium & ab\t #b4a p]c5t *lor9e4 ,!re   
are "s o!r k9ds ( atoms 9 sea wat]1 z y 
w\ld expect1 = x gets all ! sub/ances   
:i* ! wat]s ( ! e>? dissolve & c>ry d[n 
to x b !y are unimportant 9 am\nts4  ,n[
 we "k some?+ ab\t ! names ( ! important
 k9ds ( atoms & c take up aga9 ! "q ( h[

 !y are =m$ by protons & electrons4 ,no 
matt] :at k9d ( atom we are deal+ ) we  
always h a nucleus or c5t] & "s         
electrons play+ ar\nd t nucleus l t9y   
planets4 ,! only diff]5ces betwe5 "o k9d
 ( atom & any o!r k9d are diff]5ces 9 ! 
nucleus & diff]5ces 9 ! numb] &         
arrangem5t ( ! planet>y electrons :i*   
are play+ ab\t ! nucleus4  ,no matt] :at
 k9d ( atom we are consid]+ !re is      
always 9 x j z _m electrons z protons4  
,= example1 ! iron atom is =m$ by a     
nucleus & tw5ty-six electrons play+     
ar\nd x4 ,! copp] atom has tw5ty-n9e    
electrons z t9y planets to its nucleus4 
,:at does t mean ab\t its nucleus8 ,t   
!re are tw5ty-n9e m protons 9 ! nucleus 
?an !re are electrons4 ,silv] has ev5 m 
planet>y electrons1 = x has #dg4 ,radium
 has #hh & ! heavie/ atom ( all1 t (    
uranium1 has #ib4  ,we might use numb]s 
= ! diff]5t k9ds ( atoms in/ead ( names 
if we want$ to d s4 ,we c\ld describe   
any k9d ( atom by tell+ h[ _m planet>y  
electrons !re 7 9 x4 ,= example1 hydrog5

 w\ld be numb] #a1 helium numb] #b1     
li?ium ( :i* y p]haps nev] he>d1 w\ld be
 numb] #c1 & s on4 ,oxyg5 is #h1 sodium 
is #aa1 *lor9e is #ag1 iron #bf1 & copp]
 #bi4 ,= ea* k9d ( atom !re is a numb]4 
,let's call t numb] its atomic numb]4   
,n[ let's see :at ! atomic numb] tells  
u4 ,take copp]1 = example1 :i* is numb] 
#bi4 ,9 ea* atom ( copp] !re are #bi    
electrons play+ ar\nd ! nucleus4 ,!     
nucleus itself is a little inn] gr\p (  
electrons & protons1 b !re are m protons
 ?an electrons 9 x2 tw5ty-n9e m 9 fact4 
,9 an atom !re is always an extra proton
 9 ! nucleus = ea* planet>y electron4 ,t
 makes ! total numb] ( protons &        
electrons ! same4  ,ab\t ! nucleus ( a  
copp] atom !re are play+ #bi electrons j
 z if ! nucleus 0 a tea*] responsible = 
#bi *ildr5 :o 7 out 9 ! play y>d4 ,!re  
is "o v funny ?+ ab\t x all1 h[ev]1 & t 
is t we mu/ ?9k ( ! s*ol>s z if !y 7 all
 j alike s t ! tea*] c\ldn't tell "o f !
 o!r4 ,electrons are all alike1 y       
rememb]4 ,all ! tea*] or nucleus c>es = 

is t !re %all be j ! "r numb] play+     
ar\nd h]4 ,y c\ld br+ a boy 9 f "s o!r  
play gr\nd & ! tea*] c\ldn't tell t he 0
 a /rang] b %e w\ld "k t some?+ 0 !     
matt] = !re w\ld be "o too _m 9 h] gr\p4
 ,%e is responsible = j #bi s*ol>s1 & ! 
nucleus ( ! copp] atom is responsible = 
j #bi electrons4 ,x doesn't make any    
diff]5ce :]e ^! electrons come f provid$
 !re are always j #bi play+ ar\nd !     
nucleus4 ,if !re are m or less ?an #bi  
some?+ peculi> w happ54  ,we %all see   
lat] :at might happ51 b fir/ let's ?9k (
 an enorm\s lot ( atoms su* z !re w\ld  
be 9 a copp] wire4 ,a small copp] wire w
 h 9 x billions ( copp] atoms1 ea* ) its
 planet>y electrons play+ _! invisible  
game ab\t _! own nucleus4 ,!re is q a   
little di/ance 9 any atom betwe5 !      
nucleus & any ( ! electrons = :i* x is  
responsible4 ,!re is usually a great]   
di/ance /ill betwe5 "o atomic gr\p & any
 o!r4  ,on ! :ole ! electrons hold      
pretty close to _! own circles ab\t _!  
own nuclei4 ,!re is always "s t5d5cy to 

run away & play 9 "s o!r gr\p4 ,) #bi   
electrons x's no wond] if sometimes "o  
goes w&]+ (f & f9ally gets into ! game  
ab\t "s o!r nucleus4 ,( c\rse1 an       
electron f "s o!r atom may come w&]+    
along & take ! place j left vacant1 s t 
nucleus is satisfi$4  ,we don't "k all  
we might ab\t h[ ! electrons w&] ar\nd f
 atom to atom inside a copp] wire b we d
 "k t !re are always a lot ( !m mov+    
ab\t 9 ! spaces betwe5 ! atoms4 ,"s ( !m
 are go+ "o way & "s ano!r4  ,x's ^!    
w&]+ electrons :i* are affect$ :5 a     
batt]y is connect$ to a copp] wire4 ,e  
s+le electron :i* is away f its home    
gr\p1 & w&]+ ar\nd1 is s5t scamp]+ along
 t[>d ! end ( ! wire :i* is connect$ to 
! positive plate or t]m9al ( ! batt]y & 
away f ! negative plate4 ,t's :at !     
batt]y does to !m = be+ away f home2 x  
drives !m along ! wire4 ,!re's a regul> 
/ream or procession ( !m f ! negative   
end ( ! wire t[>d ! positive4 ,:5 we h a
 /ream ( electrons l ?is we say we h a  
curr5t ( electricity4  ,we'll ne$ to    

le>n m lat] ab\t a curr5t ( electricity 
b "o ( ! fir/ ?+s we ought to "k is h[ a
 batt]y is made & :y x affects ^! w&]+  
electrons 9 ! copp] wire4 ,t's :at ,i   
%all tell y 9 my next lett]4#a          
,footnote #a3 ,! read] :o wi%es ! %orte/
 pa? to ! con/ruction & op]ation ( a    
radio set %\ld omit ! next two lett]s4  
,,lett] #c  ,,h[ ,a ,,batt]y ,,works    
7,?is lett] may be omitt$ on ! fir/     
read+47  ,,my ,,de> ,,boy3  ,:5 ,i 0 a  
boy we us$ to make our own batt]ies =   
our exp]im5ts4 ,t 0 be=e /orage batt]ies
 became z widely us$ z !y are to-day :5 
ev]ybody has "o 9 ! />t+ sy/em ( 8      
automobile4 ,t 0 also be=e ! day ( !    
small dry batt]y su* z we use 9 pocket  
fla% lights4 ,! batt]ies :i* we made 7 l
 ^? :i* !y us$ on telegraph sy/ems1 & 7 
sometimes call$ gravity batt]ies4 ,(    
c\rse1 we tri$ sev]al k9ds & ,i believe 
,i got q a little acid ar\nd ! h\se at  
"o "t or ano!r4 ,i'll tell y ab\t only  
"o k9d b ,i %all use ! words electron1  
proton1 nucleus1 atom1 & molecule1 ab\t 

"s ( :i* no?+ 0 kn[n :5 ,i 0 a boy4  ,we
 us$ a /raight-sid$ glass j> :i* w\ld   
hold ab\t a gallon4 ,on ! bottom we set 
a /> %ap$ arrangem5t made ( %eets (     
copp] ) a long wire sold]$ to x s z to  
rea* up out ( ! j>4 ,!n we p\r$ 9 a     
solution ( copp] sulphate until ! j> 0  
ab\t half full4 ,?is solution 0 made by 
dissolv+ 9 wat] cry/als ( blue vitriol  
:i* we b\ght at ! drug /ore4  ,blue     
vitriol1 or copp] sulphate z ! *emi/s   
w\ld call x1 is a sub/ance :i* =ms      
glassy blue cry/als4 ,its molecules are 
=m$ ( copp] atoms1 sulphur atoms1 &     
oxyg5 atoms4 ,9 ea* molecule ( x !re is 
"o atom ( copp]1 "o ( sulphur & f\r (   
oxyg54  ,:5 x dissolves 9 wat] !        
molecules ( ! blue vitriol g w&]+ out   
into ! spaces betwe5 ! wat] molecules4  
,b t isn't all t happ5s or ! mo/        
important ?+ = "o :o is int]e/$ 9 mak+ a
 batt]y4  ,ea* molecule is =m$ by six   
atoms1 t is by six little gr\ps (       
electrons play+ ab\t six little nuclei4 
,ab\t ea* nucleus !re is go+ on a game b

 "s ( ! electrons are play+ 9 ! game    
ab\t _! own nucleus & at ! same "t tak+ 
"s "p 9 ! game :i* is go+ on ar\nd "o ( 
! o!r nuclei4 ,t's :y ! gr\ps or atoms  
/ay toge!r z a molecule4 ,:5 ! molecules
 w&] out into ! spaces betwe5 ! wat]    
molecules some?+ happ5s to ?is          
complicat$ game4  ,x w be easie/ to see 
:at sort ( ?+ happ5s if we talk ab\t a  
molecule ( ord9>y table salt1 = t has   
only two atoms 9 x4 ,"o atom is sodium &
 "o is *lor9e4 ,! sodium molecule has   
elev5 electrons play+ ar\nd its nucleus4
 ,fairly close to ! nucleus !re are two 
electrons4 ,!n f>!r away !re are eight m
 & ^! are hav+ a p]fect game4 ,!n /ill  
f>!r away f ! nucleus !re is a s+le     
lonely electron4  ,! atom ( *lor9e has  
sev5te5 electrons :i* play ab\t its     
nucleus4 ,close to ! nucleus !re are    
two4 ,a little f>!r away !re are eight j
 z !re are 9 ! sodium atom4 ,!n /ill    
f>!r away !re are sev54  ,i am go+ to   
draw a picture 7,fig4 #a7 to %[ :at ,i  
mean1 b y mu/ rememb] t ^! electrons are

 n all 9 ! same plane z if !y lay on a  
%eet ( pap]1 b are scatt]$ all ar\nd j z
 !y w\ld be if !y 7 specks on a ball4   
,illu/ration3 ,fig4 #a  ,y see t !      
sodium atom has "o lonely electron :i*  
hasn't any play fell[s & t ! *lor9e atom
 has sev5 9 its outside circle4 ,x      
appe>s t eight w\ld make a mu* bett]    
game4 ,suppose t extra electron 9 !     
sodium atom goes ov] & plays ) ^? 9 !   
*lor9e atom s z to make eight 9 !       
outside gr\p z ,i h %[n ,fig4 #b4 ,t w  
be all "r z long z x doesn't get out (  
sight ( its own nucleus because y       
rememb] t ! sodium nucleus is           
responsible = elev5 electrons4 ,! lonely
 electron ( ! sodium atom ne$n't be     
lonely any m if x c p]suade its nucleus 
to /ay s close to ! *lor9e atom t x c   
play 9 ! out] circle ( ! *lor9e atom4   
,illu/ration3 ,fig4 #b  ,! out] circle (
 ! *lor9e atom w !n h a bett] game1 = x 
w h j ! eight t makes a p]fect game4    
,?is c happ5 if ! *lor9e atom w /ay     
close en\gh to ! sodium atom s t !      

out]mo/ electron ( ! sodium atom c play 
9 ! *lor9e circle4 ,y see ev]y?+ w be   
satisfactory if an electron c be %>$ by 
! two atoms4 ,t c happ5 only if ! two   
atoms /ay toge!r2 t is1 if !y =m a      
molecule4 ,t's :y !re are molecules &   
t's :at ,i meant :5 ,i spoke ( !        
molecule z a big game play$ by !        
electrons ( two or m atoms4  ,?is       
molecule :i* is =m$ by a sodium atom & a
 *lor9e atom is call$ a molecule (      
sodium *loride by *emi/s & a molecule ( 
salt by mo/ e "o :o eats x4 ,some?+     
/range happ5s :5 x dissolves4 ,x w&]s   
ar\nd betwe5 ! wat] molecules & = "s    
reason or o!r--we don't "k exactly :y--x
 decides to split up aga9 into sodium & 
*lor9e b x c't q d x4 ,! electron :i*   
jo9$ ! game ab\t ! *lor9e nucleus won't 
leave x4 ,! result is t ! nucleus ( !   
sodium atom gets away b x leaves ?is "o 
electron beh9d4  ,:at gets away isn't a 
sodium atom = x has "o too few          
electrons2 & :at rema9s beh9d isn't a   
*lor9e atom = x has "o too _m electrons4

 ,we call ^! new gr\ps ions f a ,greek  
^w :i* means to g = !y d g1 w&]+ (f into
 ! spaces betwe5 ! wat] molecules4 ,fig4
 #c gives y an idea ( :at happ5s4  ,y   
rememb] t 9 an atom !re are always j z  
_m protons z electrons4 ,9 ?is sodium   
ion :i* is =m$ :5 ! nucleus ( ! sodium  
atom breaks away b leaves beh9d "o      
planet>y electron1 !re is !n "o m proton
 ?an !re are electrons4 ,because x has  
an extra proton1 :i* hasn't any electron
 to associate )1 we call x a plus ion or
 a positive ion4 ,simil>ly we call !    
*lor9e ion1 :i* has "o less proton ?an x
 has electrons1 a m9us or negative ion4 
,illu/ration3 ,fig4 #c  ,n[1 despite !  
fact t ^! ions broke away f ea* o!r !y  
ar5't really satisfi$4 ,any "t t !      
sodium ion c f9d an electron to take !  
place ( ! "o x lo/ x w welcome x4 ,t is1
 ! sodium ion w want to g t[>d places   
:]e !re are extra electrons4 ,9 ! same  
way ! *lor9e ion w g t[>d places :]e    
electrons are want$ z if x c\ld satisfy 
its guilty consci5ce by giv+ up !       

electron :i* x /ole f ! sodium atom1 or 
at lea/ by giv+ away "s o!r electron1 = 
!y are all alike anyway4  ,sometimes a  
positive sodium ion & a negative *lor9e 
ion meet 9 _! w&]+s 9 ! solution & bo?  
get satisfi$ by =m+ a molecule aga94    
,ev5 s !y don't /ay toge!r long be=e !y 
split ap>t & />t w&]+ aga94 ,t's :at    
goes on ov] & ov] aga91 millions (      
times1 :5 y dissolve a little salt 9 a  
glass ( wat]4  ,n[ we c see :at happ5s  
:5 copp] sulphate dissolves4 ,! copp]   
atom has tw5ty-n9e electrons ab\t its   
nucleus & all except two ( ^! are nicely
 gr\p$ = play+ _! games ab\t ! nucleus4 
,two ( ! electrons are r out ( ! game1 &
 are unsatisfi$4 ,!y play ) ! electrons 
( ! "p ( ! molecule :i* is call$        
sulphate1 t is1 ! "p =m$ by ! sulphur   
atom & ! f\r oxyg5 atoms4 ,^! five atoms
 ( ! sulphate "p /ay toge!r v well & s  
we treat !m z a gr\p4  ,! sulphate gr\p 
& ! copp] atom /ay toge!r z long z !y   
are n 9 solution b :5 !y are1 !y act v  
mu* l ! sodium & *lor9e :i* ,i j        

describ$4 ,! molecule splits up into two
 ions1 "o positive & "o negative4 ,!    
positive ion is ! copp] "p except t two 
( ! electrons :i* really belong to a    
copp] atom got left beh9d because !     
sulphate "p w\ldn't give !m up4 ,! re/ (
 ! molecule is ! negative ion4  ,! copp]
 ion is a copp] atom :i* has lo/ two    
electrons4 ,! sulphate ion is a         
comb9ation ( "o sulphur atom1 f\r oxyg5 
atoms & two electrons :i* x /ole f !    
copp] atom4 ,j z ! sodium ion is        
unsatisfi$ because 9 x !re is "o m      
proton ?an !re are electrons1 s ! copp] 
ion is unsatisfi$4 ,z a matt] ( fact x  
is twice z badly unsatisfi$4 ,x has two 
m protons ?an x has electrons4 ,we say x
 has twice ! electrical *>ge ( ! sodium 
ion4  ,j l a sodium ion ! copp] ion w   
t5d to g t[>d any place :]e !re are     
extra electrons :i* x c get to satisfy  
its own ne$s4 ,9 mu* ! same way !       
sulphate ion w g t[>d places :]e x c    
give up its two extra electrons4        
,sometimes1 ( c\rse1 z ions ( ^! two    

k9ds w&] ab\t betwe5 ! wat] molecules1  
!y meet & satisfy ea* o!r by =m+ a      
molecule ( copp] sulphate4 ,b if !y d !y
 w split ap>t lat] on2 t is1 !y w       
dissociate z we %\ld say4  ,n[ let's g  
on ) ! k9d ( batt]ies ,i us$ to make z a
 boy4 ,y c see t 9 ! solution ( copp]   
sulphate at ! bottom ( ! j> !re 0 always
 pres5t a lot ( positive copp] ions & ( 
negative sulphate ions4  ,on top ( ?is  
solution ( copp] sulphate ,i p\r$ v     
c>efully a weak solution ( sulphuric    
acid4 ,z ,i told y1 an acid always has  
hydrog5 9 its molecules4 ,sulphuric acid
 has molecules =m$ by two hydrog5 atoms 
& "o ( ! gr\ps :i* we decid$ to call    
sulphate4 ,a bett] "n = ?is acid w\ld be
 hydrog5 sulphate = t w\ld imply t its  
molecule is ! same z "o ( copp]         
sulphate1 except t ! place ( ! copp] is 
tak5 by two atoms ( hydrog54 ,x takes   
two atoms ( hydrog5 because ! copp] atom
 has two lonely electrons :ile a hydrog5
 atom only has "o4 ,x takes two         
electrons to fill up ! game :i* !       

electrons ( ! sulphate gr\p are play+4  
,if x c get ^! f a s+le atom1 all "r2 b 
if x has to get "o f ea* ( two atoms1 x 
w d x t way4  ,i rememb] :5 ,i mix$ !   
sulphuric acid ) wat] t ,i le>n$ to p\r 
! acid into ! wat] & n ! o!r way ar\nd4 
,spatt]+s ( sulphuric acid are n good = 
h&s or clo!s4 ,) ?is solution ,i fill$ !
 j> almo/ to ! top & !n hung ov] ! edge 
a sort ( a cr['s foot %ape ( ca/ z9c4 ,!
 z9c rea*$ d[n into ! sulphuric acid    
solution4 ,!re 0 a b9d+ po/ on x to :i* 
a wire c\ld be connect$4 ,?is wire & !  
"o :i* came f ! plate ( copp] at !      
bottom 7 ! two t]m9als ( ! batt]y4 ,we  
call$ ! wire f ! copp] positive & ! "o f
 ! z9c negative4  ,n[ we %all see :y &  
h[ ! batt]y work$4 ,! molecules (       
sulphuric acid dissociate 9 solution j z
 d ^? ( copp] sulphate4 ,:5 sulphuric   
acid molecules split1 ! sulphate "p goes
 away ) two electrons :i* don't belong  
to x & ea* ( ! hydrog5 atoms goes away  
by itself b )\t its electron4 ,we call  
ea* a hydrog5 ion b y c see t ea* is a  

s+le proton4  ,9 ! two solutions are    
pieces ( z9c & copp]4 ,z9c is l all !   
re/ ( ! metals 9 "o way4 ,atoms ( metals
 always h lonely electrons = :i* !re    
doesn't seem to be room 9 ! game :i* is 
go+ on ar\nd _! nuclei4 ,copp] z we saw 
has two lonely electrons 9 ea* atom4    
,z9c also has two4 ,"s metals h "o & "s 
two & "s ev5 m lonely electrons 9 ea*   
atom4  ,:at happ5s !n is ?is4 ,!        
sulphate ions w&]+ ar\nd 9 ! weak       
solution ( sulphuric acid come along    
beside ! z9c plate & beckon to its      
atoms4 ,! sulphate ions _h a great deal 
r play ! game call$ z9c sulphate ?an !  
game call$ hydrog5 sulphate4 ,s ! z9c   
atoms leave _! places to jo9 ) !        
sulphate ions4 ,b wait a m9ute6 ,!      
sulphate ions h two extra electrons :i* 
!y kept f ! hydrog5 atoms4 ,!y don't ne$
 ! two lonely electrons :i* ea* z9c atom
 c\ld br+ & s ! z9c atom leaves beh9d x 
^! unnecess>y electrons4  ,e "t a z9c   
atom leaves ! plate x fails to take all 
its electrons ) x4 ,:at leaves ! z9c    

plate1 !re=e1 to g into solution is     
really n a z9c atom b is a z9c ion2 t   
is1 x is ! nucleus ( a z9c atom & all   
except two ( ! planet>y electrons4  ,e  
"t a z9c ion leaves ! plate !re are left
 beh9d two electrons4 ,! plate doesn't  
want !m = all ! re/ ( its atoms h j !   
same numb] ( protons z ( electrons4 ,:]e
 are !y to g8 ,we %all see 9 a m9ute4   
,sometimes ! z9c ions :i* h got into    
solution meet ) sulphate ions & =m z9c  
sulphate molecules4 ,b if !y d ^!       
molecules split up soon] or lat] into   
ions aga94 ,9 ! upp] "p ( ! liquid 9 !  
j>1 !re=e1 !re are sulphate ions :i* are
 negative & two k9ds ( positive ions1   
namely1 ! hydrog5 ions & ! z9c ions4    
,be=e ! z9c ions began to cr[d 9 !re 7 j
 en\gh hydrog5 ions to g ) ! sulphate   
ions4 ,z x is1 ! entrance ( ! z9c ions  
has increas$ ! numb] ( positive ions &  
n[ !re are too _m4 ,"s ( ! positive     
ions1 !re=e1 & p>ticul>ly ! hydrog5     
ions1 because ! sulphate pref]s to      
associate ) ! z9c ions1 c't f9d en\gh   

playfell[s & s g d[n 9 ! j>4  ,d[n 9 !  
bottom ( ! j> ! hydrog5 ions f9d m      
sulphate ions to play )1 b t leaves !   
copp] ions :i* us$ to play ) ^! sulphate
 ions )\t any playmates4 ,s ! copp] ions
 g /ill fur!r d[n & jo9 ) ! copp] atoms 
( ! copp] plate4 ,!y hav5't mu* "r to d 
s1 = y rememb] t !y hav5't _! prop]     
numb] ( electrons4 ,ea* copp] ion lacks 
two electrons ( be+ a copp] atom4       
,nev]!less !y jo9 ! copp] plate4 ,!     
result is a plate ( copp] :i* has too   
few electrons4 ,x ne$s two electrons = e
 copp] ion :i* jo9s x4  ,h[ ab\t ! z9c  
plate8 ,y rememb] t x has two electrons 
m ?an x ne$s = e z9c ion :i* has left x4
 ,if only ! extra electrons on !        
negative z9c plate c\ld get ar\nd to !  
positive copp] plate4 ,!y c if we       
connect a wire f "o plate to ! o!r4 ,!n 
! electrons f ! z9c /ream into ! spaces 
betwe5 ! atoms ( ! wire & pu% ahead ( !m
 ! electrons :i* are w&]+ ar\nd 9 ^!    
spaces4 ,at ! o!r end an equal numb] (  
electrons leave ! wire to satisfy !     

positive copp] plate4 ,s we h a /ream ( 
electrons 9 ! wire1 t is1 a curr5t (    
electricity & our batt]y is work+4  ,t's
 ! sort ( a batt]y ,i us$ to play )4 ,if
 y und]/& x y c get ! g5]al idea ( all  
batt]ies4 ,let me express x 9 g5]al     
t]ms4  ,at ! negative plate ( a batt]y  
ions g into solution & electrons are    
left beh9d4 ,at ! o!r end ( ! batt]y    
positive ions are cr[d$ out ( solution &
 jo9 ! plate :]e !y cause a sc>city (   
electrons2 t is1 make ! plate positive4 
,if a wire is connect$ betwe5 ! two     
plates1 electrons w /ream ?r\gh x f !   
negative plate to ! positive2 & ?is     
/ream is a curr5t ( electricity4        
,illu/ration3 ,pl4 ,,iii4--,dry ,batt]y 
= ,use 9 ,audion ,circuits 7,c\rtesy (  
,national ,c>bon ,co41 ,inc474 ,/orage  
,batt]y 7,c\rtesy ( ! ,electric ,/orage 
,batt]y ,co474     ,,lett] #d  ,,!      
,,batt]ies ,,9 ,,y\r ,,radio ,,set      
7,?is lett] may be omitt$ on ! fir/     
read+47  ,,my ,,de> ,,"y ,,man3  ,y w   
ne$ sev]al batt]ies :5 y come to set up 

y\r radio receiv] b y won't use su*     
clumsy affairs z ! gravity cell :i* ,i  
describ$ 9 my la/ lett]4 ,"s ( y\r      
batt]ies w be dry batt]ies ( ! size us$ 
9 pocket fla% lights4  ,^! are n really 
dry1 = betwe5 ! plates !y are fill$ ) a 
moi/ pa/e :i* is !n seal$ 9 ) wax to    
keep x f dry+ out or f spill+4 ,in/ead (
 z9c & copp] ^! batt]ies use z9c &      
c>bon4 ,no glass j> is ne$$1 = ! z9c is 
=m$ into a j> %ape4 ,9 ?is is plac$ !   
pa/e & 9 ! c5t] ( ! pa/e a rod or b> (  
c>bon4 ,! pa/e doesn't conta9 sulphuric 
acid1 b in/ead has 9 x a /uff call$ sal 
ammoniac2 t is1 ammonium *loride4  ,!   
batt]y1 h[ev]1 acts v mu* l ! "o ,i     
describ$ 9 my la/ lett]4 ,ions ( z9c    
leave ! z9c & w&] into ! moi/ pa/e4 ,^! 
ions are positive1 j z 9 ! case ( !     
gravity batt]y4 ,! result is t !        
electrons :i* us$ to associate ) a z9c  
ion to =m a z9c atom are left 9 ! z9c   
plate4 ,t makes ! z9c negative = x has m
 electrons ?an protons4 ,! z9c ions take
 ! place ( ! positive ions :i* are      

already 9 ! pa/e4 ,! positive ions :i*  
orig9ally belong$ ) ! pa/e1 !re=e1 move 
along to ! c>bon rod & !re get "s       
electrons4 ,tak+ electrons away f !     
c>bon leaves x ) too _m protons2 t is1  
leaves x positive4 ,9 ! little fla%     
light batt]ies1 !re=e1 y w always f9d t 
! r\nd c>bon rod1 :i* /icks out ( !     
c5t]1 is positive & ! z9c cas+ is       
negative4  ,! tr\ble ) ! batt]y l ! "o  
,i us$ to make is t ! z9c plate wa/es   
away4 ,e "t a z9c ion leaves x t means t
 ! great] "p ( an atom is gone4 ,!n :5 !
 two electrons :i* 7 left beh9d get a   
*ance to />t along a copp] wire t[>d !  
positive plate ( ! batt]y !re goes ! re/
 ( ! atom4 ,aft] a :ile !re is no m z9c 
plate4 ,x is easy to see :at has happ5$4
 ,all ! z9c has gone into solution or   
be5 eat5 away z mo/ p say4 ,dry         
batt]ies1 h[ev]1 don't /op work+ because
 ! z9c gets us$ up1 b because ! active  
/uff 9 ! pa/e1 ! ammonium *loride1 is   
*ang$ into some?+ else4  ,!re's ano!r   
k9d ( batt]y :i* y w ne$ to use ) y\r   

radio set2 t is ! /orage batt]y4 ,/orage
 batt]ies c be us$ ov] & ov] aga9 if !y 
are *>g$ betwe5 times & w la/ = a long  
"t if prop]ly c>$ =4 ,!n too1 !y c give 
a l>ge curr5t1 t is1 a big swift-mov+   
/ream ( electrons4 ,y w ne$ t :5 y wi%  
to heat ! filam5t ( ! audion 9 y\r      
receiv+ set4  ,! ,engli% call our /orage
 batt]ies by ! "n accumulators4 ,i don't
 l t "n at all1 b ,i don't l our "n = !m
 ne>ly z well z ,i d ! "n rev]sible     
batt]ies4 ,nobody uses ?is la/ "n       
because x's too late to *ange4          
,nev]!less a /orage batt]y is rev]sible1
 = x w "w ei!r way at an in/ant's       
notice4  ,a /orage batt]y is some?+ l a 
boy's wagon on a hill side4 ,x w run d[n
 hill b x c be pu%$ up aga9 = ano!r     
desc5t4 ,y c use x to s5d a /ream (     
electrons ?r\gh a wire f its negative   
plate to its positive plate4 ,!n if y   
connect ^! plates to "s o!r batt]y or to
 a g5]ator1 7t is1 a dynamo7 y c make a 
/ream ( electrons g 9 ! o!r direction4  
,:5 y h done s long en\gh ! batt]y is   

*>g$ aga9 & ready to dis*>ge4  ,i am n  
go+ to tell y v mu* ab\t ! /orage batt]y
 b y ought to "k a little ab\t x if y   
are to own & run "o ) y\r radio set4 ,:5
 x is all *>g$ & ready to "w1 ! negative
 plate is a lot ( s(t spongy lead held 9
 place by a frame ( h>d] lead4 ,!       
positive plate is a lead frame ) small  
squ>es :i* are fill$ ) lead p]oxide1 z x
 is call$4 ,?is is a sub/ance )         
molecules =m$ ( "o lead atom & two oxyg5
 atoms4 ,:y ! *emi/s call x lead p]oxide
 in/ead ( j lead oxide ,i'll tell y "s  
o!r "t1 b n 9 ^! lett]s4  ,betwe5 ! two 
plates is a wood sep>ator to keep pieces
 ( lead f fall+ d[n betwe5 & t\*+ bo?   
plates4 ,y "k :at w\ld happ5 if a piece 
( metal t\*$ bo? plates4 ,!re w\ld be a 
%ort circuit1 t is1 a sort ( a %ort cut 
across lots by :i* "s ( ! electrons f ! 
negative plate c\ld get to ! positive   
plate )\t go+ along ! wires :i* we want 
!m to travel4 ,t's :y !re are sep>ators4
  ,! two plates are 9 a j> ( sulphuric  
acid solution4 ,! sulphuric acid has    

molecules :i* split up 9 solution1 z y  
rememb]1 into hydrog5 ions & ! ions :i* 
we call$ sulphate4 ,9 my gravity batt]y 
! sulphate ions us$ to coax ! z9c ions  
away into ! solution4 ,9 ! /orage batt]y
 on ! o!r h& ! sulphate ions c get to   
mo/ ( ! lead atoms because ! lead is s  
spongy4 ,:5 !y d1 !y =m lead sulphate "r
 :]e ! lead atoms are4 ,!y don't really 
ne$ :ole lead atoms1 because !y h two m 
electrons ?an !y des]ve1 s !re are two  
extra electrons = e molecule ( lead     
sulphate :i* is =m$4 ,t's :y ! spongy   
lead plate is negative4  ,! lead        
sulphate won't dissolve1 s x /ays !re on
 ! plate z a :iti% coat+4 ,n[ see :at t 
means4 ,:at are ! hydrog5 ions go+ to d8
 ,z long z !re 0 sulphuric acid 9 ! wat]
 !re 0 pl5ty ( sulphate ions = !m to    
associate ) z (t5 z !y met2 & !y w\ld   
meet pretty (t54 ,b if ! sulphate ions  
get ti$ up ) ! lead ( ! plate !re w be  
too _m hydrog5 ions left 9 ! solution4  
,n[ :at are ! hydrog5 ions to d8 ,!y are
 go+ to get z f> away f ea* o!r z !y c1 

= !y are no?+ b protons2 & protons don't
 l to associate4 ,!y only /ay$ ar\nd 9 !
 fir/ place because !re 0 always pl5ty (
 sulphate ions ) :om !y lik$ to play4   
,:5 ! hydrog5 ions try to get away f ea*
 o!r !y g to ! o!r plate ( ! batt]y1 &  
!re !y w get "s electrons1 if !y h to   
/eal 9 _! turn4  ,i won't try to tell y 
all t happ5s at ! o!r plate4 ,! hydrog5 
ions get ! electrons :i* !y ne$1 b !y   
get some?+ m4 ,!y get "s ( ! oxyg5 away 
f ! plate & s =m molecules ( wat]4 ,y   
rememb] t wat] molecules are made ( two 
atoms ( hydrog5 & "o ( oxyg54 ,mean:ile1
 ! lead atoms1 :i* h lo/ _! oxyg5       
companions1 comb9e ) "s ( ! sulphate    
ions :i* are 9 t neighborhood4 ,dur+ !  
mix-up electrons are c>ri$ away f !     
plate & t leaves x positive4  ,! result 
( all ?is is a little lead sulphate on  
ea* plate1 a negative plate :]e ! spongy
 lead 01 & a positive plate :]e ! lead  
p]oxide 04  ,notice v c>efully t ,i said
 a little lead sulphate on ea* plate4 ,!
 sort ( ?+ ,i h be5 describ+ doesn't g  

on v long4 ,if x did ! batt]y w\ld run  
d[n inside itself & !n :5 we came to />t
 our automobile we w\ld h to get out &  
crank4  ,h[ long does x g on8 ,answ]    
ano!r "q fir/4 ,s f> we hav5't connect$ 
any wire betwe5 ! two plates ( ! batt]y1
 & s none ( ! electrons on ! negative   
plate h any way ( gett+ ar\nd to !      
positive plate :]e electrons are badly  
ne$$4 ,e "t a negative sulphate ion     
comb9es ) ! spongy lead ( ! negative    
plate !re are two m electrons add$ to t 
plate4 ,y "k h[ well electrons l ea*    
o!r4 ,d !y let ! sulphate ions keep giv+
 t plate m electrons8 ,!re is ! o!r "q2 
& ! answ] is t !y d n4 ,e electron t is 
add$ to t plate makes x j s mu* h>d] =  
ano!r sulphate ion to get ne> en\gh to d
 bus9ess at all4 ,t's :y aft] a few     
extra electrons h accumulat$ on ! spongy
 lead plate ! actions :i* ,i 0 describ+ 
come to a /op4  ,d !y ev] beg9 aga98 ,!y
 d j z soon z !re is any r$uction 9 !   
numb] ( electrons :i* are hopp+ ar\nd 9 
! negative plate try+ to keep out ( ea* 

o!r's way4 ,:5 we connect a wire betwe5 
! plates we let "s ( ^! extra electrons 
( ! negative plate pass along to !      
positive plate :]e !y w be welcome4 ,& !
 mom5t a c\ple ( !m />t (f on t err&    
along comes ano!r sulphate ion 9 !      
solution & l&s two m electrons on !     
plate4 ,t's h[ ! batt]y keeps on        
dis*>g+4  ,we mu/n't let x get too mu*  
dis*>g$ = ! lead sulphate is n soluble1 
z ,i j told y1 & x w coat up t plate    
until !re isn't mu* *ance ( gett+ !     
process to rev]se4 ,t's :y we are s     
c>eful n to let ! dis*>ge process g on  
too long be=e we rev]se x & *>ge4 ,t's  
:y1 :5 ! c> batt]y has be5 us$ pretty   
h>d to />t ! c>1 ,i l to run q a :ile to
 let ! g5]ator *>ge ! batt]y aga94 ,:5 !
 batt]y *>ges1 ! process rev]ses & we   
get spongy lead on ! negative plate &   
lead p]oxide on ! positive plate4  ,y've
 le>n$ en\gh = "o day4 ,write me y\r    
que/ions & ,i'll answ] & !n g on 9 my   
next lett] to tell h[ ! audion works4 ,y
 "k ab\t conduction ( electricity 9     

wires2 t is1 ab\t ! electron /ream1 &   
ab\t batt]ies :i* c cause ! /ream4 ,n[ y
 are ready = ! mo/ wond]ful little      
device kn[n to sci5ce3 ! audion4        
,,lett] #e  ,,gett+ ,,electrons ,,f ,a  
,,heat$ ,,wire   ,,de> ,,son3  ,i 0     
pleas$ to get y\r lett] & its que/ions4 
,yes1 a proton is a speck ( electricity 
( ! k9d we call positive & an electron  
is ( ! k9d we call negative4 ,y might   
rememb] ?is simple law2 ,l k9ds (       
electricity repel1 & unlike attract4  ,!
 ^w ion#b is us$ to describe any atom1  
or "p ( a molecule :i* c travel by      
itself & has m or less ?an its prop]    
numb] ( electrons4 ,by prop] numb] (    
electrons ,i mean prop] = ! numb] (     
protons :i* x has4 ,if an ion has m     
electrons ?an protons x is negative2 if 
! inequality is ! o!r way ar\nd x is    
positive4 ,an atom or molecule has nei!r
 m nor less protons ?an electrons4 ,x is
 neutral or un*>g$1 z we say4  ,no1 n e 
sub/ance :i* w dissolve w dissociate or 
split up into positive & negative ions4 

,! salt :i* y eat w1 b ! sug> w n4 ,if y
 want a "n = ^? sub/ances :i* w         
dissociate 9 solution1 call !m          
electrolytes4 ,to make a batt]y we mu/  
always use an electrolyte4  ,yes1 x is  
h>d to ?9k ( a smoo? piece ( metal or a 
wire z full ( holes4 ,ev5 9 ! d5se/     
solids l lead ! atoms are q f> ap>t &   
!re are l>ge spaces betwe5 ! nuclei & ! 
planet>y electrons ( ea* atom4  ,i hope 
?is cle>s up ! que/ions 9 y\r m9d = ,i  
want to get along to ! vacuum tube4 ,by 
a vacuum we mean a space :i* has v few  
atoms or molecules 9 x1 j z few z we c  
possibly get1 ) ! be/ me?ods ( pump+ &  
exhau/+4 ,= ! pres5t let's suppose t we 
c get all ! gas molecules1 t is1 all !  
air1 out ( a little glass bulb4  ,!     
audion is a glass bulb l an electric    
light bulb :i* has 9 x a ?read1 or      
filam5t1 ( metal4 ,! ends ( ?is filam5t 
ext5d out ?r\gh ! glass s t we may      
connect a batt]y to !m & pass a curr5t (
 electricity ?r\gh ! wire4 ,if we d s ! 
wire gets hot4  ,:at d we mean :5 we say

 ! wire gets hot8 ,we mean t x feels    
hot4 ,x heats ! glass bulb & we c feel  
x4 ,b :at d we mean 9 words ( electrons 
& atoms8 ,to answ] ?is we mu/ />t back a
 little way4  ,9 e bit ( matt] 9 our _w 
! atoms & molecules are 9 v rapid       
motion4 ,9 gases !y c move any:]e2 & d4 
,t's :y odors travel s fa/4 ,9 liquids  
mo/ ( ! molecules or atoms h to d _!    
mov+ )\t gett+ out ( ! di% or above !   
surface4 ,n all ( !m /ay 91 h[ev]1 = "s 
are always gett+ away f ! liquid & go+  
out into ! air above4 ,t is :y a di% (  
wat] w dry up s quickly4 ,! fa/] !      
molecules are go+ ! bett] *ance !y h (  
jump+ cle> away f ! wat] l fi% jump+ 9 !
 lake at sund[n4 ,heat+ ! liquid makes  
its molecules move fa/] & s m ( !m are  
able to jump cle> ( ! re/ ( ! liquid4   
,t's :y :5 we come 9 wet we hang our    
clo!s :]e !y w get w>m4 ,! wat] 9 !m    
evaporates m quickly :5 x is heat$      
because all we mean by heat+ is spe$+ up
 ! molecules4  ,9 a solid body !        
molecules c't get v f> away f :]e !y />t

 b !y keep mov+ back & =? & ar\nd &     
ar\nd4 ,! hott] ! body is1 ! fa/] are   
its molecules mov+4 ,g5]ally !y move a  
little f>!r :5 ! body is hot ?an :5 x is
 cold4 ,t means !y mu/ h a little m room
 & t is :y a body is l>g] :5 hot ?an :5 
cold4 ,x exp&s ) heat+ because its      
molecules are mov+ m rapidly & slightly 
f>!r4  ,:5 a wire is heat$ its molecules
 & atoms are hurri$ up & !y da% back &  
=? fa/] ?an be=e4 ,n[ y "k t a wire1 l !
 filam5t ( a lamp1 gets hot :5 !        
electricity is turn$ on1 t is1 :5 !re is
 a /ream ( electrons pass+ ?r\gh x4 ,:y 
does x get hot8 ,because :5 ! electrons 
/ream ?r\gh x !y bump & jo/le _! way    
along l rude boys on a cr[d$ sidewalk4  
,! atoms h to /ep a bit m lively to keep
 out ( ! way4 ,^! m rapid motions ( !   
atoms we recognize by ! wire gr[+ hott]4
  ,t is :y an electric curr5t heats a   
wire ?r\gh :i* x is fl[+4 ,n[ :at happ5s
 to ! electrons1 ! rude boys :o are     
dodg+ _! way along ! sidewalk8 ,"s ( !m 
are go+ s fa/ & s c>elessly t !y w h to 

dodge out into ! gutt] & (f ! sidewalk  
entirely4 ,! m boys t are ru%+ along & !
 fa/] !y are go+ ! m ( !m w be turn$    
aside & plunge (f ! sidewalks4  ,!      
great] & fa/] ! /ream ( electrons1 t is 
! m curr5t :i* is fl[+ ?r\gh ! wire1 ! m
 electrons w be emitt$1 t is1 ?r[n out (
 ! wire4 ,if y c\ld wat* !m y w\ld see  
!m %oot+ out ( ! wire1 "h1 !re1 & all   
along its l5g?1 & go+ 9 e direction4 ,! 
numb] :i* %oot out ea* second isn't v   
l>ge until !y h /irr$ ?+s up s t ! wire 
is j ab\t r$ hot4  ,:at becomes ( !m8   
,sometimes !y don't get v f> away f !   
wire & s come back inside aga94 ,!y     
scoot (f ! sidewalk & on aga9 j z boys d
 9 dodg+ _! way along4 ,"s ( !m />t away
 z if !y 7 go+ = good4  ,if ! wire is 9 
a vacuum tube1 z x is 9 ! case ( !      
audion1 !y c't get v f> away4 ,( c\rse  
!re is lots ( room2 b !y are go+ s fa/ t
 !y ne$ m room j z old] boys :o run fa/ 
ne$ a l>g] play gr\nd ?an d ! little    
tots4 ,by & by !re gets to be s _m ( !m 
outside t !y h to dodge ea* o!r & "s (  

!m are always dodg+ back into ! wire    
:ile new electrons are %oot+ out f x4   
,:5 !re are j z _m electrons dodg+ back 
into ! wire ea* second z are be+ emitt$ 
f x ! vacuum 9 ! tube has all !         
electrons x c hold4 ,we might say x is  
saturat$ ) electrons1 :i* means1 9      
slang1 full up4 ,if any m electrons are 
to get out ( ! filam5t j z _m o!rs :i*  
are already outside h to g back inside4 
,or else !y h got to be tak5 away       
some:]e else4  ,:at ,i h j told y ab\t  
electrons gett+ away f a heat$ wire is v
 mu* l :at happ5s :5 a liquid is heat$4 
,! molecules ( ! liquid get away f !    
surface4 ,if we cov] a di% ( liquid :i* 
is be+ heat$ ! liquid molecules c't get 
f> away & v soon ! space betwe5 !       
surface ( ! liquid & ! cov] gets        
saturat$ ) !m4 ,!n e "t ano!r molecule  
escapes f ! surface ( ! liquid !re mu/  
be "s molecule :i* goes back into !     
liquid4 ,!re is !n j z mu* cond5sation  
back into liquid z !re is evaporation f 
x4 ,t's :y 9 cook+ !y put cov]s ov] !   

vessels :5 !y don't want ! liquid all to
 boil away4  ,sometimes we speak ( !    
vacuum tube 9 ! same words we w\ld use 9
 describ+ evaporation ( a liquid4 ,!    
molecules ( ! liquid :i* h escap$ =m :at
 is call$ a vapor ( ! liquid4 ,z y "k   
!re is usually consid]able wat] vapor 9 
! air4 ,we say !n t electrons are boil$ 
out ( ! filam5t & t !re is a vapor (    
electrons 9 ! tube4  ,t is en\gh = ?is  
lett]4 ,next "t ,i %all tell y h[ use is
 made ( ^! electrons :i* h be5 boil$ out
 & are free 9 ! space ar\nd ! filam5t4  
,footnote #b3 ,if ! read] has omitt$    
,lett]s #c & #d he %\ld omit ?is        
p>agraph & ! next4   ,,lett] #f  ,,!    
,,audion   ,,de> ,,son3  ,9 my la/ lett]
 ,i told h[ electrons are boil$ out ( a 
heat$ filam5t4 ,! hott] ! filam5t ! m   
electrons are emitt$ ea* second4 ,if !  
temp]ature is kept /eady1 or con/ant z  
we say1 !n !re are emitt$ ea* second j !
 same numb] ( electrons4 ,:5 ! filam5t  
is enclos$ 9 a vessel or glass bulb ^!  
electrons :i* get free f x _c g v f>    

away4 ,"s ( !m1 !re=e1 h to come back to
 ! filam5t & ! numb] :i* returns ea*    
second is j equal to ! numb] :i* is     
leav+4 ,y realize t ?is is :at is happ5+
 inside an ord9>y electric light bulb :5
 its filam5t is be+ heat$4              
,illu/ration3 ,fig #d  ,an ord9>y       
electric light bulb1 h[ev]1 is n an     
audion al?\gh x is l "o 9 ! emission (  
electrons f its filam5t4 ,t rem9ds me t 
la/ night z ,i 0 wait+ = a tra9 ,i pick$
 up "o ( ! ,radio ,supplem5ts :i* s _m  
newspap]s are n[ runn+4 ,!re 0 a column 
( enquiries4 ,"o lett] told h[ its writ]
 _h tri$ to use an ord9>y electric light
 bulb to receive radio signals4  ,he _h 
pl5ty ( electrons 9 x b no way to       
control !m & make _! motions useful4 ,9 
an audion besides ! filam5t !re are two 
o!r ?+s4 ,"o is a little %eet or plate (
 metal ) a connect+ wire lead+ out ?r\gh
 ! glass walls & ! o!r is a little wire 
scre5 %ap$ l a gridiron & s call$ a     
grid4 ,x also has a connect+ wire lead+ 
?r\gh ! glass4 ,fig4 #d %[s an audion4  

,x w be mo/ conv5i5t1 h[ev]1 to repres5t
 an audion z 9 ,fig4 #e4 ,!re y see !   
filam5t1 ,f1 ) its two t]m9als br\ght   
out f ! tube1 ! plate1 ,p1 & betwe5 ^! !
 grid1 ,g4  ,illu/ration3 ,fig #e  ,^!  
?ree p>ts ( ! tube are sometimes call$  
elem5ts4 ,usually1 h[ev]1 !y are call$  
electrodes & t is :y ! audion is spok5 (
 z ! ?ree-electrode vacuum tube4 ,an    
electrode is :at we call any piece (    
metal or wire :i* is s plac$ z to let u 
get at electrons 7or ions7 to control _!
 motions4 ,let u see h[ x does s4  ,to  
/>t )1 we %all =get ! grid & ?9k ( a    
tube ) only a filam5t & a plate 9 x--a  
two-electrode tube4 ,we %all repres5t x 
z 9 ,fig4 #f & %[ ! batt]y :i* heats !  
filam5t by "s l9es z at ,a4 ,9 ?is way (
 repres5t+ a batt]y ea* cell is         
repres5t$ by a %ort heavy l9e & a long] 
light] l9e4 ,! heavy l9e /&s = !        
negative plate & ! long] l9e = !        
positive plate4 ,we %all call ! batt]y  
:i* heats ! filam5t ! filam5t batt]y or 
sometimes ! ,a-batt]y4 ,z y see1 x is   

=m$ by sev]al batt]y cells connect$ 9   
s]ies4  ,illu/ration3 ,fig #f  ,sometime
 lat] ,i may tell y h[ to connect batt]y
 cells toge!r & :y4 ,= ! pres5t all y   
ne$ to rememb] is t two batt]ies are 9  
s]ies if ! positive plate ( "o is       
connect$ to ! negative plate ( ! o!r4   
,if ! batt]ies are alike !y w pull an   
electron j twice z h>d z ei!r c\ld      
alone4  ,illu/ration3 ,pl4              
,,iv4--,radiotron 7,c\rtesy ( ,radio    
,corporation ( ,am]ica74  ,to heat !    
filam5t ( an audion1 su* z y w probably 
use 9 y\r set1 w require ?ree           
/orage-batt]y cells1 l ! "o ,i describ$ 
9 my f\r? lett]1 all connect$ 9 s]ies4  
,we g5]ally use /orage batt]ies ( ab\t !
 same size z ^? 9 ! automobile4 ,if y w 
look at ! automobile batt]y y w see t x 
is made ( ?ree cells connect$ 9 s]ies4  
,t batt]y w\ld d v well = ! filam5t     
circuit4  ,by ! way1 d y "k :at a       
circuit is8 ,! ^w comes f ! same ,lat9  
^w z our ^w circus4 ,! ,romans 7 v fond 
( *>iot rac+ at _! circuses & built race

 tracks ar\nd :i* ! *>iots c\ld g4 ,a   
circuit1 !re=e1 is a pa? or track ar\nd 
:i* some?+ c race2 & an electrical      
circuit is a pa? ar\nd :i* electrons c  
race4 ,! filam5t1 ! ,a-batt]y & !       
connect+ wires ( ,fig4 #f =m a circuit4 
,illu/ration3 ,fig #g  ,let u imag9e    
ano!r batt]y =m$ by sev]al cells 9 s]ies
 :i* we %all connect to ! tube z 9 ,fig4
 #g4 ,all ! positive & negative t]m9als 
( ^! batt]ies are connect$ 9 pairs1 !   
positive ( "o cell to ! negative ( !    
next1 except = "o positive & "o         
negative4 ,! rema9+ positive t]m9al is !
 positive t]m9al ( ! batt]y :i* we are  
mak+ by ?is s]ies connection4 ,we !n    
connect ?is positive t]m9al to ! plate &
 ! negative t]m9al to ! filam5t z %[n 9 
! figure4 ,?is new batt]y we %all call !
 plate batt]y or ! ,b-batt]y4  ,n[ :at's
 go+ to happ58 ,! ,b-batt]y w want to   
take 9 electrons at its positive t]m9al 
& to s5d !m out at its negative t]m9al4 
,! positive is connect$ to ! plate 9 !  
vacuum tube ( ! figure & s draws "s ( ! 

electrons ( ! plate away f x4 ,:]e d ^! 
electrons come f8 ,!y us$ to belong to !
 atoms ( ! plate b !y 7 out play+ 9 !   
space betwe5 ! atoms1 s t !y came "r    
along :5 ! batt]y call$ !m4 ,t leaves ! 
plate ) less ?an its prop] numb] (      
electrons2 t is1 leaves x positive4 ,s !
 plate imm$iately draws to itself "s ( !
 electrons :i* are dodg+ ab\t 9 ! vacuum
 ar\nd x4  ,d y rememb] :at 0 happ5+ 9 !
 tube8 ,! filam5t 0 /eadily go+ on      
emitt+ electrons al?\gh !re 7 already 9 
! tube s _m electrons t j z _m cr[d$    
back into ! filam5t ea* second z !      
filam5t s5t out4 ,! filam5t 0 nei!r ga9+
 nor los+ electrons1 al?\gh x 0 busy    
s5d+ !m out & welcom+ !m home aga94  ,:5
 ! ,b-batt]y gets to "w all ?is is      
*ang$4 ,! ,b-batt]y attracts electrons  
to ! plate & s r$uces ! cr[d 9 ! tube4  
,!n !re are n z _m electrons cr[d+ back 
into ! filam5t z !re 7 be=e & s !       
filam5t loses m ?an x gets back4        
,suppose t1 be=e ! ,b-batt]y 0 connect$ 
to ! plate1 ea* t9y l5g? ( ! filam5t 0  

emitt+ #ajjj electrons ea* second b 0   
gett+ #ajjj back ea* second4 ,!re 0 no  
net *ange4 ,n[1 suppose t ! ,b-batt]y   
takes away #ajj ( ^! ea* second4 ,!n    
only #ijj get back to ! filam5t & !re is
 a net loss f ! filam5t ( #ajj4 ,ea*    
second ?is t9y l5g? ( filam5t s5ds into 
! vacuum #ajj electrons :i* are tak5 out
 at ! plate4 ,f ea* little bit ( filam5t
 !re is a /ream ( electrons to ! plate4 
,millions ( electrons1 !re=e1 /ream     
across f filam5t to plate4 ,t is1 !re is
 a curr5t ( electricity betwe5 filam5t &
 plate & ?is curr5t cont9ues to fl[ z   
long z ! ,a-batt]y & ! ,b-batt]y d _!   
"w4  ,! negative t]m9al ( ! ,b-batt]y is
 connect$ to ! filam5t4 ,e "t ?is batt]y
 pulls an electron f ! plate its        
negative t]m9al %oves "o out to !       
filam5t4 ,y "k f my ?ird & f\r? lett]s t
 electrons are c>ri$ ?r\gh a batt]y f   
its positive to its negative t]m9al4 ,y 
see1 !n1 t !re is ! same /ream (        
electrons ?r\gh ! ,b-batt]y z !re is    
?r\gh ! vacuum betwe5 filam5t & plate4  

,?is same /ream passes also ?r\gh !     
wires :i* connect ! batt]y to ! tube4 ,!
 pa? foll[$ by ! /ream ( electrons      
includes ! wires1 ! vacuum & ! batt]y 9 
s]ies4 ,we call ?is pa? ! plate circuit4
  ,we c connect a telephone receiv]1 or 
a curr5t-measur+ in/rum5t1 or any ?+ we 
wi% :i* w pass a /ream ( electrons1 s z 
to let ?is same /ream ( electrons pass  
?r\gh x also4 ,all we h to d is to      
connect ! in/rum5t 9 s]ies ) ! o!r p>ts 
( ! plate circuit4 ,i'll %[ y h[ 9 a    
m9ute1 b j n[ ,i want y to und]/& t we h
 a /ream ( electrons1 = ,i want to tell 
y h[ x may be controll$4  ,suppose we   
use ano!r batt]y & connect x betwe5 !   
grid & ! filam5t s z to make ! grid     
positive4 ,t w\ld mean connect+ !       
positive t]m9al ( ! batt]y to ! grid & !
 negative to ! filam5t z %[n by !       
,c-batt]y ( ,fig4 #h4 ,?is figure also  
%[s a curr5t-measur+ in/rum5t 9 ! plate 
circuit4  ,:at effect is ?is ,c-batt]y1 
or grid-batt]y1 go+ to h on ! curr5t 9 !
 plate circuit8 ,mak+ ! grid positive   

makes x want electrons4 ,x w !re=e act j
 z we saw t ! plate did & pull electrons
 across ! vacuum t[>ds itself4          
,illu/ration3 ,fig #h  ,:at happ5s !n is
 some?+ l ?is3 ,electrons are fre$ at ! 
filam5t2 ! plate & ! grid bo? call !m & 
!y />t (f 9 a ru%4 ,"s ( !m are /opp$ by
 ! wires ( ! grid b mo/ ( !m g on by to 
! plate4 ,! grid is mo/ly op5 space1 y  
"k1 & ! electrons move z fa/ z lightn+4 
,!y are go+ too fa/ 9 ! g5]al direction 
( ! grid to /op & look = its few & small
 wires4  ,:5 ! grid is positive ! grid  
helps ! plate to call electrons away f !
 filam5t4 ,mak+ ! grid positive1 !re=e1 
increases ! /ream ( electrons betwe5    
filam5t & plate2 t is1 increases !      
curr5t 9 ! plate circuit4  ,we c\ld get 
! same effect s f> z conc]ns an increas$
 plate curr5t by us+ m batt]ies 9 s]ies 
9 ! plate circuit s z to pull h>d]4 ,b !
 grid is s close to ! filam5t t a s+le  
batt]y cell 9 ! grid circuit c call     
electrons s /rongly t x w\ld take sev]al
 extra batt]y cells 9 ! plate circuit to

 produce ! same effect4  ,illu/ration3  
,fig #i  ,if we rev]se ! grid batt]y1 z 
9 ,fig4 #i1 s z to make ! grid negative1
 !n1 in/ead ( attract+ electrons ! grid 
repels !m4 ,n[h]e ne> z _m electrons w  
/ream across to ! plate :5 ! grid says1 
,no1 g back4 ,! grid is 9 a /rategic    
position & :at x says has a great       
effect4  ,:5 !re is no batt]y connect$  
to ! grid x has no possibility (        
influ5c+ ! electron /ream :i* ! plate is
 attract+ to itself4 ,we say1 !n1 t !   
grid is un*>g$ or is at z]o pot5tial1   
mean+ t x is z]o or no?+ 9 possibility4 
,b :5 ! grid is *>g$1 no matt] h[       
little1 x makes a *ange 9 ! plate       
curr5t4 ,:5 ! grid says ,come on1 ev5   
?\gh v s(tly1 x has z mu* effect on !   
electrons z if ! plate %\t$ at !m1 & a  
lot ( extra electrons ru% = ! plate4 ,b 
:5 ! grid :isp]s ,g back1 _m electrons  
:i* w\ld o!rwise h gone /reak+ (f to !  
plate cr[d back t[>d ! filam5t4 ,t's h[ 
! audion works4 ,!re is an electron     
/ream & a wond]fully s5sitive way (     

controll+ ! /ream4     ,,lett] #g  ,,h[ 
,,to ,,measure ,,an ,,electron ,,/ream  
7,?is lett] may be omitt$ on ! fir/     
read+47  ,,de> ,,y\?3  ,if we are to    
talk ab\t ! audion & h[ its grid        
controls ! curr5t 9 ! plate circuit we  
mu/ "k some?+ ( h[ to measure curr5ts4  
,an electric curr5t is a /ream (        
electrons4 ,we measure x by f9d+ ! rate 
at :i* electrons are travel+ along ?r\gh
 ! circuit4  ,:at d we mean by ! ^w     
rate8 ,y "k :at x means :5 a spe$omet]  
says tw5ty miles an h\r4 ,if ! c> %\ld  
keep go+ j z x 0 do+ at ! in/ant y look$
 at ! spe$omet] x w\ld g tw5ty miles 9 !
 next h\r4 ,its rate is tw5ty miles an  
h\r ev5 ?\gh x runs into a sma% ! next  
m9ute & nev] goes any:]e aga9 except to 
! junk heap4  ,x's ! same :5 we talk (  
electric curr5ts4 ,we say !re is a      
curr5t ( su* & su* a numb] ( electrons a
 second go+ by ea* po9t 9 ! circuit4 ,we
 don't mean t ! curr5t isn't go+ to     
*ange1 = x may get l>g] or small]1 b we 
d mean t if ! /ream ( electrons keeps   

go+ j z x is !re w be su* & su* a numb] 
( electrons pass by 9 ! next second4  ,9
 mo/ ( ! electrical circuits ) :i* y w  
deal y w f9d t electrons mu/ be pass+   
along 9 ! circuit at a mo/ amaz+ rate if
 !re is to be any appreciable effect4   
,:5 y turn on ! #dj-watt light at y\r   
desk y />t !m go+ ?r\gh ! filam5t ( !   
lamp at ! rate ( ab\t two & a half      
billion billion ea* second4 ,y h /ood on
 ! sidewalk 9 ! city & wat*$ ! p /ream  
pa/ y4 ,j suppose y c\ld /& beside t    
n>r[ little sidewalk :i* ! filam5t (f]s 
to ! electrons & c\nt !m z !y g by4 ,we 
don't try to c\nt !m al?\gh we d to-day 
"k ab\t h[ _m g by 9 a second if !      
curr5t is /eady4  ,if "s "o asks y h[   
old y are y don't say ,ab\t five hundr$ 
million seconds2 y tell him 9 ye>s4 ,:5 
"s "o asks h[ l>ge a curr5t is fl[+ 9 a 
wire we don't tell him six billion      
billion electrons ea* second2 we tell   
him "o amp]e4 ,j z we use ye>s z ! units
 9 :i* to c\nt up "t s we use amp]es z !
 units 9 :i* to c\nt up /reams (        

electrons4 ,:5 a wire is c>ry+ a curr5t 
( "o amp]e ! electrons are /ream+ ?r\gh 
x at ! rate ( ab\t                      
#f1jjj1jjj1jjj1jjj1jjj1jjj a second4    
,don't try to rememb] ?is numb] b d     
rememb] t an amp]e is a unit 9 :i* we   
measure curr5ts j z a ye> is a unit 9   
:i* we measure "t4 ,an amp]e is a unit 9
 :i* we measure /reams ( electrons j z  
miles p] h\r is a unit 9 :i* we measure 
! spe$ ( tra9s or automobiles4  ,if y   
want$ to f9d ! weight ( some?+ y w\ld   
take a scale & weigh x1 w\ldn't y8 ,y   
might take t spr+ balance :i* hangs out 
9 ! kit*54 ,b if ! spr+ balance said !  
?+ weigh$ five p\nds h[ w\ld y "k if x 0
 "r8 ,( c\rse y might take :at ev] x 0  
d[n t[n & weigh x on "s o!r scales b h[ 
w\ld y "k ^? scales gave correct weight8
  ,! only way to f9d out w\ld be to try 
! scales ) weights :i* y 7 sure 7 "r &  
see if ! read+s on ! scale correspond to
 ! kn[n weights4 ,!n y c\ld tru/ x to   
tell y ! weight ( some?+ else4 ,t's !   
way scales are te/$4 ,9 fact t's ! way t

 ! mak]s "k h[ to m>k !m 9 ! fir/ place4
 ,!y put on kn[n weights & m>k$ ! l9es &
 figures :i* y see4 ,:at !y did 0 call$ 
calibrat+ ! scale4 ,y c\ld make a scale 
= y\rself if y wi%$1 b if x 0 to be     
reliable y w\ld h to f9d ! places = !   
m>k+s by apply+ kn[n weights1 t is1 by  
calibration4  ,h[ w\ld y "k t ! weights 
y us$ to calibrate y\r scale 7 really   
:at y ?\ght !m to be8 ,y w\ld h to f9d  
"s place :]e !y _h a weight t ev]ybody  
w\ld agree 0 correct & !n comp>e y\r    
weight ) t4 ,y might1 = example1 s5d y\r
 p\nd weight to ! ,bureau ( ,/&>ds 9    
,wa%+ton & = a small paym5t h ! ,bureau 
comp>e x ) ! p\nd :i* x keeps z a /&>d4 
,t is easy :]e "o is int]e/$ 9 a p\nd4  
,b x is a little diff]5t :5 "o is       
int]e/$ 9 an amp]e4 ,y c't make an amp]e
 out ( a piece ( plat9um z y c a /&>d   
p\nd weight4 ,an amp]e is a /ream (     
electrons at ab\t ! rate ( six billion  
billion a second4 ,no "o c\ld ev] c\nt  
any:]e ne> t _m1 & yet ev]ybody :o is   
conc]n$ ) electricity wants to be able  

to measure curr5ts 9 amp]es4 ,h[ is x   
done8  ,fir/ !re is made an in/rum5t :i*
 w h some?+ 9 x to move :5 electrons are
 fl[+ ?r\gh ! in/rum5t4 ,we want a met] 
= ! fl[ ( electrons4 ,9 ! basem5t we h a
 met] = ! fl[ ( gas & ano!r = ! fl[ (   
wat]4 ,ea* ( ^! has "s "p :i* w move :5 
! wat] or ! gas passes ?r\gh4 ,b !y are 
bo? arrang$ ) little ge> :eels s z to   
keep track ( all ! wat] or gas :i* has  
fl[$ ?r\gh2 !y won't tell ! rate at :i* 
! gas or wat] is fl[+4 ,!y are l !      
odomet] on ! c> :i* gives ! trip mileage
 or ! total mileage4 ,we want a met] l !
 spe$omet] :i* w indicate at ea* in/ant 
j h[ fa/ ! electrons are /ream+ ?r\gh x4
  ,!re are sev]al k9ds ( met]s b ,i %all
 n try to tell y n[ ( m ?an "o4 ,!      
simple/ to und]/& is call$ a hot-wire   
met]4 ,y already "k t an electron /ream 
heats a wire4 ,suppose a piece ( f9e    
wire is fa/5$ at ! two ends & t !re are 
b9d+ po/s also fa/5$ to ^! ends ( ! wire
 s t ! wire may be made "p ( ! circuit  
:]e we want to "k ! electron /ream4 ,!n 

! same /ream ( electrons w fl[ ?r\gh !  
f9e wire z ?r\gh ! o!r p>ts ( ! circuit4
 ,because ! wire is f9e x acts l a v    
n>r[ sidewalk = ! /ream ( electrons & !y
 h to bump & jo/le pretty h>d to get    
?r\gh4 ,t's :y ! wire gets heat$4  ,y "k
 t a heat$ wire exp&s4 ,?is wire exp&s4 
,x gr[s long] & because x is held firmly
 at ! ends x mu/ b[ out at ! c5t]4 ,!   
bigg] ! rate ( fl[ ( electrons ! hott] x
 gets2 & ! hott] x gets ! m x b[s out4  
,at ! c5t] we might fa/5 "o end--! %ort 
end--( a little lev]4 ,a small motion ( 
?is %ort end ( ! lev] w mean a l>ge     
motion ( ! o!r end1 j l a teet] bo>d :5 
"o end is long] ?an ! o!r2 ! *ild on !  
long end travels fur!r ?an ! *ild on !  
%ort end4 ,! lev] magnifies ! motion ( !
 c5t] ( ! hot wire "p ( our met] s t we 
c see x easi]4  ,illu/ration3 ,fig #aj  
,!re are sev]al ways to make su* a met]4
 ,! "o %[n 9 ,fig4 #aj is z easy to     
und]/& z any4 ,we %ape ! long end ( !   
lev] l a po9t]4 ,!n ! hott] ! wire !    
f>!r ! po9t] moves4  ,if we c\ld put ?is

 met] 9 an electric circuit :]e we knew 
"o amp]e 0 fl[+ we c\ld put a num]al #a 
opposite :]e ! po9t] /ood4 ,!n if we    
c\ld increase ! curr5t until !re 7 two  
amp]es fl[+ ?r\gh ! met] we c\ld m>k t  
position ( ! po9t] #b & s on4 ,t's ! way
 we w\ld calibrate ! met]4 ,aft] we _h  
done s we w\ld call x an ammet] because 
x measures amp]es4 ,ye>s ago p w\ld h   
call$ x an amp]emet] b no "o :o is      
up-to-date w\ld call x s to-day4        
,illu/ration3 ,fig #aa  ,if we _h a v   
c>efully made ammet] we w\ld s5d x to ! 
,bureau ( ,/&>ds to be calibrat$4 ,at ! 
,bureau !y h a numb] ( met]s :i* !y "k  
are correct 9 _! read+s4 ,!y w\ld put "o
 ( _! met]s & ours into ! same circuit s
 t bo? c>ry ! same /ream ( electrons z 9
 ,fig4 #aa4 ,!n :atev] ! read+ 0 on _!  
met] c\ld be m>k$ opposite ! po9t] on   
ours4  ,n[ ,i want to tell y h[ !       
physici/s at ! ,bureau "k :at is an     
amp]e4 ,sev]al ye>s ago !re 0 a meet+ or
 congress ( physici/s & electrical      
eng9e]s f all ov] ! _w :o discuss$ :at  

!y ?\ght %\ld be ! unit 9 :i* to measure
 curr5t4 ,!y decid$ j :at !y w\ld call  
an amp]e & !n all ! c\ntries f :i* !y   
came pass$ laws say+ t an amp]e %\ld be 
:at ^! sci5ti/s _h recomm5d$4 ,to-day1  
!re=e1 an amp]e is def9$ by law4  ,to   
tell :5 an amp]e ( curr5t is fl[+       
requires ! use ( two silv] plates & a   
solution ( silv] nitrate4 ,silv] nitrate
 has molecules made up ( "o atom ( silv]
 comb9$ ) a gr\p ( atoms call$ nitrate4 
,y rememb] t ! molecule ( copp]         
sulphate1 discuss$ 9 our ?ird lett]1 0  
=m$ by a copp] atom & a gr\p call$      
sulphate4 ,nitrate is ano!r gr\p some?+ 
l sulphate = x has oxyg5 atoms 9 x1 b x 
has ?ree in/ead ( f\r1 & in/ead ( a     
sulphur atom !re is an atom ( nitrog54  
,:5 silv] nitrate molecules g into      
solution !y break up into ions j z copp]
 sulphate does4 ,"o ion is a silv] atom 
:i* has lo/ "o electron4 ,?is electron 0
 /ol5 f x by ! nitrate "p ( ! molecule  
:5 !y dissociat$4 ,! nitrate ion1 !re=e1
 is =m$ by a nitrog5 atom1 ?ree oxyg5   

atoms1 & "o extra electron4  ,if we put 
two plates ( silv] into su* a solution  
no?+ w happ5 until we connect a batt]y  
to ! plates4 ,!n ! batt]y takes         
electrons away f "o plate & gives       
electrons to ! o!r4 ,"s ( ! atoms 9 !   
plate :i* ! batt]y is robb+ ( electrons 
are j l ! silv] ions :i* are mov+ ar\nd 
9 ! solution4 ,t's :y !y c g out into ! 
solution & play ) ! nitrate ions ea* (  
:i* has an extra electron :i* x /ole f  
"s silv] atom4 ,b ! mom5t silv] ions    
leave _! plate we h m silv] ions 9 !    
solution ?an we d sulphate ions4  ,!    
only ?+ t c happ5 is = "s ( ! silv] ions
 to get out ( ! solution4 ,!y ar5't go+ 
back to ! positive silv] plate f :i* !y 
j came4 ,!y g on t[>d ! negative plate  
:]e ! batt]y is s5d+ an electron = e "o 
:i* x takes away f ! positive plate4    
,!re />t (f t[>ds ! negative plate1 n   
only ! ions :i* j came f ! positive     
plate1 b all ! ions t are 9 ! solution4 
,! fir/ "o to arrive gets an electron b 
x c't take x away f ! silv] plate4 ,& :y

 %\ld x8 ,z soon z x has got ?is        
electron x is aga9 a normal silv] atom4 
,s x /ays ) ! o!r atoms 9 ! silv] plate4
 ,t's :at happ5s "r along4 ,= e atom :i*
 is lo/ f ! positive plate !re is "o    
add$ to ! negative plate4 ,! silv] ( !  
positive plate gradually wa/es away & ! 
negative plate gradually gets an extra  
coat+ ( silv]4  ,e "t ! batt]y takes an 
electron away f ! positive plate & gives
 x to ! negative plate !re is add$ to ! 
negative plate an atom ( silv]4 ,if !   
negative plate is weigh$ be=e ! batt]y  
is connect$ & aga9 aft] ! batt]y is     
disconnect$ we c tell h[ mu* silv] has  
be5 add$ to x4 ,suppose ! curr5t has be5
 p]fectly /eady1 t is1 ! same numb] (   
electrons /ream+ ?r\gh ! circuit ea*    
second4 ,!n if we "k h[ long ! curr5t   
has be5 runn+ we c tell h[ mu* silv] has
 be5 deposit$ ea* second4  ,! law says t
 if silv] is be+ deposit$ at ! rate (   
#j4jjaaah gram ea* second !n ! curr5t is
 "o amp]e4 ,t's a small am\nt ( silv]1  
only ab\t a ?\s&? "p ( a gram1 & y "k t 

x takes #bh4ce grams to make an ounce4  
,x's a v small am\nt ( silv] b x's an   
enorm\s numb] ( atoms4 ,h[ _m8 ,six     
billion billion1 ( c\rse1 = !re is      
deposit$ "o atom = ea* electron 9 !     
/ream4  ,9 my next lett] ,i'll tell y h[
 we measure ! pull :i* batt]ies c give  
to electrons1 & !n we %all be ready to g
 on ) m ab\t ! audion4     ,,lett] #h   
,,electron-,,mov+-,,=ces   7,?is lett]  
may be omitt$ on ! fir/ read+47  ,,de>  
,,"y ,,man3  ,i tru/ y h a fairly good  
idea t an amp]e means a /ream (         
electrons at a c]ta9 def9ite rate & h5ce
 t a curr5t ( say #c amp]es means a     
/ream ) ?ree times z _m electrons pass+ 
along ea* second4  ,9 ! ?ird & f\r?     
lett]s y f\nd out :y a batt]y drives    
electrons ar\nd a conduct+ circuit4 ,y  
also f\nd t !re are sev]al diff]5t k9ds 
( batt]ies4 ,batt]ies diff] 9 _!        
abilities to drive electrons & x is     
!re=e conv5i5t to h "s way ( comp>+ !m4 
,we d ?is by measur+ ! electron-mov+-=ce
 or electromotive =ce :i* ea* batt]y c  

ex]t4 ,to express electromotive =ce &   
give ! results ( our measurem5ts we mu/ 
h "s unit4 ,! unit we use is call$ !    
volt4  ,! volt is def9$ by law & is bas$
 on ! sugge/ions ( ! same body (        
sci5ti/s :o recomm5d$ ! amp]e ( our la/ 
lett]4 ,!y def9$ x by tell+ h[ to make a
 p>ticul> k9d ( batt]y & !n say+ t ?is  
batt]y _h an electromotive =ce ( a c]ta9
 numb] ( volts4 ,"o c buy su* /&>d      
batt]ies1 or /&>d cells z !y are call$1 
or he c make !m = himself4 ,to be sure t
 !y are j "r he c !n s5d !m to ! ,bureau
 ( ,/&>ds & h !m comp>$ ) ! /&>d cells  
:i* ! ,bureau has4  ,i don't propose to 
tell y mu* ab\t /&>d cells = y won't h  
to use !m until y come to /udy physics 9
 real e>ne/4 ,!y are n good = ord9>y    
purposes because ! mom5t !y g to "w     
driv+ electrons ! conditions inside !m  
*ange s _! electromotive =ce is *ang$4  
,!y are delicate little affairs & are   
useful only z /&>ds ) :i* to comp>e o!r 
batt]ies4 ,& ev5 z /&>d batt]ies !y mu/ 
be us$ 9 su* a way t !y are n requir$ to

 drive any electrons4  ,illu/ration3    
,fig #ab  ,let's see h[ x c be done4    
,suppose two boys sit opposite ea* o!r  
on ! floor & brace _! feet toge!r4 ,!n )
 _! h&s !y take hold ( a /ick & pull 9  
opposite directions4 ,if bo? h ! same   
/ick-motive-=ce ! /ick w n move4  ,n[   
suppose we connect ! negative feet--,i  
mean negative t]m9als--( two batt]ies   
toge!r z 9 ,fig4 #ab4 ,!n we connect _! 
positive t]m9als toge!r by a wire4 ,9 ! 
wire !re w be lots ( free electrons     
ready to g to ! positive plate ( !      
batt]y :i* pulls ! h>d]4 ,if ! batt]ies 
are equal 9 electromotive =ce ^!        
electrons w /ay "r :]e !y are4 ,!re w be
 no /ream ( electrons & yet we'll be us+
 "o ( ! batt]ies to comp>e ) ! o!r4  ,t 
is all "r1 y ?9k1 b :at are we to d :5 !
 batt]ies are n j equal 9 e4 m4 f48 7e4 
m4 f4 is code = electromotive =ce74     
,i'll tell y1 because ! tell+ includes  
"s o!r ideas :i* w be valuable 9 y\r    
lat] read+4  ,illu/ration3 ,fig #ac     
,suppose we take batt]ies :i* ar5't go+ 

to be injur$ by be+ made to "w--/orage  
batt]ies w d nicely--& connect !m 9     
s]ies z 9 ,fig4 #ac4 ,:5 batt]ies are 9 
s]ies !y act l a s+le /rong] batt]y1 "o 
^: e4 m4 f4 is ! sum ( ! e4 m4 f4's ( ! 
sep>ate batt]ies4 ,connect ^! batt]ies  
to a long f9e wire z 9 ,fig4 #ad4 ,!re  
is a /ream ( electrons along ?is wire4  
,next connect ! negative t]m9al ( ! /&>d
 cell to ! negative t]m9al ( ! /orage   
batt]ies1 t is1 brace _! feet aga9/ ea* 
o!r4 ,!n connect a wire to ! positive   
t]m9al ( ! /&>d cell4 ,?is wire acts j l
 a long arm /ick+ out f ! positive plate
 ( ?is cell4  ,illu/ration3 ,fig #ad    
,t\* ! end ( ! wire1 :i* is p ( ,fig4   
#ad1 to "s po9t z a on ! f9e wire4 ,n[  
:at d we h8 ,"r at a1 ( c\rse1 !re are  
"s free electrons & !y he> ! calls ( bo?
 batt]ies4 ,if ! /&>d batt]y1 ,s ( !    
figure1 calls ! /rong] !y g to x4 ,9 t  
case move ! end p ne>] ! positive plate 
( ! batt]y ,b1 s t x w h a *ance to ex]t
 a /rong] pull4 ,suppose we try at c &  
f9d ! batt]y ,b is !re ! /rong]4 ,!n we 

c move back to "s po9t1 say b1 :]e !    
pulls are equal4  ,to make a te/ l ?is  
we put a s5sitive curr5t-measur+        
in/rum5t 9 ! wire :i* leads f ! positive
 t]m9al ( ! /&>d cell4 ,we also use a   
long f9e wire s t !re c nev] be mu* ( an
 electron /ream anyway4 ,:5 ! pulls are 
equal !re w be no curr5t ?r\gh ?is      
in/rum5t4  ,z soon z we f9d out :]e !   
prop] sett+ is we c replace ,s by "s o!r
 batt]y1 say ,x1 :i* we wi% to comp>e ) 
,s4 ,we f9d ! sett+ = t batt]y 9 ! same 
way z we j did = ,s4 ,suppose x is at d 
9 ,fig4 #ad :ile ! sett+ = ,s 0 at b4   
,we c see at once t ,x is /rong] ?an ,s4
 ,! "q1 h[ev]1 is h[ mu* /rong]4        
,p]haps x w\ld be bett] to try to answ] 
?is "q by talk+ ab\t e4 m4 f4's4 ,x     
isn't fair to speak only ( ! positive   
plate :i* calls1 we mu/ speak also ( !  
negative plate :i* is %oo+ electrons    
away f itself4 ,! idea ( e4 m4 f4 takes 
c>e ( bo? ^! actions4 ,! /eady /ream (  
electrons 9 ! f9e wire is due to ! e4 m4
 f4 ( ! batt]y ,b1 t is to ! pull ( !   

positive t]m9al & ! %ove ( ! negative4  
,if ! wire is uni=m1 t is ! same ?r\gh\t
 its l5g?1 !n ea* in* ( x requires j z  
mu* e4 m4 f4 z any o!r in*4 ,two in*es  
require twice ! e4 m4 f4 :i* "o in*     
requires4 ,we "k h[ mu* e4 m4 f4 x takes
 to keep ! electron /ream go+ 9 ! "p ( !
 wire f n to b4 ,x takes j ! e4 m4 f4 ( 
! /&>d cell1 ,s1 because :5 t _h its    
feet brac$ at n x pull$ j z h>d at b z  
did ! big batt]y ,b4  ,suppose ! di/ance
 n to d 7usually writt5 nd7 is twice z  
great z t f n to b 7nb74 ,t means t     
batt]y ,x has twice ! e4 m4 f4 ( batt]y 
,s4 ,y rememb] t ,x c\ld ex]t ! same =ce
 ?r\gh ! l5g? ( wire nd1 z c\ld ! l>ge  
batt]y4 ,t is twice :at cell ,s c d4    
,!re=e if we "k h[ _m volts to call ! e4
 m4 f4 ( ! /&>d cell we c say t ,x has  
an e4 m4 f4 ( twice z _m volts4  ,if we 
measur$ dry batt]ies ?is way we %\ld f9d
 t !y ea* _h an e4 m4 f4 ( ab\t #a4df   
volts4 ,a /orage batt]y w\ld be f\nd to 
h ab\t #b4d volts :5 fully *>g$ & p]haps
 z l[ z #b4a volts :5 we _h run x = a   

:ile4  ,t is ! way 9 :i* to comp>e      
batt]ies & to measure _! e4 m4 f4's1 b y
 see x takes a lot ( "t4 ,x is easi] to 
use a voltmet] :i* is an in/rum5t =     
measur+ e4 m4 f4's4 ,"h is h[ "o c\ld be
 made4  ,fir/ !re is made a             
curr5t-measur+ in/rum5t :i* is q        
s5sitive1 s t its po9t] w %[ a          
deflection :5 only a v small /ream (    
electrons is pass+ ?r\gh ! in/rum5t4 ,we
 c\ld make "o 9 ! same way z we made !  
ammet] ( ! la/ lett] b !re are o!r bett]
 ways ( :i* ,i'll tell y lat]4 ,!n we   
connect a good deal ( f9e wire 9 s]ies )
 ! in/rum5t = a reason :i* ,i'll tell y 
9 a m9ute4 ,! next & la/ /ep is to      
calibrate4  ,we "k h[ _m volts ( e4 m4  
f4 are requir$ to keep go+ ! electron   
/ream betwe5 n & b--we "k t f ! e4 m4 f4
 ( our /&>d cell4 ,suppose !n t we      
connect ?is new in/rum5t1 :i* we h j    
made1 to ! wire at n & b z 9 ,fig4 #ae4 
,"s ( ! electrons at n :i* are s anxi\s 
to get away f ! negative plate ( batt]y 
,b c n[ travel z f> z b ?r\gh ! wire ( !

 new in/rum5t4 ,!y d s & ! po9t] sw+s   
ar\nd to "s new position4 ,opposite t we
 m>k ! numb] ( volts :i* ! /&>d batt]y  
told u !re 0 betwe5 n & b4              
,illu/ration3 ,fig #ae  ,if we move !   
end ( ! wire f b to d ! po9t] w take a  
new position4 ,opposite ?is we m>k twice
 ! numb] ( volts ( ! /&>d cell4 ,we c   
run x to a po9t e :]e ! di/ance ne is   
"o-half nb1 & m>k our scale ) half !    
numb] ( volts ( ! /&>d cell1 & s on =   
o!r positions along ! wire4 ,t's ! way  
we calibrate a s5sitive curr5t-measur+  
in/rum5t 7) its add$ wire1 ( c\rse7 s t 
x w read volts4 ,x is n[ a voltmet]4    
,if we connect a voltmet] to ! batt]y ,x
 z 9 ,fig4 #af ! po9t] w tell u ! numb] 
( volts 9 ! e4 m4 f4 ( ,x1 = ! po9t] w  
take ! same position z x did :5 !       
voltmet] 0 connect$ betwe5 n & d4  ,!re 
is only "o ?+ to wat* out = 9 all ?is4  
,we mu/ be c>eful t ! voltmet] is s made
 t x won't (f] too easy a pa? =         
electrons to foll[4 ,we only want to f9d
 h[ h>d a batt]y c pull an electron1 = t

 is :at we mean by e4 m4 f4 ,( c\rse1 we
 mu/ let a small /ream ( electrons fl[  
?r\gh ! voltmet] s z to make ! po9t]    
move4 ,t is :y voltmet]s ( ?is k9d are  
made out ( a long piece ( f9e wire or   
else h a coil ( f9e wire 9 s]ies ) !    
curr5t-measur+ "p4 ,! f9e wire makes a  
long & n>r[ pa? = ! electrons & s !re c 
be only a small /ream4 ,usually we      
describe ?is condition by say+ t a      
voltmet] has a high resi/ance4          
,illu/ration3 ,fig #af  ,f9e wires (f] m
 resi/ance to electron /reams ?an d     
heavy wires ( ! same l5g?4 ,if a wire is
 ! same diamet] all along1 ! long] !    
l5g? ( x :i* we use ! great] is !       
resi/ance :i* is (f]$ to an electron    
/ream4  ,y w ne$ to "k h[ to describe ! 
resi/ance ( a wire or ( any "p ( an     
electric circuit4 ,to d s y tell h[ _m  
ohms ( resi/ance x has4 ,! ohm is ! unit
 9 :i* we measure ! resi/ance ( a       
circuit to an electron /ream4  ,i c %[ y
 :at an ohm is if ,i tell y a simple way
 to measure a resi/ance4 ,suppose y h a 

wire or coil ( wire & want to "k its    
resi/ance4 ,connect x 9 s]ies ) a batt]y
 & an ammet] z %[n 9 ,fig4 #ag4 ,! same 
electron /ream passes ?r\gh all p>ts (  
?is circuit & ! ammet] tells u :at ?is  
/ream is 9 amp]es4 ,n[ connect a        
voltmet] to ! two ends ( ! coil z %[n 9 
! figure4 ,! voltmet] tells 9 volts h[  
mu* e4 m4 f4 is be+ appli$ to =ce !     
curr5t ?r\gh ! coil4 ,divide ! numb] (  
volts by ! numb] ( amp]es & ! quoti5t   
7answ]7 is ! numb] ( ohms ( resi/ance 9 
! coil4  ,illu/ration3 ,fig #ag         
,suppose ! ammet] %[s a curr5t ( "o     
amp]e & ! voltmet] an e4 m4 f4 ( "o     
volt4 ,!n divid+ #a by #a gives #a4 ,t  
means t ! coil has a resi/ance ( "o ohm4
 ,x also means "o ohm is su* a resi/ance
 t "o volt w s5d ?r\gh x a curr5t ( "o  
amp]e4 ,y c get lots ( mean+ out ( ?is4 
,= example1 x means also t "o volt w s5d
 a curr5t ( "o amp]e ?r\gh a resi/ance (
 "o ohm4  ,h[ _m ohms w\ld ! coil h if x
 took #e volts to s5d #b amp]es ?r\gh x4
 ,solution3 ,divide #e by #b & y get    

#b4e4 ,!re=e ! coil w\ld h a resi/ance (
 #b4e ohms4  ,try ano!r4 ,if a coil (   
resi/ance ?ree ohms is c>ry+ two amp]es 
:at is ! voltage across ! t]m9als ( !   
coil8 ,= #a ohm x w\ld take #a volt to  
give a curr5t ( #a amp]e1 w\ldn't x8 ,= 
#c ohms x takes ?ree times z mu* to give
 "o amp]e4 ,to give twice ?is curr5t    
w\ld take twice #c volts4 ,t is1 #b     
amp]es 9 #c ohms requires #bx#c volts4  
,"h's "o = y to try by y\rself4 ,if an  
e4 m4 f4 ( #h volts is s5d+ curr5t ?r\gh
 a resi/ance ( #b ohms1 h[ mu* curr5t is
 fl[+8 ,notice t ,i told ! numb] ( ohms 
& ! numb] ( volts1 :at are y go+ to     
tell8 ,don't tell j ! numb]2 tell h[ _m 
& :at4     ,,lett] #i  ,,!              
,,audion-,,*>act]i/ic   ,,my ,,de> ,,"y 
,,/ud5t3  ,al?\gh !re is mu* 9 ,lett]s  
#g & #h :i* x is well to le>n & to ?9k  
ab\t1 !re are only ?ree ( ! ideas :i* y 
mu/ h firmly grasp$ to get ! mo/ out (  
?is lett] :i* ,i am n[ go+ to write y   
ab\t ! audion4  ,fir/3 ,electric curr5ts
 are /reams ( electrons4 ,we measure    

curr5ts 9 amp]es4 ,to measure a curr5t  
we may connect into ! circuit an ammet]4
  ,second3 ,electrons move 9 a circuit  
:5 !re is an electron-mov+-=ce1 t is an 
electromotive =ce or e4 m4 f4 ,we       
measure e4 m4 f4's 9 volts4 ,to measure 
an e4 m4 f4 we connect a voltmet] to !  
two po9ts betwe5 :i* ! e4 m4 f4 is      
active4  ,?ird3 ,:at curr5t any p>ticul>
 e4 m4 f4 w cause dep5ds ^u ! circuit 9 
:i* x is active4 ,circuits diff] 9 !    
resi/ance :i* !y (f] to e4 m4 f4's4 ,=  
any p>ticul> e4 m4 f4 7t is = any giv5  
e4 m4 f47 ! result+ curr5t w be small] !
 great] ! resi/ance ( ! circuit4 ,we    
measure resi/ance 9 ohms4 ,to measure x 
we f9d ! quoti5t ( ! numb] ( volts      
appli$ to ! circuit by ! numb] ( amp]es 
:i* fl[4  ,9 my six? lett] ,i told y    
some?+ ( h[ ! audion works4 ,x w\ld be  
wor? :ile to read aga9 t lett]4 ,y      
rememb] t ! curr5t 9 ! plate circuit c  
be controll$ by ! e4 m4 f4 :i* is appli$
 to ! grid circuit4 ,!re is a           
relation%ip betwe5 ! plate curr5t & !   

grid voltage :i* is peculi> or          
*>act]i/ic to ! tube4 ,s we call su* a  
relation%ip a *>act]i/ic4 ,let u see h[ 
x may be f\nd & :at x w be4  ,connect an
 ammet] 9 ! plate- or ,b-circuit1 ( !   
tube s z to measure ! plate-circuit     
curr5t4 ,y w f9d t almo/ all books use !
 lett] ,i to /& = curr5t4 ,! reason is t
 sci5ti/s us$ to speak ( ! int5sity ( an
 electric curr5t s t ,i really /&s =    
int5sity4 ,we use ,i to /& = some?+ m   
?an ! ^w curr5t4 ,x is our symbol =     
:atev] an ammet] w\ld read1 t is = !    
am\nt ( curr5t4  ,illu/ration3 ,fig #ah 
,ano!r conv5i5ce 9 symbols is ?is3 ,we  
%all frequ5tly want to speak ( ! curr5ts
 9 sev]al diff]5t circuits4 ,x saves "t 
to use ano!r lett] along ) ! lett] ,i to
 %[ ! circuit to :i* we ref]4 ,=        
example1 we are go+ to talk ab\t !      
curr5t 9 ! ,b-circuit ( ! audion1 s we  
call t curr5t ,i,b4 ,we write ! lett] ,b
 bel[ ! l9e on :i* ,i /&s4 ,t is :y we  
say ! ,b is subscript1 mean+ writt5     
bel[4 ,:5 y are read+ to y\rself be sure

 to read ,i,b z eye-bee or else z       
eye-subscript-bee4 ,i,b !re=e w /& = !  
numb] ( amp]es 9 ! plate circuit ( !    
audion4 ,9 ! same way ,ia w\ld /& = !   
curr5t 9 ! filam5t circuit4  ,we are go+
 to talk ab\t e4 m4 f4's also4 ,! lett] 
,e /&s = ! numb] ( volts ( e4 m4 f4 9 a 
circuit4 ,9 ! filam5t circuit ! batt]y  
has ,e,a volts4 ,9 ! plate circuit ! e4 
m4 f4 is ,e,b volts4 ,if we put a batt]y
 9 ! grid circuit we c let ,e,c repres5t
 ! numb] ( volts appli$ to !            
grid-filam5t or ,c-circuit4  ,!         
*>act]i/ic relation :i* we are aft] is  
"o betwe5 grid voltage1 t is ,e,c1 &    
plate curr5t1 t is ,i,b4 ,s we call x ! 
,e,c--,i,b *>act]i/ic4 ,! da% betwe5 !  
lett]s is n a subtraction sign b m]ely a
 da% to sep>ate ! lett]s4 ,n[ we'll f9d 
! ee-see-eye-bee *>act]i/ic4  ,connect  
"s small dry cells 9 s]ies = use 9 !    
grid circuit4 ,!n connect ! filam5t to !
 middle cell z 9 ,fig4 #ai4 ,take ! wire
 :i* comes f ! grid & put a batt]y clip 
on x1 !n y c connect ! grid any:]e y    

want along ?is s]ies ( batt]ies4 ,see   
,fig4 #ah4 ,9 ! figure ?is movable clip 
is repres5t$ by an arr[ head4 ,y c see t
 if x is at a ! batt]y w make ! grid    
positive4 ,if x is mov$ to b ! grid w be
 m positive4 ,on ! o!r h& if ! clip is  
at o !re w be no e4 m4 f4 appli$ to !   
grid4 ,if x is at c ! grid w be made    
negative4  ,betwe5 grid & filam5t !re is
 plac$ a voltmet] :i* w tell h[ mu* e4  
m4 f4 is appli$ to ! grid1 t is1 tell ! 
value ( ,e,c1 = any position :atev] ( ! 
clip4  ,we %all />t ) ! filam5t heat$ to
 a deep r$4 ,! manufactur]s ( ! audion  
tell ! pur*as] :at curr5t %\ld fl[ ?r\gh
 ! filam5t s t !re w be ! prop] emission
 ( electrons4 ,!re are easy ways ( f9d+ 
out = "o's self b we %all n /op to      
describe !m4 ,! mak]s also tell h[ _m   
volts to apply to ! plate1 t is :at     
value ,e,b %\ld h4 ,we c\ld f9d ?is out 
also = ourselves b we %all n /op to d s4
  ,illu/ration3 ,fig #ai  ,n[ we set !  
batt]y clip s t !re is no voltage appli$
 to ! grid2 t is1 we />t ) ,e,c equal to

 z]o4 ,!n we read ! ammet] 9 ! plate    
circuit to f9d ! value ( ,i,b :i*       
corresponds to ?is condition ( ! grid4  
,next we move ! clip s z to make ! grid 
z positive z "o batt]y w make x1 t is we
 move ! clip to a 9 ,fig4 #ai4 ,we n[ h 
a diff]5t value ( ,e,c & w f9d a diff]5t
 value ( ,i,b :5 we read ! ammet]4 ,next
 move ! clip to apply two batt]ies to ! 
grid4 ,we get a new pair ( values = ,e,c
 & ,i,b1 gett+ ,e,c f ! voltmet] & ,i,b 
f ! ammet]4 ,z we cont9ue 9 ?is way1    
increas+ ,e,c1 we f9d t ! curr5t ,i,b   
increases = a :ile & !n aft] we h rea*$ 
a c]ta9 value ( ,e,c ! curr5t ,i,b /ops 
increas+4 ,add+ m batt]ies & mak+ ! grid
 m positive doesn't h any effect on !   
plate curr5t4  ,illu/ration3 ,fig #bj   
,be=e ,i tell y :y ?is happ5s ,i want to
 %[ y h[ to make a picture ( ! pairs (  
values ( ,e,c & ,i,b :i* we h be5 read+ 
on ! voltmet] & ammet]4  ,imag9e a city 
:]e all ! /reets are at "r angles & !   
nor? & s\? /reets are call$ /reets &    
numb]$ :ile ! ea/ & we/ ?or(>es are     

call$ av5ues4 ,i'll draw ! map z 9 ,fig4
 #bj4 ,"r ?r\gh ! c5t] ( ! city goes    
,ma9 ,/reet4 ,b ! p :o laid out ! roads 
7 ma!maticians & in/ead ( call+ x ,ma9  
,/reet !y call$ x ,z]o ,/reet4 ,! fir/  
/reet ea/ ( ,z]o ,/4 we %\ld h call$    
,ea/ ,fir/ ,/reet b !y call$ x ,positive
 #a ,/4 & ! next beyond ,positive #b    
,/41 & s on4 ,we/ ( ! ma9 /reet !y call$
 ! fir/ /reet ,negative #a ,/4 & s on4  
,:5 !y came to "n ! av5ues !y 7 j z     
precise & ma!matical4 ,!y call$ ! ma9   
av5ue ,z]o ,ave4 & ^? nor? ( x ,positive
 #a ,ave41 ,positive #b ,ave4 & s on4 ,(
 c\rse1 ! av5ues s\? ( ,z]o ,ave4 !y    
call$ ,negative4  ,! ,t[n ,c\ncil w5t   
almo/ crazy on ! subject ( numb]+2 !y   
numb]$ ev]y?+4 ,! sil5t policeman :i*   
/ood at ! corn] ( ,positive #b ,/4 &    
,positive #a ,ave4 0 m>k$ t way4 ,half  
way betwe5 ,positive #b ,/4 & ,positive 
#c ,/4 !re 0 a g>age :i* set back ab\t  
two-t5?s ( a block f ,positive #a ,ave4 
,! ,c\ncil numb]$ x & call$ x ,positive 
#b4e ,/4 & ,positive #a4b ,ave4 ,mo/ ( !

 p spoke ( x z ,plus #b4e ,/4 & ,plus   
#a4b ,ave4  ,sometime lat] !re 0 an     
election 9 ! city & a new ,c\ncil 0     
elect$4 ,! memb]s 7 mo/ly "y            
electricians & ! new ,highway           
,commission] 0 a radio en?usia/4 ,at !  
fir/ meet+ ! ,c\ncil *ang$ ! names ( all
 ! av5ues to ,mil-amp]es#c & ( all !    
/reets to ,volts4  ,!n ! ,highway       
,commission] :o _h j be5 tak+ a set (   
voltmet] & ammet] read+s on an audion   
mov$ t !re %\ld be a new road kn[n z    
,audion ,*>act]i/ic4 ,he said ! road    
%\ld pass ?r\gh ! foll[+ po9ts3         
,z]o ,volt & ,plus #a4j ,mil-amp]e      
,plus #b4j ,volts & ,plus #a4g          
,mil-amp]es     ,plus #d4j ,volts &     
,plus #b4f ,mil-amp]es     ,plus #f4j   
,volts & ,plus #c4d ,mil-amp]es         
,plus #h4j ,volts & ,plus #d4c          
,mil-amp]es  ,& s on4 ,fig4 #ba %[s !   
new road4  ,illu/ration3 ,fig #ba  ,"o  
memb] ( ! ,c\ncil jump$ up & said ,b :at
 if ! grid is made negative8 ,!         
,commission] _h =gott5 to see :at happ5$

 s he w5t home to take m read+s4  ,he   
%ift$ ! batt]y clip along1 />t+ at c (  
,fig4 #bb4 ,at ! next meet+ ( ! ,c\ncil 
he br\ght 9 ! foll[+ li/ ( read+s & h5ce
 ( po9ts on 8 propos$ road4      ,m9us  
#a4j ,volts & ,plus #j4f ,mil-amp]e     
#b4j             #j4d                   
#c4j             #j4b                   
#d4j             #j4a                   
#e4j             #j4j         ,!n he %[$
 ! o!r memb]s ( ! ,c\ncil on ! map (    
,fig4 #bc h[ ! ,audion ,*>act]i/ic w\ld 
look4  ,illu/ration3 ,fig #bb  ,!re 0   
consid]able discussion aft] t & x appe>$
 t diff]5t designs & makes ( audions    
w\ld h diff]5t *>act]i/ic curves4 ,!y   
all _h ! same g5]al =m ( curve b !y w\ld
 pass ?r\gh diff]5t sets ( po9ts dep5d+ 
^u ! design & ^u ! ,b-batt]y voltage4 ,x
 0 sev]al meet+s lat]1 h[ev]1 be=e !y   
f\nd out :at effects 7 due to ! =m ( !  
curve4 ,"r aft] ?is !y f\nd t !y c\ld   
get mu* bett] results ) _! radio sets4  
,n[ look at ! audion *>act]i/ic4 ,mak+ !
 grid positive1 t is go+ on ! positive  

side ( ! z]o volts 9 our map1 makes !   
plate curr5t l>g]4 ,y rememb] t ,i told 
y 9 ,lett] #f h[ ! grid1 :5 positive1   
help$ call electrons away f ! filam5t & 
s made a l>g] /ream ( electrons 9 !     
plate circuit4 ,! grid calls electrons  
away f ! filam5t4 ,x c't call !m out (  
x2 !y h to come out !mselves z ,i       
expla9$ to y 9 ! fif? lett]4            
,illu/ration3 ,fig #bc  ,y c see t z we 
make ! grid m & m positive1 t is1 make x
 call l\d] & l\d]1 a condition w be     
rea*$ :]e x won't d x any good to call  
any l\d]1 = x w already be gett+ all !  
electrons away f ! filam5t j z fa/ z !y 
are emitt$4 ,mak+ ! grid m positive aft]
 t w n increase ! plate curr5t any4 ,t's
 :y ! *>act]i/ic flatt5s (f z y see at  
high values ( grid voltage4  ,!         
arrangem5t :i* we pictur$ 9 ,fig4 #bb = 
mak+ *anges 9 ! grid voltage is simple b
 x doesn't let u *ange ! voltage by less
 ?an t ( a s+le batt]y cell4 ,i want to 
%[ y a way :i* w4 ,y'll f9d x v useful  
to "k & x is easily und]/ood = x is     

some?+ l ! arrangem5t ( ,fig4 #ad 9 !   
prec$+ lett]4  ,illu/ration3 ,fig #bd   
,connect ! cells z 9 ,fig4 #bd to a f9e 
wire4 ,ab\t ! middle ( ?is wire connect 
! filam5t4 ,z be=e use a clip on ! end (
 ! wire f ! grid4 ,if ! grid is connect$
 to a 9 ! figure !re is appli$ to ! grid
 circuit t "p ( ! e4 m4 f4 ( ! batt]y   
:i* is active 9 ! l5g? ( wire betwe5 o &
 a4 ,! po9t a is ne>] ! positive plate (
 ! batt]y ?an is ! po9t o4 ,s ! grid w  
be positive & ! filam5t negative4  ,on !
 o!r h&1 if ! clip is connect$ at b !   
grid w be negative ) respect to !       
filam5t4 ,we c1 !re=e1 make ! grid      
positive or negative dep5d+ on :i* side 
( o we connect ! clip4 ,h[ l>ge ! e4 m4 
f4 is :i* w be appli$ to ! grid dep5ds1 
( c\rse1 ^u h[ f> away f o ! clip is    
connect$4  ,suppose y took ! clip 9 y\r 
h& & slid x along 9 contact ) ! wire1   
fir/ f o to a & !n back aga9 ?r\gh o to 
b & s on back & =?4 ,y w\ld be mak+ !   
grid alt]nately positive & negative1    
w\ldn't y8 ,t is1 y w\ld be apply+ to ! 

grid an e4 m4 f4 :i* increases to "s    
positive value & !n1 decreas+ to z]o1   
rev]ses1 & increases j z mu*1 only to   
decrease to z]o1 :]e x />t$4 ,if y d ?is
 ov] & ov] aga91 tak+ always ! same "t =
 "o r\nd trip ( ! clip y w be impress+  
on ! grid circuit an alt]nat+ e4 m4 f4  
,:at's go+ to happ5 9 ! plate circuit8  
,:5 !re is no e4 m4 f4 appli$ to ! grid 
circuit1 t is :5 ! grid pot5tial        
7possibilities7 is z]o1 !re is a def9ite
 curr5t 9 ! plate circuit4 ,t curr5t we 
c f9d f our *>act]i/ic ( ,fig4 #bc = x  
is :]e ! curve crosses ,z]o ,volts4 ,z !
 grid becomes positive ! curr5t rises   
above ?is value4 ,:5 ! grid is made     
negative ! curr5t falls bel[ ?is value4 
,! curr5t1 ,i,b1 !n is made alt]nately  
great] & less ?an ! curr5t :5 ,e,c is   
z]o4  ,y might sp5d a little "t ?9k+ ov]
 ?is1 see+ :at happ5s :5 an alt]nat+ e4 
m4 f4 is appli$ to ! grid ( an audion1 =
 t is go+ to be fundam5tal to our /udy (
 radio4  ,footnote #c3 ,a mil-amp]e is a
 ?\s&? ( an amp]e j z a millimet] is a  

?\s&? ( a met]4     ,,lett] #aj         
,,cond5s]s ,,& ,,coils   ,,de> ,,son3   
,9 ! la/ lett] we le>n$ ( an alt]nat+ e4
 m4 f4 ,! way ( produc+ x1 :i* ,i       
describ$1 is v crude & ,i want to tell  
h[ to make ! audion develop an alt]nat+ 
e4 m4 f4 = itself4 ,t is :at ! audion   
does 9 ! transmitt+ set ( a radio       
telephone4 ,b an audion c't d x all     
alone4 ,x mu/ h associat$ ) x "s coils &
 a cond5s]4 ,y "k :at ,i mean by coils b
 y h yet to le>n ab\t cond5s]s4  ,a     
cond5s] is m]ely a gap 9 an o!rwise     
conduct+ circuit4 ,x's a gap across :i* 
electrons _c pass s t if !re is an e4 m4
 f4 9 ! circuit1 electrons w be v       
pl5tiful on "o side ( ! gap & sc>ce on !
 o!r side4 ,if !re are to be _m         
electrons wait+ beside ! gap !re mu/ be 
room = !m4 ,= t reason we usually       
provide wait+-rooms = ! electrons on ea*
 side ( ! gap4 ,metal plates or %eets ( 
t9foil s]ve nicely = ?is purpose4 ,look 
at ,fig4 #be4 ,y see a batt]y & a       
circuit :i* w\ld be conduct+ except = ! 

gap at ,c4 ,on ea* side ( ! gap !re is a
 %eet ( metal4 ,! metal %eets may be    
sep>at$ by air or mica or p>aff9$ pap]4 
,! comb9ation ( gap1 plates1 & :atev] is
 betwe51 provid$ x is n conduct+1 is    
call$ a cond5s]4  ,let u see :at happ5s 
:5 we connect a batt]y to a cond5s] z 9 
! figure4 ,! positive t]m9al ( ! batt]y 
calls electrons f "o plate ( ! cond5s]  
:ile ! negative batt]y-t]m9al drives    
electrons away f itself t[>d ! o!r plate
 ( ! cond5s]4 ,"o plate ( ! cond5s]1    
!re=e1 becomes positive :ile ! o!r plate
 becomes negative4  ,illu/ration3 ,fig  
#be  ,y "k t ?is action ( ! batt]y w g  
on until !re are s _m electrons 9 !     
negative plate ( ! cond5s] t !y prev5t !
 batt]y f add+ any m electrons to t     
plate4 ,! same ?+ happ5s at ! o!r       
cond5s] plate4 ,! positive t]m9al ( !   
batt]y calls electrons away f ! cond5s] 
plate :i* x is mak+ positive until s _m 
electrons h left t ! protons 9 ! atoms (
 ! plate are call+ = electrons to /ay   
home j z l\dly & effectively z !        

positive batt]y-t]m9al is call+ !m away4
  ,:5 bo? ^! conditions are rea*$--& !y 
are bo? rea*$ at ! same "t--!n ! batt]y 
has to /op driv+ electrons ar\nd !      
circuit4 ,! batt]y has n en\gh e4 m4 f4 
to drive any m electrons4 ,:y8 ,because 
! cond5s] has n[ j en\gh e4 m4 f4 ) :i* 
to oppose ! batt]y4  ,x w\ld be well to 
le>n at once ! "r words to use 9        
describ+ ?is action4 ,we say t ! batt]y 
s5ds a *>g+ curr5t ar\nd its circuit &  
*>ges ! cond5s] until x has ! same e4 m4
 f4 ,:5 ! batt]y is fir/ connect$ to !  
cond5s] !re is lots ( space 9 !         
wait+-rooms s !re is a great ru% or     
surge ( electrons into "o plate & away f
 ! o!r4 ,j at ?is fir/ in/ant ! *>g+    
curr5t1 !re=e1 is l>ge b x decreases    
rapidly1 = ! mom5t electrons />t to pile
 up on "o plate ( ! cond5s] & to leave !
 o!r1 an e4 m4 f4 builds up on !        
cond5s]4 ,?is e4 m4 f41 ( c\rse1 opposes
 t ( ! batt]y s t ! net e4 m4 f4 act+ to
 move electrons r\nd ! circuit is no    
long] t ( ! batt]y1 b is ! diff]5ce     

betwe5 ! e4 m4 f4 ( ! batt]y & t ( !    
cond5s]4 ,& s1 ) ea* add$ electron1 ! e4
 m4 f4 ( ! cond5s] increases until      
f9ally x is j equal to t ( ! batt]y &   
!re is no net e4 m4 f4 to act4  ,:at    
w\ld happ5 if we %\ld !n disconnect !   
batt]y8 ,! cond5s] w\ld be left ) its   
extra electrons 9 ! negative plate & )  
its positive plate lack+ ! same numb] ( 
electrons4 ,t is1 ! cond5s] w\ld be left
 *>g$ & its e4 m4 f4 w\ld be ( ! same   
numb] ( volts z ! batt]y4  ,illu/ration3
 ,fig #bf  ,n[ suppose we connect a %ort
 wire betwe5 ! plates ( ! cond5s] z 9   
,fig4 #bf4 ,! electrons ru% home f !    
negative plate to ! positive plate4 ,z  
fa/ z electrons get home ! e4 m4 f4     
decreases4 ,:5 !y are all back ! e4 m4  
f4 has be5 r$uc$ to z]o4 ,sometimes we  
say t ! cond5s] dis*>ges4 ,! dis*>ge    
curr5t />ts ) a ru% ! mom5t ! conduct+  
pa? is (f]$ betwe5 ! two plates4 ,! e4  
m4 f4 ( ! cond5s] falls1 ! dis*>ge      
curr5t gr[s small]1 & 9 a v %ort "t !   
cond5s] is completely dis*>g$4          

,illu/ration3 ,fig #bg  ,t's :at happ5s 
:5 !re is a %ort conduct+ pa? = !       
dis*>ge curr5t4 ,if t 7 all t c\ld happ5
 ,i d\bt if !re w\ld be any radio       
communication to-day4 ,b if we connect a
 coil ( wire betwe5 two plates ( a *>g$ 
cond5s]1 z 9 ,fig4 #bg1 !n some?+ (     
great int]e/ happ5s4 ,to und]/& y mu/ "k
 some?+ m ab\t electron /reams4         
,suppose we %\ld w9d a few turns ( wire 
on a cyl9drical core1 say on a /iff     
c>dbo>d tube4 ,we %all use insulat$     
wire4 ,n[ />t f "o end ( ! coil1 say a1 
& foll[ along ! coil$ wire = a few turns
 & !n scrat* (f ! insulation & sold]    
onto ! coil two wires1 b1 & c1 z %[n 9  
,fig4 #bh4 ,! fur!r end ( ! coil we %all
 call d4 ,n[ let's arrange a batt]y &   
swit* s t we c s5d a curr5t ?r\gh ! "p (
 ! coil betwe5 a & b4 ,arrange also a   
curr5t-measur+ in/rum5t s z to %[ if any
 curr5t is fl[+ 9 ! "p ( ! coil betwe5 c
 & d4 ,= ?is purpose we %all use a k9d (
 curr5t-measur+ in/rum5t :i* ,i h n yet 
expla9$4 ,x is diff]5t f ! hot-wire type

 describ$ 9 ,lett] #g = x w %[ 9 :i*    
direction electrons are /ream+ ?r\gh x4 
,! diagram ( ,fig4 #bh indicates !      
app>atus ( our exp]im5t4 ,:5 we close ! 
swit*1 ,s1 ! batt]y />ts a /ream (      
electrons f a t[>ds b4 ,j at t in/ant ! 
ne$le1 or po9t]1 ( ! curr5t in/rum5t    
moves4 ,! ne$le moves1 & ?us %[s a      
curr5t 9 ! coil cd2 b x comes "r back   
aga91 %[+ t ! curr5t is only mom5t>y4   
,let's say ?is aga9 9 diff]5t words4 ,! 
batt]y keeps /eadily =c+ electrons ?r\gh
 ! circuit ab b ! in/rum5t 9 ! circuit  
cd %[s no curr5t 9 t circuit except j at
 ! in/ant :5 curr5t />ts to fl[ 9 !     
neighbor+ circuit ab4  ,illu/ration3    
,fig #bh  ,"o ?+ ?is curr5t-measur+     
in/rum5t tells u is ! direction ( !     
electron /ream ?r\gh itself4 ,x %[s t ! 
mom5t>y /ream ( electrons goes ?r\gh !  
coil f d to c1 t is 9 ! opposite        
direction to ! /ream 9 ! "p ab4  ,n[    
prep>e to d a little close ?9k+4 ,read  
ov] c>efully all ,i h told y ab\t ?is   
exp]im5t4 ,y see t ! mom5t ! batt]y />ts

 a /ream ( electrons f a t[>ds b1 some?+
 causes a mom5t>y1 t is a tempor>y1     
movem5t ( electrons f d to c4 ,we say t 
/>t+ a /ream ( electrons f a to b sets  
up or induces a /ream ( electrons f d to
 c4  ,:at w happ5 !n if we connect !    
batt]y betwe5 a & d z 9 ,fig4 #bi8      
,electrons w />t /ream+ away f a t[>ds  
b1 t is t[>ds d4 ,b t means !re w be a  
mom5t>y /ream f d t[>ds c1 t is t[>ds a4
 ,our /ream f ! batt]y causes ?is       
oppositely direct$ /ream4 ,9 ! usual    
words we say x induces 9 ! coil an      
oppos+ /ream ( electrons4 ,?is oppos+   
/ream doesn't la/ long1 z we saw1 b :ile
 x does la/ x h9d]s ! /ream :i* ! batt]y
 is try+ to e/abli%4  ,illu/ration3 ,fig
 #bi  ,! /ream ( electrons :i* ! batt]y 
causes w at fir/ meet an opposition s x 
takes a little "t be=e ! batt]y c get ! 
full-siz$ /ream ( electrons fl[+        
/eadily4 ,9 o!r words a curr5t 9 a coil 
builds up sl[ly1 because :ile x is      
build+ up x induces an effect :i*       
opposes some:at its own build+ up4  ,did

 y ev] see a small boy />t (f some:]e1  
p]haps :]e he %\ldn't be go+1 & f9d 8   
consci5ce />t+ to tr\ble him at once4 ,=
 a "t he goes a little sl[ly b 9 a mom5t
 or two 8 consci5ce /ops oppos+ him & he
 goes on /eadily at 8 full pace4 ,:5 he 
/>t$ he /irr$ up 8 consci5ce & t oppos$ 
him4 ,nobody else 0 h9d]+ 8 go+4 ,x 0   
all br\ght ab\t by 8 own actions4 ,!    
opposition :i* he met 0 self-induc$4 ,he
 0 h9d]$ at fir/ by a self-induc$ effect
 ( 8 own consci5ce4 ,if he 0 a /ream (  
electrons />t+ (f to travel ar\nd ! coil
 we w\ld say t he 0 oppos$ by a         
self-induc$ e4 m4 f4 ,& any pa? 9 :i*   
su* an effect w be produc$ we say has   
self-inductance4 ,usually we %ort5 ?is  
t]m & speak ( inductance4  ,!re is ano!r
 way ( look+ at x4 ,we "k habits are h>d
 to =m & equally h>d to break4 ,x's h>d 
to get electrons go+ ar\nd a coil & !   
self-inductance ( a circuit tells u h[  
h>d x is4 ,! h>d] x is ! m              
self-inductance we say t ! coil or      
circuit has4 ,( c\rse1 we ne$ a unit 9  

:i* to measure self-inductance4 ,! unit 
is call$ ! h5ry4 ,b t is m              
self-inductance ?an we c /& 9 mo/ radio 
circuits1 s we f9d x conv5i5t to measure
 9 small] units call$ mil-h5ries :i* are
 ?\s&?s ( a h5ry4  ,y ought to "k :at a 
h5ry#d is1 if we are to use ! ^w1 b x   
isn't necess>y j n[ to sp5d mu* "t on x4
 ,! opposition :i* "o's self-induc$     
consci5ce (f]s dep5ds ^u h[ rapidly "o  
/>ts4 ,x's volts :i* make electrons move
 & s ! consci5ce :i* opposes !m w be    
measur$ 9 volts4 ,!re=e we say t a coil 
has "o h5ry ( inductance :5 an electron 
/ream :i* is increas+ "o amp]e's wor?   
ea* second /irs up 9 ! coil a           
consci5ti\s objection ( "o volt4 ,don't 
try to rememb] ?is n[2 y c come back to 
x lat]4  ,!re is "o m effect (          
inductance :i* we mu/ "k be=e we c get v
 f> ) our radio4 ,suppose an electron   
/ream is fl[+ ?r\gh a coil because a    
batt]y is driv+ ! electrons along4 ,n[  
let ! batt]y be remov$ or disconnect$4  
,y'd expect ! electron /ream to /op at  

once b x doesn't4 ,x keeps on = a mom5t 
because ! electrons h got ! habit4      
,illu/ration3 ,fig #bh  ,if y look aga9 
at ,fig4 #bh y w see :at ,i mean4       
,suppose ! swit* is clos$ & a /eady     
/ream ( electrons is fl[+ ?r\gh ! coil f
 a to b4 ,!re w be no curr5t 9 ! o!r "p 
( ! coil4 ,n[ op5 ! swit*4 ,!re w be a  
motion ( ! ne$le ( ! curr5t-measur+     
in/rum5t1 %[+ a mom5t>y curr5t4 ,!      
direction ( ?is motion1 h[ev]1 %[s t !  
mom5t>y /ream ( electrons goes ?r\gh !  
coil f c to d4  ,d y see :at ?is means8 
,! mom5t ! batt]y is disconnect$ !re is 
no?+ driv+ ! electrons 9 ! "p ab & !y   
sl[ d[n4 ,imm$iately1 & j = an in/ant1 a
 /ream ( electrons />ts (f 9 ! "p cd 9 !
 same direction z if ! batt]y 0 driv+ !m
 along4  ,n[ look aga9 at ,fig4 #bi4 ,if
 ! batt]y is sudd5ly disconnect$ !re is 
a mom5t>y ru% ( electrons 9 ! same      
direction z ! batt]y 0 driv+ !m4 ,j z ! 
self-inductance ( a coil opposes ! />t+ 
( a /ream ( electrons1 s x opposes !    
/opp+ ( a /ream :i* is already go+4     

,illu/ration3 ,fig #bi  ,s f> we hav5't 
said mu* ab\t mak+ an audion produce    
alt]nat+ e4 m4 f4's & ?us mak+ x useful 
= radio-telephony4 ,be=e radio 0        
possible all ^! ?+s t ,i h j told y1 &  
"s m too1 _h to be kn[n4 ,x took hundr$s
 ( good sci5ti/s ye>s ( pati5t /udy &   
exp]im5t to f9d out ^? ideas ab\t       
electricity :i* h made possible         
radio-telephony4  ,two ( ^! ideas are   
absolutely necess>y = ! /ud5t (         
radio-communication4 ,fir/3 ,a cond5s]  
is a gap 9 a circuit :]e !re are        
wait+-rooms = ! electrons4 ,second3     
,electrons =m habits4 ,x's h>d to get !m
 go+ ?r\gh a coil ( wire1 h>d] ?an ?r\gh
 a /raight wire1 b aft] !y are go+ !y   
don't l to /op4 ,!y l x mu* less if !y  
are go+ ?r\gh a coil in/ead ( a /raight 
wire4  ,9 my next lett] ,i'll tell y :at
 happ5s :5 we h a coil & a cond5s]      
toge!r 9 a circuit4  ,footnote #d3 ,!   
h5ry has no?+ to d ) a well-kn[n        
automobile4 ,x 0 nam$ aft] ,joseph      
,h5ry1 a pr(essor ye>s ago at ,pr9ceton 

,univ]sity4     ,,lett] #aa  ,a ,c-,w   
,,transmitt]   ,,de> ,,son3             
,illu/ration3 ,fig #bh  ,let's look aga9
 at ! coils ( ,fig4 #bh :i* we /udi$ 9 !
 la/ lett]4 ,i h reproduc$ !m "h s y    
won't h to turn back4 ,:5 electrons />t 
f a t[>ds b !re is a mom5t>y /ream (    
electrons f d t[>ds c4 ,if ! electron   
/ream ?r\gh ab 7 />t$ 9 ! opposite      
direction1 t is f b to a ! induc$ /ream 
9 ! coil cd w\ld be f c t[>ds d4        
,illu/ration3 ,fig #cj  ,x all rem9ds me
 ( two boys ) a h$ge or f5ce betwe5 !m z
 9 ,fig4 #cj4 ,"o boy is aft] ! o!r4    
,suppose y 7 be+ *as$2 y "k :at y'd d4  
,if y\r pursu] />t$ (f ) a ru% t[>ds "o 
end ( ! h$ge y'd beat x t[>ds ! o!r4 ,b 
if he />t$ sl[ly & cauti\sly y w\ld />t 
sl[ly too4 ,y always g 9 ! opposite     
direction1 dodg+ back & =? along ! pa?s 
:i* y are we>+ 9 ! grass on opposite    
sides ( ! h$ge4 ,if he />ts to ! "r & !n
 sl[s up & />ts back1 y w />t to y\r "r1
 sl[ up1 & />t back4 ,suppose he />ts at
 ! c5t] ( ! h$ge4 ,fir/ he dodges to !  

"r1 & !n back ?r\gh ! c5t] z f> to !    
left1 !n back aga9 & s on4 ,y foll[ 8 e 
*ange4  ,illu/ration3 ,fig #ca  ,i am   
go+ to make a picture ( :at y two d4    
,let's />t ) ! o!r fell[4 ,he dodges or 
alt]nates back & =?4 ,"s p]sons w\ld say
 he oscillates back & =? 9 ! same pa?4  
,z he does s he induces y to move4 ,i am
 on y\r side ( ! h$ge ) a mov+-picture  
cam]a4 ,my cam]a cat*es bo? ( y4 ,fig4  
#ca %[s ! way ! film w\ld look if x     
caught only y\r heads4 ,! :ite circle   
repres5ts ! t[-head on my side ( ! h$ge 
& ! black circle1 "y ,br[n :o lives next
 door4 ,( c\rse1 ! cam]a only cat*es y  
ea* "t ! %utt] op5s b x is easy to draw 
a complete picture ( :at takes place z  
"t goes on4 ,see ,fig4 #cb4             
,illu/ration3 ,fig #cb  ,n[ suppose y   
are an electron 9 coil cd ( ,fig4 #cc & 
,br[nie is "o 9 coil ab4 ,y\r motions   
are induc$ by 84 ,:at's true ( y two is 
true ( all ! o!r electrons4 ,i h sep>at$
 ! coils a little 9 ?is sket* s t y c   
?9k ( a h$ge betwe54 ,i don't "k h[ "o  

electron c affect ano!r on ! opposite   
side ( ?is h$ge b x c4 ,& ,i don't "k   
any?+ really ab\t ! h$ge1 :i* is g5]ally
 call$ ! e!r4 ,! h$ge isn't air4 ,!     
effect w\ld be ! same if ! coils 7 9 a  
vacuum4 ,! e!r is j a "n = :atev] is    
left 9 ! space ab\t u :5 we h tak5 out  
ev]y?+ :i* we c see or feel--e molecule1
 e proton & e electron4  ,illu/ration3  
,fig #cc  ,:y & h[ electrons c affect "o
 ano!r :5 !y are widely sep>at$ is "o ( 
! great my/]ies ( sci5ce4 ,we don't "k  
any m ab\t x ?an ab\t :y !re are        
electrons4 ,let's accept x z a          
fundam5tal fact :i* we c't z yet expla94
  ,illu/ration3 ,fig #cd  ,& n[ we c see
 h[ to make an audion produce an        
alt]nat+ curr5t or z we sometimes say   
make an audion oscillator4 ,we %all set 
up an audion ) its ,a-batt]y z 9 ,fig4  
#cd4 ,betwe5 ! grid & ! filam5t we put a
 coil & a cond5s]4 ,notice t !y are 9   
p>allel1 z we say4 ,9 ! plate-filam5t   
circuit we connect ! ,b-batt]y & a      
swit*1 ,s1 & ano!r coil4 ,?is coil 9 !  

plate circuit ( ! audion we place close 
to ! o!r coil s t ! two coils are j l ! 
coils ab & cd ( :i* ,i h be5 tell+ y4 ,!
 mom5t any curr5t fl[s 9 coil ab !re w  
be a curr5t fl[ 9 ! coil cd4 7,an induc$
 electron /ream47 ,( c\rse1 z long z !  
swit* 9 ! ,b-batt]y is op5 no curr5t c  
fl[4  ,! mom5t ! swit* ,s is clos$ !    
,b-batt]y makes ! plate positive )      
respect to ! filam5t & !re is a sudd5   
surge ( electrons r\nd ! plate circuit &
 ?r\gh ! coil f a to b4 ,y "k :at t does
 to ! coil cd4 ,x induces an electron   
/ream f d t[>ds c4 ,:]e d ^! electrons  
come f8 ,:y1 f ! grid & ! plate #a ( !  
cond5s]4 ,:]e d !y g8 ,mo/ ( !m g to !  
wait+-room (f]$ by plate #b ( ! cond5s] 
& "s1 ( c\rse1 to ! filam5t4 ,:at is !  
result8 ,! grid becomes positive & !    
filam5t negative4  ,illu/ration3 ,fig   
#ce  ,?is is ! crucial mom5t 9 our /udy4
 ,c y tell me :at is go+ to happ5 to !  
/ream ( electrons 9 ! plate circuit8    
,rememb] t j at ! in/ant :5 we clos$ !  
swit* ! grid 0 nei!r positive nor       

negative4 ,we 7 at ! po9t ( z]o volts on
 ! audion *>act]i/ic ( ,fig4 #ce4 ,:5 we
 close ! swit* ! curr5t 9 ! plate       
circuit />ts to jump f z]o mil-amp]es to
 ! numb] ( mil-amp]es :i* repres5ts !   
po9t :]e ,z]o ,volt ,/4 crosses ,audion 
,*>act]i/ic4 ,b ?is jump 9 plate curr5t 
makes ! grid positive z we h j se54 ,s !
 grid w help ! plate call electrons & t 
w make ! curr5t 9 ! plate circuit /ill  
l>g]1 t is1 result 9 a l>g] /ream (     
electrons f a to b4  ,?is increase 9    
curr5t w be mat*$ by an increas$ effect 
9 ! coil cd1 = y rememb] h[ y & ,br[nie 
behav$4 ,& t w pull m electrons away f  
plate #a ( ! cond5s] & s5d !m to !      
wait+-room ( #b4 ,all ?is makes ! grid m
 positive & s makes x call all ! m      
effectively to help ! plate move        
electrons4  ,illu/ration3 ,pl4          
,v4--,v>iomet] 7top7 & ,v>iable ,cond5s]
 7bottom7 ( ! ,g5]al ,radio ,company4   
,voltmet] & ,ammet] ( ! ,we/on ,in/rum5t
 ,company4  ,we />t$ some?+ t "t4 ,x's  
go+ on all by itself4 ,! grid is gett+ m

 positive1 ! plate curr5t is gett+      
bigg]1 & s ! grid is gett+ m positive & 
! plate curr5t /ill bigg]4 ,is x ev] go+
 to /op8 ,yes4 ,look at ! audion        
*>act]i/ic4 ,!re comes a "t :5 mak+ !   
grid a little m positive won't h any    
effect on ! plate-circuit curr5t4 ,s !  
plate curr5t /ops increas+4  ,!re is    
no?+ n[ to keep pull+ electrons away f  
plate #a & cr[d+ !m into wait+-room #b4 
,:y %\ldn't ! electrons 9 ?is wait+-room
 g home to t ( plate #a8 ,!re is n[ no  
reason & s !y />t (f ) a ru%4  ,( c\rse1
 "s ( !m came f ! grid & z fa/ z        
electrons get back to ! grid x becomes  
less & less positive4 ,z ! grid becomes 
less & less positive x becomes less &   
less helpful to ! plate4  ,if ! grid    
doesn't help1 ! plate alone c't keep up 
?is /ream ( electrons4 ,all ! plate c d 
by itself is to ma9ta9 ! curr5t         
repres5t$ by ! int]section ( z]o volts &
 ! audion *>act]i/ic4 ,! result is t !  
curr5t 9 ! plate circuit1 t is1 ( c\rse1
 ! curr5t 9 coil ab1 becomes gradually  

less4 ,ab\t ! "t all ! electrons1 :i* _h
 left ! grid & plate #a ( ! cond5s]1 h  
got home ! plate curr5t is back to !    
value correspond+ to ,e,c#j4  ,! plate  
curr5t fir/ increases & !n decreases1 b 
x doesn't /op decreas+ :5 x gets back to
 z]o-grid value4 ,& ! reason is all due 
to ! habit =m+ t5d5cies ( electrons 9   
coils4 ,to see h[ ?is comes ab\t1 let's 
tell ! :ole /ory ov] aga94 ,9 o!r words 
let's make a review & s get a sort (    
fly+ />t4  ,illu/ration3 ,fig #cd  ,:5  
we close ! batt]y swit*1 ,s 9 ,fig4 #cd1
 we all[ a curr5t to fl[ 9 ! plate      
circuit4 ,?is curr5t induces a curr5t 9 
! coil cd & *>ges ! cond5s] :i* is      
across x1 mak+ plate #a positive & plate
 #b negative4 ,a positive grid helps !  
plate s t ! curr5t 9 ! plate circuit    
builds up to ! greate/ possible value z 
%[n by ! audion *>act]i/ic4 ,t's ! end (
 ! increase 9 curr5t4 ,n[ ! cond5s]     
dis*>ges1 s5d+ electrons ?r\gh ! coil cd
 & mak+ ! grid less positive until      
f9ally x is at z]o pot5tial1 t is nei!r 

positive nor negative4  ,:ile ! cond5s] 
is dis*>g+ ! electrons 9 ! coil cd get a
 habit ( fl[+ f c t[>d d1 t is f plate  
#b to plate #a4 ,if x wasn't = ?is habit
 ! electron /ream 9 cd w\ld /op z soon z
 ! grid _h r$uc$ to z]o voltage4        
,because ( ! habit1 h[ev]1 a lot (      
electrons t ought to /ay on plate #b get
 hurri$ along & l& on plate #a4 ,x is a 
little l ! old game ( crack ! :ip4 ,"s  
electrons get ! habit & c't /op quickly 
en\gh s !y g tumbl+ into wait+-room #a &
 make x negative4  ,t means t ! cond5s] 
n only dis*>ges b />ts to get *>g$ 9 !  
o!r direction ) plate #a negative &     
plate #b positive4 ,! grid feels !      
effect ( all ?is1 because x gets extra  
electrons if plate #a gets !m4 ,9 fact !
 voltage effective betwe5 grid & filam5t
 is always ! voltage betwe5 ! plates ( !
 cond5s]4  ,! audion *>act]i/ic tells u 
:at is ! result4 ,z ! grid becomes      
negative x opposes ! plate1 %oo+        
electrons back t[>ds ! filam5t & r$uc+ !
 plate curr5t /ill fur!r4 ,b y h already

 se5 9 my previ\s lett] :at happ5s :5 we
 r$uce ! curr5t 9 coil ab4 ,!re is !n   
induc$ 9 coil cd an electron /ream f c  
to d4 ,?is induc$ curr5t is 9 j ! "r    
direction to s5d m electrons into       
wait+-room #a & s to make ! grid /ill m 
negative4 ,& ! m negative ! grid gets ! 
small] becomes ! plate curr5t until     
f9ally ! plate curr5t is r$uc$ to z]o4  
,look at ! audion *>act]i/ic aga9 & see 
t mak+ ! grid suffici5tly negative      
entirely /ops ! plate curr5t4  ,:5 !    
plate curr5t /ops1 ! cond5s] 9 ! grid   
circuit is *>g$1 ) plate #a negative &  
#b positive4 ,x 0 ! plate curr5t :i* 0 !
 ma9 cause ( ?is *ange = x induc$ ! *>g+
 curr5t 9 coil cd4 ,s1 :5 ! plate curr5t
 becomes z]o !re is no?+ to prev5t !    
cond5s] f dis*>g+4  ,its dis*>ge makes !
 grid less & less negative until x is   
z]o volts & !re we are--back practically
 :]e we />t$4 ,! plate curr5t is        
increas+ & ! grid is gett+ positive1 &  
we're (f on ano!r cycle z we say4 ,dur+ 
a cycle ! plate curr5t increases to a   

maximum1 decreases to z]o1 & !n         
increases aga9 to its initial value4    
,illu/ration3 ,fig #cf  ,?is lett] has a
 long] cont9u\s tra9 ( ?\ght ?an ,i     
usually ask y to foll[4 ,b be=e ,i /op  
,i want to give y "s idea ( :at good ?is
 is 9 radio4  ,:at ab\t ! curr5t :i*    
fl[s 9 coil cd8 ,x's an alt]nat+ curr5t1
 isn't x8 ,fir/ ! electrons /ream f d   
t[>ds c1 & !n back aga9 f c t[>ds d4    
,suppose we set up ano!r coil l ,,cd 9  
,fig4 #cf4 ,x w\ld h an alt]nat+ curr5t 
induc$ 9 x4 ,if ?is coil 0 connect$ to  
an ant5na !re w\ld be radio waves s5t   
out4 ,! swit* ,s c\ld be us$ = a key &  
kept clos$ long] or %ort] int]vals      
dep5d+ ^u :e!r da%es or dots 7 be+ set4 
,i'll tell y m ab\t ?is lat]1 b 9 ?is   
diagram are ! mak+s ( a ,c-,w           
,transmitt]1 t is a cont9u\s wave       
transmitt] = radio-telegraphy4  ,x w\ld 
be wor? :ile to g ov] ?is lett] aga9 us+
 a p5cil & trac+ 9 ! v>i\s circuits !   
electron /reams :i* ,i h describ$4      
,,lett] #ab  ,,inductance ,,& ,,capacity

   ,,de> ,,sir3  ,9 ! la/ lett] ,i      
didn't /op to draw y a picture ( !      
action ( ! audion oscillator :i* ,i     
describ$4 ,i am go+ to d x n[ & y are to
 imag9e me z us+ two p5cils & draw+     
simultane\sly two curves4 ,"o curve %[s 
:at happ5s to ! curr5t 9 ! plate        
circuit4 ,! o!r %[s h[ ! voltage ( !    
grid *anges4 ,bo? curves />t f ! in/ant 
:5 ! swit* is clos$2 & ! two tak5 toge!r
 %[ j :at happ5s 9 ! tube f in/ant to   
in/ant4  ,fig4 #cg %[s ! two curves4 ,y 
w notice h[ ,i h drawn !m beside & bel[ 
! audion *>act]i/ic4 ,! grid voltage & !
 plate curr5t are relat$1 z ,i h told y1
 & ! audion *>act]i/ic is j a conv5i5t  
way ( %[+ ! relation%ip4 ,if we "k !    
curr5t 9 ! plate circuit we c f9d !     
voltage ( ! grid & vice v]sa4  ,z "t    
goes on1 ! plate curr5t gr[s to its     
maximum & decreases to z]o & !n goes on 
climb+ up & d[n betwe5 ^! two extremes4 
,! grid voltage mean:ile is v>y+        
alt]nately1 hav+ its maximum positive   
value :5 ! plate curr5t is a maximum &  

its maximum negative value :5 ! plate   
curr5t is z]o4 ,look at ! two curves &  
see ?is = y\rself4  ,illu/ration3 ,fig  
#cg  ,n[ ,i want to tell y some?+ ab\t  
h[ fa/ ^! oscillations occur4 ,we />t by
 le>n+ two words4 ,"o is cycle ) :i* y  
are already p>tly famili> & ! o!r is    
frequ5cy4 ,take cycle fir/4 ,/>t+ f z]o 
! curr5t increases to a maximum1        
decreases to z]o1 & is ready aga9 = !   
same s]ies ( *anges4 ,we say ! curr5t   
has pass$ ?r\gh a cycle ( values4 ,x    
doesn't make any diff]5ce :]e we />t f4 
,if we foll[ ! curr5t ?r\gh all its     
diff]5t values until we are back at !   
same value z we />t$ ) & ready to />t   
all ov]1 !n we h foll[$ ?r\gh a cycle ( 
values4  ,once y get ! idea ( a cycle1 &
 ! m>k+s on ! curves 9 ,fig4 #ca w help 
y to und]/&1 !n ! o!r idea is easy4 ,by 
frequ5cy we mean ! numb] ( cycles ea*   
second4 ,! electric curr5t :i* we use 9 
light+ our h\se goes ?r\gh sixty cycles 
a second4 ,t means ! curr5t rev]ses its 
direction #abj times a second4  ,9 radio

 we use alt]nat+ curr5ts :i* h v high   
frequ5cies4 ,9 %ip sets ! frequ5cy is   
ei!r #ejj1jjj or #a1jjj1jjj cycles p]   
second4 ,amateur transmitt+ sets usually
 h oscillators :i* run at well ov] a    
million cycles p] second4 ,! long] range
 /ations use l[] frequ5cies4  ,y'll f9d1
 h[ev]1 t ! newspap] ann\ncem5ts ( !    
v>i\s broadca/ /ations d n tell !       
frequ5cy b in/ead tell ! wave l5g?4 ,i  
am n go+ to /op n[ to expla9 :at t means
 b ,i am go+ to give y a simple rule4   
,divide #cjj1jjj1jjj by ! wave l5g? &   
y'll h ! frequ5cy4 ,= example1 %ips are 
suppos$ to use wave l5g?s ( #cjj met]s  
or #fjj met]s4 ,divid+ ?ree hundr$      
million by ?ree hundr$ gives "o million 
& t is "o ( ! frequ5cies :i* ,i told y 7
 us$ by %ip sets4 ,divid+ by six hundr$ 
gives #ejj1jjj or j half ! frequ5cy4 ,y 
c rememb] t sets transmitt+ ) long waves
 h l[ frequ5cies1 b sets ) %ort waves h 
high frequ5cies4 ,! frequ5cy & ! wave   
l5g? don't *ange 9 ! same way4 ,!y *ange
 9 opposite ways or inv]sely1 z we say4 

,! high] ! frequ5cy ! %ort] ! wave l5g?4
  ,i'll tell y ab\t wave l5g?s lat]4    
,fir/ let's see h[ to control ! frequ5cy
 ( an audion oscillator l t ( ,fig4 #ch4
  ,illu/ration3 ,fig #ch  ,x takes "t to
 get a full-siz$ /ream go+ ?r\gh a coil 
because ( ! inductance ( ! coil4 ,t y h 
le>n$4 ,& also x takes "t = su* a curr5t
 to /op completely4 ,!re=e1 if we make !
 inductance ( ! coil small1 keep+ !     
cond5s] ! same1 we %all make ! "t       
requir$ = ! curr5t to />t & /op small]4 
,t w mean a high] frequ5cy = !re w be m 
oscillations ea* second4 ,"o rule1 !n1 =
 increas+ ! frequ5cy ( an audion        
oscillator is to decrease ! inductance4 
,lat] 9 ?is lett] ,i %all tell y h[ to  
increase or decrease ! inductance ( a   
coil4 ,be=e ,i d s1 h[ev]1 ,i want to   
call y\r att5tion to ! o!r way 9 :i* we 
c *ange ! frequ5cy ( an audion          
oscillator4  ,let's see h[ ! frequ5cy w 
dep5d ^u ! capacity ( ! cond5s]4 ,if a  
cond5s] has a l>ge capacity x means t x 
c accommodate 9 its wait+-room a l>ge   

numb] ( electrons be=e ! e4 m4 f4 ( !   
cond5s] becomes l>ge en\gh to /op !     
/ream ( electrons :i* is *>g+ ! cond5s]4
 ,if ! cond5s] 9 ! grid circuit ( ,fig4 
#ch is ( l>ge capacity x means t x mu/  
receive 9 its upp] wait+-room a l>ge    
numb] ( electrons be=e ! grid w be      
negative en\gh to make ! plate curr5t   
z]o4 ,!re=e1 ! *>g+ curr5t w h to fl[ a 
long "t to /ore up ! necess>y numb] (   
electrons4  ,y w get ! same idea1 (     
c\rse1 if y ?9k ab\t ! electrons 9 ! l[]
 room4 ,! curr5t 9 ! plate circuit w n  
/op increas+ until ! voltage ( ! grid   
has become positive en\gh to make !     
plate curr5t a maximum4 ,x c't d t until
 en\gh electrons h left ! upp] room &   
be5 /or$ away 9 ! l[]4 ,!re=e ! *>g+    
curr5t w h to fl[ = a long "t if !      
capacity is l>ge4 ,we h1 !re=e1 ! o!r   
rule = increas+ ! frequ5cy ( an audion  
oscillator1 t is1 decrease ! capacity4  
,^! rules c be /at$ ! o!r way ar\nd4 ,to
 decrease ! frequ5cy we c ei!r increase 
! capacity or increase ! inductance or d

 bo?4  ,b :at w\ld happ5 if we %\ld     
decrease ! capacity & increase !        
inductance8 ,decreas+ ! capacity w\ld   
make ! frequ5cy high]1 b increas+ !     
inductance w\ld make x l[]4 ,:at w\ld be
 ! net effect8 ,t w\ld dep5d ^u h[ mu*  
we decreas$ ! capacity & h[ mu* we      
increas$ ! inductance4 ,x w\ld be       
possible to decrease ! capacity & !n if 
we increas$ ! inductance j ! "r am\nt to
 h no *ange 9 ! frequ5cy4 ,no matt] h[  
l>ge or h[ small we make ! capacity we c
 always make ! inductance su* t !re     
isn't any *ange 9 frequ5cy4 ,i'll give y
 a rule = ?is1 aft] ,i h told y "s m ?+s
 ab\t capacities & inductances4  ,fir/ z
 to inductances4 ,a %ort /raight wire   
has a v small inductance1 inde$4 ,!     
long] ! wire ! l>g] w be ! inductance b 
unless ! l5g? is hundr$s ( feet !re     
isn't mu* inductance anyway4 ,a coil$   
wire is v diff]5t4  ,a coil ( wire w h m
 inductance ! m turns !re are to x4 ,t  
isn't ! :ole /ory b x's en\gh = ! mom5t4
 ,let's see :y4 ,! reason :y a /ream (  

electrons has an oppos+ consci5ce :5 !y 
are />t$ (f 9 a coil ( wire is because  
ea* electron affects e o!r electron :i* 
c move 9 a p>allel pa?4 ,look aga9 at ! 
coils ( ,figs4 #bh & #bi :i* we discuss$
 9 ! t5? lett]4 ,^? sket*es pla9ly br+  
out ! fact t ! electrons 9 "p cd travel 
9 pa?s :i* are p>allel to ^? ( !        
electrons 9 "p ab4  ,illu/ration3 ,fig  
#ci  ,if we %\ld turn ^! coils z 9 ,fig4
 #ci s t all ! pa?s 9 cd are at "r      
angles to ^? 9 ab !re w\ldn't be any    
effect 9 cd :5 a curr5t 9 ab />t$ or    
/opp$4 ,look at ! circuit ( ! oscillat+ 
audion 9 ,fig4 #ch4 ,if we %\ld turn ^! 
coils at "r angles to ea* o!r we w\ld   
/op ! oscillation4 ,electrons only      
influ5ce o!r electrons :i* are 9 p>allel
 pa?s4  ,:5 we want a l>ge inductance we
 w9d ! coil s t !re are _m p>allel pa?s4
 ,!n :5 ! batt]y />ts to drive an       
electron along1 ?is electron affects all
 its fell[s :o are 9 p>allel pa?s &     
tries to />t !m (f 9 ! opposite         
direction to t 9 :i* x is be+ driv54 ,! 

batt]y1 ( c\rse1 />ts to drive all !    
electrons1 n only ^? ne>e/ its negative 
t]m9al b ^? all along ! wire4 ,& e "o ( 
^! electrons makes up = ! fact t !      
batt]y is driv+ x along by urg+ all its 
fell[s 9 ! opposite direction4  ,x is n 
an exceptional /ate ( affairs4 ,suppose 
a lot ( boys are be+ driv5 out ( a y>d  
:]e !y _h no "r to be play+4 ,suppose   
also t a boy c resi/ & lag back twice z 
mu* if "s o!r boy urges him to d s4     
,make x easy & imag9e ?ree boys4 ,! fir/
 boy lags back n only on 8 own acc\nt b 
because ( ! urg+ ( ! o!r boys4 ,t makes 
him ?ree times z h>d to />t z if ! o!r  
boys didn't influ5ce him4 ,! same is    
true ( ! second boy & also ( ! ?ird4 ,! 
result is ! un=tunate prop]ty own] has  
n9e times z h>d a job gett+ t gang />t$ 
z if only "o boy 7 to be dealt )4 ,if   
!re 7 two boys x w\ld be f\r times z h>d
 z = "o boy4 ,if !re 7 f\r 9 ! gr\p x   
w\ld be sixte5 times1 & if five x w\ld  
be tw5ty-five times4 ,! difficulty      
increases mu* m rapidly ?an ! numb] (   

boys4  ,n[ all we h to d to get ! "r    
idea ( inductance is to ?9k ( ea* boy z 
/&+ = ! electrons 9 "o turn ( ! coil4   
,if !re are five turns !re w be         
tw5ty-five times z mu* inductance1 z = a
 s+le turn2 & s on4 ,y see t we c *ange 
! inductance ( a coil v easily by *ang+ 
! numb] ( turns4  ,i'll tell y two ?+s m
 ab\t inductance because !y w come 9    
h&y4 ,! fir/ is t ! inductance w be l>g]
 if ! turns are l>ge circles4 ,y c see t
 = y\rself because if ! circles 7 v     
small we w\ld h practically a /raight   
wire4  ,! o!r fact is ?is4 ,if t prop]ty
 own] _h be5 an electrical eng9e] & !   
boys _h be5 electrons he w\ld h fix$ x s
 t :ile half ( !m said1 ,aw1 don't g2 he
 c't put y (f2 ! o!r half w\ld h said   
,come on1 let's get out4 ,if he did t he
 w\ld h a coil )\t any inductance1 t is1
 he w\ld h only ! natural in]tia ( !    
electrons to deal )4 ,we w\ld say t he  
_h made a coil ) pure resi/ance or else 
t he _h made a non-inductive resi/ance4 
,illu/ration3 ,fig #dj  ,h[ w\ld he d x8

 ,easy en\gh aft] "o le>ns h[1 b q      
+5i\s4 ,take ! wire & fold x at !       
middle4 ,/>t ) ! middle & w9d ! coil ) !
 d\bl$ wire4 ,fig4 #dj %[s h[ ! coil    
w\ld look & y c see t "p ( ! way !      
electrons are go+ ar\nd ! coil 9 "o     
direction & ! re/ ( ! way 9 ! opposite  
direction4 ,x is j z if ! boys 7 pair$  
(f1 a goody-goody & a t\gh nut toge!r4  
,!y bo? %\t at once opposite advice &   
nei!r has any effect4  ,i h told y all  
except "o ( ! ways 9 :i* we c affect !  
inductance ( a circuit4 ,y "k n[ all !  
me?ods :i* are important 9 radio4 ,s    
let's consid] h[ to make l>ge or small  
capacities4  ,fir/ ,i want to tell y h[ 
we measure ! capacity ( a cond5s]4 ,we  
use units call$ micr(>ads4 ,y rememb] t 
an amp]e means an electron /ream at !   
rate ( ab\t six billion billion         
electrons a second4 ,a million? ( an    
amp]e w\ld1 !re=e1 be a /ream at ! rate 
( ab\t six million million electrons a  
second--q a sizable little /ream = any  
"o :o want$ to c\nt !m z !y w5t by4 ,if 

a curr5t ( "o million? ( an amp]e %\ld  
fl[ = j "o second six million million   
electrons w\ld pass along by e po9t 9 ! 
pa? or circuit4  ,t is :at w\ld happ5 if
 !re w]5't any wait+-rooms 9 ! circuit4 
,if !re 0 a cond5s] !n t numb] (        
electrons w\ld leave "o wait+-room &    
w\ld ent] ! o!r4 ,well1 suppose t j z ! 
la/ electron ( ?is enorm\s numb]#e ent]$
 its wait+-room we %\ld "k t ! voltage (
 ! cond5s] 0 j "o volt4 ,!n we w\ld say 
t ! cond5s] _h a capacity ( "o micr(>ad4
 ,if x takes half t numb] to make !     
cond5s] oppose fur!r *anges 9 ! cont5ts 
( its wait+-rooms1 ) "o volt's wor? (   
opposition1 t is1 "o volt ( e4 m4 f41 !n
 ! cond5s] has only half a micr(>ad (   
capacity4 ,! numb] ( micr(>ads (        
capacity 7abbreviat$ mf47 is a measure (
 h[ _m electrons we c get away f "o     
plate & into ! o!r be=e ! voltage rises 
to "o volt4  ,:at mu/ we d !n to make a 
cond5s] ) l>ge capacity8 ,ei!r ( two    
?+s2 ei!r make ! wait+-rooms l>ge or put
 !m close toge!r4  ,if we make ! plates 

( a cond5s] l>g]1 keep+ ! sep>ation     
betwe5 !m ! same1 x means m space 9 !   
wait+-rooms & h5ce less cr[d+4 ,y "k t !
 m cr[d$ ! electrons become ! m !y pu%  
back aga9/ any o!r electron :i* "s      
batt]y is try+ to =ce into _!           
wait+-room1 t is ! high] ! e4 m4 f4 ( ! 
cond5s]4  ,! o!r way to get a l>g]      
capacity is to br+ ! plates clos]       
toge!r1 t is to %ort5 ! gap4 ,look at x 
?is way3 ,! clos] ! plates are toge!r ! 
ne>] home ! electrons are4 ,_! home is  
only j across a little gap2 !y c almo/  
see ! electronic games go+ on ar\nd !   
nuclei !y left4 ,!y =get ! long         
r\nd-ab\t j\rney !y took to get to ?is  
new wait+-room & !y cr[d ov] to "o side 
( ?is room to get j z close z !y c to _!
 old homes4 ,t's :y x's always easi]1 & 
takes less voltage1 to get ! same numb] 
( electrons mov$ f "o plate to ! o!r ( a
 cond5s] :i* has only a small space     
betwe5 plates4 ,x takes less voltage & t
 means t ! cond5s] has a small] e4 m4 f4
 = ! same numb] ( electrons4 ,x also    

means t be=e ! e4 m4 f4 rises to "o volt
 we c get m electrons mov$ ar\nd if !   
plates are close toge!r4 ,& t means l>g]
 capacity4  ,!re is "o ?+ to rememb] 9  
all ?is3 ,x doesn't make any diff]5ce h[
 ?ick ! plates are4 ,x all dep5ds ^u h[ 
mu* surface !y h & h[ close toge!r !y   
are4 ,mo/ ( ! electrons 9 ! plate :i* is
 be+ made negative are way ov] on ! side
 t[>d _! old homes1 t is1 t[>d ! plate  
:i* is be+ made positive4 ,& mo/ ( !    
homes1 t is1 atoms :i* h lo/ electrons1 
are on ! side ( ! positive plate :i* is 
next to ! gap4 ,t's :y ,i said !        
electrons c\ld almo/ see _! old homes4  
,illu/ration3 ,fig #da  ,all ?is leads  
to two v simple rules = build+ cond5s]s4
 ,if y h a cond5s] ) too small a        
capacity & want "o1 say1 twice z l>ge1 y
 c ei!r use twice z l>ge plates or br+ !
 plates y already h twice z close       
toge!r2 t is1 make ! gap half z l>ge4   
,g5]ally1 ( c\rse1 ! gap is pretty well 
fix$4 ,= example1 if we make a cond5s]  
by us+ two pieces ( metal & sep>at+ !m  

by a %eet ( mica we don't want ! job (  
splitt+ ! mica4 ,s we increase ! size ( 
! plates4 ,we c d t ei!r by us+ l>g]    
plates or o!r plates & connect+ x z 9   
,fig4 #da s t ! total wait+-room space =
 electrons is increas$4  ,illu/ration3  
,pl4 ,,vi4--,l[-p[] ,transmitt+ ,tube1  
,u ,v #bjb 7,c\rtesy ( ,radio           
,corporation ( ,am]ica74  ,illu/ration3 
,fig #db  ,if y h got ^! ideas y c      
und]/& h[ we use bo? sides ( ! same     
plate 9 "s types ( cond5s]s4 ,look at   
,fig4 #db4 ,!re are two plates connect$ 
toge!r & a ?ird betwe5 !m4 ,suppose     
electrons are pull$ f ! outside plates &
 cr[d$ into ! middle plate4 ,"s ( !m g  
on "o side & "s on ! o!r1 z ,i h %[n4 ,!
 negative signs indicate electrons & !  
plus signs _! old homes4 ,if we use m   
plates z 9 ,fig4 #dc we h a l>g]        
capacity4  ,illu/ration3 ,fig #dc       
,illu/ration3 ,fig #dd  ,:at if we h two
 plates :i* are n directly opposite "o  
ano!r1 l ^? ( ,fig4 #dd8 ,:at does !    
capacity dep5d ^u8 ,imag9e y\rself an   

electron on ! negative plate4 ,look (f  
t[>d ! positive plate & see h[ big x    
seems to y4 ,! bigg] x looks ! m        
capacity ! cond5s] has4 ,:5 ! plates are
 "r opposite "o ano!r ! positive plate  
looms up pretty l>ge4 ,b if !y slide    
ap>t y don't see s mu* ( x2 & if x is (f
 to "o side ab\t all y see is ! edge4   
,if y c't see lots ( atoms :i* h lo/    
electrons & s w\ld make good homes = y1 
!re is no use ( y\r /ay+ ar\nd on t side
 ( ! plate2 y might j z well be try+ to 
g back home ! long way :i* y orig9ally  
came4  ,t's :y 9 a v>iable plate cond5s]
 !re is v little capacity :5 no p>ts ( !
 plates are opposite ea* o!r1 & !re is !
 greate/ capacity :5 !y are exactly     
opposite "o ano!r4  ,illu/ration3 ,fig  
#de  ,:ile we are at x we might j z well
 clean up ?is :ole bus9ess ( v>iable    
capacities & inductances by consid]+ two
 ways 9 :i* to make a v>iable           
inductance4 ,fig4 #de %[s ! simple/ way 
b x has "s disadvantages :i* ,i won't   
try n[ to expla94 ,we make a long coil &

 !n take (f taps4 ,we c make connections
 betwe5 "o end ( ! coil & any ( ! taps4 
,! m turns !re are includ$ 9 ! "p ( !   
coil :i* we are us+ ! great] is !       
inductance4 ,if we want to d a real job 
we c br+ ea* ( ^! taps to a little /ud &
 arrange a slid+ or rotat+ contact ) !m4
 ,!n we h an inductance ! value ( :i* we
 c v>y /ep-by-/ep 9 a conv5i5t mann]4   
,ano!r way to make a v>iable inductance 
is to make :at is call$ a v>iomet]4 ,i  
dislike ! "n because x doesn't met]     
any?+4 ,if prop]ly calibrat$ x w\ld (   
c\rse met] inductance1 b !n x %\ld be   
call$ an inducto-met]4  ,d y rememb] !  
gang ( boys t fell[ _h to drive (f 8    
prop]ty8 ,:at if !re _h be5 two diff]5t 
gangs play+ !re8 ,h[ mu* tr\ble he has  
dep5ds ^u :e!r !re is any?+ 9 common    
betwe5 ! gangs4 ,suppose !y are play+ 9 
diff]5t p>ts ( 8 prop]ty & s act j z if 
! o!r cr[d wasn't also trespass+4 ,he   
c\ld j add ! tr\ble ( />t+ "o gang to ! 
tr\ble ( />t+ ! o!r4  ,x w\ld be v      
diff]5t if ! gangs h any?+ 9 common4 ,!n

 "o w\ld enc\rage ! o!r mu* z ! v>i\s   
boys ( ! same gang enc\rage ea* o!r4 ,he
 w\ld h a lot m tr\ble4 ,& ?is extra    
tr\ble w\ld be because ( ! relations    
betwe5 gangs1 t is1 because ( _! mutual 
inductance4  ,on ! o!r h& suppose !     
gangs came f diff]5t p>ts ( ! t[n &     
dislik$ ea* o!r4 ,he w\ldn't h ne>ly !  
tr\ble4 ,ea* gang w\ld be yell+ at ! o!r
 z !y w5t along3 ,y'd bett] beat x4 ,he 
kn[s all "r1 all "r1 :o broke t bu% d[n 
by ! gate4 ,j wait till he cat*es y4    
,!y'd get out a little easi]1 ea* 9 !   
hope ! o!r cr[d w\ld cat* x f ! own]4   
,!re's a case :]e _! mutual relations1  
_! mutual inductance1 makes ! job easi]4
  ,t's true ( coils ) inductance4       
,suppose y w9d two inductance coils &   
connect !m 9 s]ies4 ,if !y are at "r    
angles to ea* o!r z 9 ,fig4 #df;a !y h  
no effect on ea* o!r4 ,!re is no mutual 
inductance4 ,b if !y are p>allel & w\nd 
! same way l ! coils ( ,fig4 #df;b !y w 
act l a s+le coil ( great] inductance4  
,if ! coils are p>allel b w\nd 9        

opposite directions z 9 ,fig4 #df;c !y w
 h less inductance because ( _! mutual  
inductance4 ,y c *eck ^! /atem5ts =     
y\rself if y'll ref] back to ,lett] #aj 
& see :at happ5s 9 ! same way z ,i told 
y 9 discuss+ ,fig4 #bh4  ,illu/ration3  
,fig #df;a  ,illu/ration3 ,fig #df;b    
,if ! coils are nei!r p>allel nor at "r 
angles !re w be "s mutual inductance b n
 z mu* z if !y 7 p>allel4 ,by turn+ !   
coils we c get all ! v>iations 9 mutual 
relations f ! case ( ,fig4 #df;b to t ( 
,fig4 #df;c4 ,t's :at we arrange to d 9 
a v>iable inductance ( ! v>iomet] type4 
,illu/ration3 ,fig #df;c  ,!re is ano!r 
way ( v>y+ ! mutual inductance4 ,we c   
make "o coil slide inside ano!r4 ,if x  
is way inside1 ! total inductance :i* ! 
two coils (f] is ei!r l>g] ?an ! sum (  
:at !y c (f] sep>ately or less1 dep5d+  
^u :e!r ! w9d+s are 9 ! same direction  
or opposite4 ,z we pull ! coil out !    
mutual effect becomes less & f9ally :5 x
 is well outside ! mutual inductance is 
v small4  ,n[ we h sev]al me?ods ( v>y+ 

capacity & inductance & !re=e we are    
ready to v>y ! frequ5cy ( our audion    
oscillator2 t is1 tune x1 z we say4 ,9  
my next lett] ,i %all %[ y :y we tune4  
,n[ = ! rule :i* ,i promis$4 ,! frequ5cy
 to :i* a circuit is tun$ dep5ds ^u !   
product ( ! numb] ( mil-h5ries 9 ! coil 
& ! numb] ( micr(>ads 9 ! cond5s]4      
,*ange ! coil & ! cond5s] z mu* z y want
 b keep ?is product ! same & ! frequ5cy 
w be ! same4  ,footnote #e3 ,m          
accurately ! numb] is                   
#f1bhf1jjj1jjj1jjj4     ,,lett] #ac     
,,tun+   ,,de> ,,radio ,,en?usia/3  ,i  
want to tell y ab\t receiv+ sets & _!   
tun+4 ,9 ! la/ lett] ,i told y :at      
det]m9es ! frequ5cy ( oscillation ( an  
audion oscillator4 ,x 0 ! cond5s] &     
inductance :i* y /udi$ 9 connection )   
,fig4 #cf4 ,t's :at det]m9es ! frequ5cy 
& also :at makes ! oscillations4 ,all ! 
tube does is to keep !m go+4 ,let's see 
:y ?is is s4  ,illu/ration3 ,fig #dg;a  
,/>t fir/1 z 9 ,fig4 #dg;a1 ) a v simple
 circuit ( a batt]y & a non-inductive   

resi/ance1 t is1 a wire w\nd l t ( ,fig4
 #dj 9 ! previ\s lett]1 s t x has no    
inductance4 ,! batt]y mu/ d "w =c+      
electrons ?r\gh t wire4 ,x has !        
ability1 or ! en]gy z we say4           
,illu/ration3 ,fig #dg;b  ,n[ connect a 
cond5s] to ! batt]y z 9 ,fig4 #dg;b4 ,! 
connect+ wires are v %ort2 & s          
practically all ! "w :i* ! batt]y does  
is 9 /or+ electrons 9 ! negative plate (
 ! cond5s] & robb+ ! positive plate4 ,! 
batt]y displaces a c]ta9 numb] (        
electrons 9 ! wait+-rooms ( ! cond5s]4  
,h[ _m1 dep5ds ^u h[ h>d x c pu% & pull1
 t is on its e4 m4 f41 & ^u h[ mu*      
capacity ! cond5s] has4  ,illu/ration3  
,fig #dg;c  ,remove ! batt]y & connect !
 *>g$ cond5s] to ! resi/ance z 9 ,fig4  
#dg;c4 ,! electrons ru% home4 ,!y bump &
 jo/le _! way along1 heat+ ! wire z !y  
g4 ,!y h a c]ta9 am\nt ( en]gy or       
ability to d "w because !y are away f   
home & !y use x all up1 b\nc+ along on  
_! way4 ,:5 once !y are home !y h us$ up
 all ! surplus en]gy :i* ! batt]y gave  

!m4  ,try x aga91 b ?is "t1 z 9 ,fig4   
#dg;d1 connect ! *>g$ cond5s] to a coil 
:i* has inductance4 ,! electrons don't  
get />t$ z fa/ because ( ! inductance4  
,b !y keep go+ because ! electrons 9 !  
wire =m ! habit4 ,! result is t ab\t !  
"t en\gh electrons h got into plate #b  
7:i* 0 positive71 to satisfy all its    
lonely protons1 ! electrons 9 ! wire are
 /ream+ along at a great rate4 ,a lot ( 
!m keep go+ until !y l& on ?is plate & s
 make x negative4  ,illu/ration3 ,fig   
#dg;d  ,t's ! same sort ( ?+ t happ5s 9 
! case ( ! inductance & cond5s] 9 !     
oscillat+ audion circuit except = "o    
important fact4 ,!re is no?+ to keep    
electrons go+ to ! #b plate except ?is  
habit4 ,& !re are pl5ty ( /ay-at-home   
electrons to /op !m z !y ru% along4 ,!y 
bump & jo/le1 b "s ( !m are /opp$ or    
else div]t$ s t !y g bump+ ar\nd )\t    
gett+ any ne>] plate #b4 ,( c\rse1 !y   
sp5d all _! en]gy ?is way1 gett+ e "o   
all /irr$ up & heat+ ! wire4  ,"s ( !   
en]gy :i* ! electrons _h :5 !y 7 on     

plate #a is sp5t1 !re=e1 & !re ar5't z  
_m electrons gett+ to plate #b4 ,:5 !y  
turn ar\nd & />t back1 z y "k !y d1 !   
same ?+ happ5s4 ,! result is t ea*      
successive surge ( electrons is small]  
?an ! prec$+4 ,_! en]gy is be+ wa/$ 9   
heat+ ! wire4 ,! /ream ( electrons gets 
small] & small]1 & ! voltage ( ! cond5s]
 gets small] & small]1 until by-&-by !re
 isn't any /ream & ! cond5s] is left    
un*>g$4 ,:5 t happ5s1 we say !          
oscillations h damp$ out4  ,illu/ration3
 ,fig #dh  ,t's "o way ( />t+           
oscillations :i* damp out--to />t ) a   
*>g$ cond5s] & connect an inductance    
across x4 ,!re is ano!r way :i* leads u 
to "s important ideas4 ,look at ,fig4   
#dh4 ,!re is an inductance & a cond5s]4 
,ne> ! coil is ano!r coil :i* has a     
batt]y & a key 9 circuit ) x4 ,! coils  
are our old fri5ds ( ,fig4 #cc 9 ,lett] 
#aj4 ,suppose we close ! swit* ,s4 ,x   
/>ts a curr5t ?r\gh ! coil ab :i* goes  
on /eadily z soon z x really gets go+4  
,:ile x is />t+1 h[ev]1 x induces an    

electron /ream 9 coil cd4 ,!re is only a
 mom5t>y or transi5t curr5t b x s]ves to
 *>ge ! cond5s] & !n ev5ts happ5 j z !y 
did 9 ! case :]e we *>g$ ! cond5s] ) a  
batt]y4  ,illu/ration3 ,fig #di  ,n[    
take away ?is coil ab ) its batt]y &    
sub/itute ! oscillator ( ,fig4 #cf4     
,:at's go+ to happ58 ,we h two circuits 
9 :i* oscillations c occur4 ,see ,fig4  
#di4 ,"o circuit is associat$ ) an      
audion & "s batt]ies :i* keep supply+ x 
) en]gy s t its oscillations are        
cont9u\s4 ,! o!r circuit is ne> en\gh to
 ! fir/ to be influ5c$ by :at happ5s 9 t
 circuit4 ,we say x is c\pl$ to x1      
because :atev] happ5s 9 ! fir/ circuit  
induces an effect 9 ! second circuit4   
,suppose fir/ t 9 ea* circuit !         
inductance & capacity h su* values z to 
produce oscillations ( ! same frequ5cy4 
,!n ! mom5t we />t ! oscillator we h !  
same effect 9 bo? circuits4 ,let me draw
 ! picture a little diff]5tly 7,fig4    
#ej7 s t y c see ?is m easily4 ,i h     
m]ely made ! coil ab 9 two p>ts1 "o (   

:i* c affect cd 9 ! oscillator & ! o!r !
 coil ,l ( ! second circuit4  ,b suppose
 t ! two circuits d n h ! same natural  
frequ5cies1 t is ! cond5s] & inductance 
9 "o circuit are s l>ge t x j naturally 
takes m "t = an oscillation 9 t circuit 
?an 9 ! o!r4 ,x is l le>n+ to dance4 ,y 
"k ab\t h[ well y & y\r p>tn] w\ld get  
along if y _h "o frequ5cy ( oscillation 
& %e _h ano!r4 ,t's :at happ5s 9 a case 
l ?is4  ,illu/ration3 ,fig #ej  ,if     
circuit ,l-,c takes long] = ea*         
oscillation ?an does circuit ab its     
electron /ream is always work+ at cross 
purposes ) ! electron /ream 9 ab :i* is 
try+ to lead x4 ,its electrons />t (f f 
"o cond5s] plate to ! o!r & be=e !y h   
mu* m ?an got />t$ ! /ream 9 ab tries to
 call !m back to g 9 ! o!r direction4 ,x
 is practically impossible "u ^!        
conditions to get a /ream ( any size go+
 9 circuit ,l-,c4 ,x is equally h>d if  
,l-,c has small] capacity & inductance  
?an ab s t x naturally oscillates fa/]4 
,i'll tell y exactly :at x is l4        

,suppose y & y\r p>tn] are try+ to dance
 )\t any piano or o!r s\rce ( music4 ,%e
 has "o tune runn+ ?r\gh h] head & %e   
dances to t1 except z y drag h] ar\nd ! 
floor4 ,y are try+ to foll[ ano!r tune4 
,z a c\ple y h a difficult "t go+ any:]e
 "u ^! conditions4 ,b x w\ld be all "r  
if y bo? _h ! same tune4  ,if we want ! 
electron /ream 9 coil ab to h a l>ge    
guid+ effect on ! /ream 9 coil ,l-,c we 
mu/ see t bo? circuits h ! same tune1 t 
is ! same natural frequ5cy (            
oscillation4  ,illu/ration3 ,fig #ea    
,?is c be %[n v easily by a simple      
exp]im5t4 ,suppose we set up our circuit
 ,l-,c ) an ammet] 9 x1 s z to be able  
to tell h[ l>ge an electron /ream is    
oscillat+ 9 t circuit4 ,let u also make 
! cond5s] a v>iable "o s t we c *ange ! 
natural frequ5cy or tune ( ! circuit4   
,n[ let's see :at happ5s to ! curr5t z  
we v>y ?is cond5s]1 *ang+ ! capacity &  
?us *ang+ ! tune ( ! circuit4 ,if we use
 a v>iable plate cond5s] x w h a scale  
on top graduat$ 9 degrees & we c note ! 

read+ ( ! ammet] = ea* position ( !     
movable plates4 ,if we d1 we f9d "o     
position ( ^! plates1 t is "o sett+1    
correspond+ to "o value ( capacity 9 !  
cond5s]1 :]e ! curr5t 9 ! circuit is a  
maximum4 ,?is is ! sett+ ( ! cond5s] =  
:i* ! circuit has ! same tune or natural
 frequ5cy z ! circuit cd4 ,sometimes we 
say t ! circuits are n[ 9 resonance4 ,we
 also ref] to ! curve ( values ( curr5t 
& cond5s] positions z a tun+ curve4 ,su*
 a curve is %[n 9 ,fig4 #ea4            
,illu/ration3 ,fig #eb  ,t's all !re is 
to tun+--adju/+ ! capacity & inductance 
( a circuit until x has ! same natural  
frequ5cy z "s o!r circuit ) :i* we want 
x to "w4 ,we c ei!r adju/ ! capacity z  
we j did1 or we c adju/ ! inductance4 ,9
 t case we use a v>iable inductance z 9 
,fig4 #eb4  ,if we want to be able to   
tune to any ( a l>ge range ( frequ5cies 
we usually h to take out or put into !  
circuit a :ole lot ( mil-h5ries at a "t4
 ,:5 we d we get ^! mil-h5ries (        
inductance f a coil :i* we call a load+ 

coil4 ,t's :y y\r fri5ds add a load+    
coil :5 !y want to tune = ! long        
wave-l5g? /ations1 t is1 ^? ) a l[      
frequ5cy4  ,:5 our circuit ,l-,c ( ,fig4
 #di is tun$ to ! frequ5cy ( !          
oscillator we get 9 x a maximum curr5t4 
,!re is a maximum /ream ( electrons1 &  
h5ce a maximum numb] ( !m cr[d$ fir/    
into "o & !n into ! o!r plate ( !       
cond5s]4 ,& s ! cond5s] is *>g$ to a    
maximum voltage1 fir/ 9 "o direction &  
!n 9 ! o!r4  ,illu/ration3 ,fig #ec  ,n[
 connect ! circuit ,l-,c to ! grid ( an 
audion4 ,if ! circuit is tun$ we'll h ! 
maximum possible voltage appli$ betwe5  
grid & filam5t4 ,9 ! plate circuit we'll
 get an increase & !n a decrease (      
curr5t4 ,y "k t w happ5 = ,i prep>$ y = 
?is mom5t by ! la/ page ( my n9? lett]4 
,i'll tell y m ab\t t curr5t 9 ! plate  
circuit 9 a lat] lett]4 ,i am connect+ a
 telephone receiv] 9 ! plate circuit1 & 
also a cond5s]1 ! latt] = a reason to be
 expla9$ lat]4 ,! comb9ation appe>s !n z
 9 ,fig4 #ec4 ,t figure %[s a ,c-,w     

transmitt] & an audion detector4 ,?is is
 ! sort ( a detector we w\ld use =      
radio-telephony1 b ! transmitt] is !    
sort we w\ld use = radio-telegraphy4 ,we
 %all make "s *anges 9 !m lat]4         
,illu/ration3 ,fig #ed  ,:5ev] we />t ! 
oscillat+ curr5t 9 ! transmitt] we get  
an effect 9 ! detector circuit1 ( :i*   
,i'll tell y m lat]4 ,= ! mom5t ,i am   
int]e/$ 9 %[+ y h[ ! transmitt] & !     
detector may be sep>at$ by miles & /ill 
!re w be an effect 9 ! detector circuit 
e "t ! key 9 ! transmitt] circuit is    
clos$4  ,?is is h[ we d x4 ,at ! s5d+   
/ation1 t is1 :]ev] we locate !         
transmitt]1 we make a cond5s] us+ ! e>?1
 or gr\nd1 z "o plate4 ,we d ! same ?+  
at ! receiv+ /ation :]e ! detector      
circuit is locat$4 ,to ^! cond5s]s we   
connect inductances & ^! inductances we 
c\ple to our transmitt] & receiv] z %[n 
9 ,fig4 #ed4 ,! upp] plate ( ! cond5s] 9
 ea* case is a few horizontal wires4 ,! 
l[] plate is ! moi/ e>? ( ! gr\nd & we  
arrange to get 9 contact ) t 9 v>i\s    

ways4 ,"o ( ! simple/ me?ods is to      
connect to ! wat] pipes ( ! city        
wat]-sy/em4  ,n[ we h our radio         
transmitt+-/ation & a /ation = receiv+  
its signals4 ,y rememb] we c make dots &
 da%es by ! key or swit* 9 ! oscillator 
circuit4 ,:5 we depress ! key we />t !  
oscillator go+4 ,t sets up oscillations 
9 ! circuit ) ! inductance & ! capacity 
=m$ by ! ant5na4 ,if we want a real-siz$
 /ream ( electrons up & d[n ?is ant5na  
lead 7! v]tical wire71 we mu/ tune t    
circuit4 ,t is :y ,i h %[n a v>iable    
inductance 9 ! circuit ( ! transmitt+   
ant5na4  ,:at happ5s :5 ^! electrons    
surge back & =? betwe5 ! horizontal     
wires & ! gr\nd1 ,i don't "k4 ,i d "k1  
h[ev]1 t if we tune ! ant5na circuit at 
! receiv+ /ation !re w be a small /ream 
( electrons surg+ back & =? 9 t circuit4
  ,usually sci5ti/s expla9 :at happ5s by
 say+ t ! transmitt+ /ation s5ds out    
waves 9 ! e!r & t ^! waves are receiv$  
by ! ant5na sy/em at ! di/ant /ation4   
,:]ev] y put up a receiv+ /ation y w get

 ! effect4 ,x w be mu* small]1 h[ev]1 ! 
f>!r ! two /ations are ap>t4  ,i am n   
go+ to tell y any?+ ab\t wave motion 9 !
 e!r because ,i don't believe we "k     
en\gh ab\t ! e!r to try to expla91 b ,i 
%all tell y :at we mean by wave l5g?4   
,someh[ en]gy1 ! ability to d "w1       
travels out f ! s5d+ ant5na 9 all       
directions4 ,:]ev] y put up y\r receiv+ 
/ation y get m or less ( ?is en]gy4 ,(  
c\rse1 en]gy is be+ s5t out only :ile ! 
key is depress$ & ! oscillator go+4 ,?is
 en]gy travels j z fa/ z light1 t is at 
! enorm\s spe$ ( #ahf1jjj miles a       
second4 ,if y use met]s in/ead ( miles !
 spe$ is #cjj1jjj1jjj met]s a second4   
,n[1 h[ f> w ! en]gy :i* is s5t out f ! 
ant5na travel dur+ ! "t x takes = "o    
oscillation ( ! curr5t 9 ! ant5na8      
,suppose ! curr5t is oscillat+ "o       
million times a second4 ,!n x takes     
"o-million? ( a second = "o oscillation4
 ,9 t "t ! en]gy w h travel$ away f !   
ant5na "o-million? "p ( ! di/ance x w   
travel 9 a :ole second4 ,t is           

"o-million? ( #cjj million met]s or #cjj
 met]s4  ,! di/ance :i* en]gy w g 9 ! "t
 tak5 by "o oscillation ( ! s\rce ( t   
en]gy is ! wave l5g?4 ,9 ! case j giv5 t
 di/ance is #cjj met]s4 ,! wave l5g?1   
!n1 ( #cjj met]s corresponds to a       
frequ5cy ( "o million4 ,9 fact if we    
divide #cjj million met]s by ! frequ5cy 
we get ! wave l5g?1 & t's ! same rule z 
,i gave y 9 ! la/ lett]4  ,9 fur!r      
lett]s ,i'll tell y h[ ! audion works z 
a detector & h[ we connect a telephone  
transmitt] to ! oscillator to make x s5d
 out en]gy ) a spee* significance in/ead
 ( a m]e dot & da% significance1 or     
signal significance4 ,we %all h to le>n 
q a little ab\t ! telephone itself &    
ab\t ! human voice4     ,,lett] #ad     
,,:y ,,& ,,h[ ,,to ,,use ,a ,,detector  
,,de> ,,son3  ,9 ! la/ lett] we got f>  
en\gh to sket*1 9 ,fig4 #ed1 a radio    
transmitt+ /ation & a receiv+ /ation4   
,we %\ld nev]1 h[ev]1 use j ?is         
comb9ation because ! transmitt+ /ation  
is int5d$ to s5d telegraph signals & !  

receiv+ /ation is be/ suit$ to receiv+  
telephonic transmission4 ,b let u see   
:at happ5s4  ,illu/ration3 ,fig #ed  ,:5
 ! key 9 ! plate circuit ( ! audion at !
 s5d+ /ation is depress$ an alt]nat+    
curr5t is />t$4 ,?is induces an alt]nat+
 curr5t 9 ! neighbor+ ant5na circuit4   
,if ?is ant5na circuit1 :i* is =m$ by a 
coil & a cond5s]1 is tun$ to ! frequ5cy 
( oscillations :i* are be+ produc$ 9 !  
audion circuit !n !re is a maximum      
curr5t induc$ 9 ! ant5na4  ,z soon z ?is
 />ts ! ant5na />ts to s5d out en]gy 9  
all directions1 or radiate en]gy z we   
say4 ,h[ ?is en]gy1 or ability to d "w1 
gets across space we don't "k4 ,h[ev] x 
may be1 x does get to ! receiv+ /ation4 
,x only takes a small fraction ( a      
second be=e ! ant5na at ! receiv+ /ation
 />ts to receive en]gy1 because en]gy   
travels at ! rate ( #ahf1jjj miles a    
second4  ,! en]gy :i* is receiv$ does   
its "w 9 mak+ ! electrons 9 t ant5na    
oscillate back & =?4 ,if ! receiv+      
ant5na is tun$ to ! frequ5cy :i* ! s5d+ 

/ation is produc+1 !n ! electrons 9 !   
receiv+ ant5na oscillate back & =? mo/  
widely & !re is a maximum curr5t 9 ?is  
circuit4  ,! oscillations ( ! electrons 
9 ! receiv+ ant5na induce simil>        
oscillations 9 ! tun$ circuit :i* is    
c\pl$ to x4 ,?is circuit also is tun$ to
 ! frequ5cy :i* ! di/ant oscillator is  
produc+ & s 9 x we h ! maximum          
oscillation ( ! electrons4 ,! cond5s] 9 
t circuit *>ges & dis*>ges alt]nately4  
,! grid ( ! receiv+ audion always has ! 
same voltage z ! cond5s] to :i* x is    
connect$ & s x becomes alt]nately       
positive & negative4 ,?is /ate ( affairs
 />ts almo/ z soon z ! key at ! s5d+    
/ation is depress$ & cont9ues z long z x
 is held d[n4  ,n[ :at happ5s inside !  
audion8 ,z ! grid becomes m & m positive
 ! curr5t 9 ! plate circuit increases4  
,:5 ! grid no long] gr[s m positive b r 
becomes less & less positive ! curr5t 9 
! plate circuit decreases4 ,z ! grid    
becomes ( z]o voltage & !n negative1 t  
is z ! grid rev]ses its pol>ity1 ! plate

 curr5t cont9ues to decrease4 ,:5 ! grid
 /ops gr[+ m negative & />ts to become  
less s1 ! plate curr5t /ops decreas+ &  
/>ts to increase4  ,all ?is y "k1 = y h 
foll[$ ?r\gh su* a cycle ( *anges be=e4 
,y "k also h[ we c use ! audion         
*>act]i/ic to tell u :at sort ( *anges  
take place 9 ! plate curr5t :5 ! grid   
voltage *anges4 ,! plate curr5t         
increases & decreases alt]nately1 becom+
 great] & less ?an x w\ld be if ! grid 7
 n int]f]+4 ,^! v>iations 9 its int5sity
 take place v rapidly1 t is ) :atev]    
high frequ5cy ! s5d+ /ation op]ates4    
,:at happ5s to ! plate curr5t on !      
av]age8  ,! plate curr5t1 y rememb]1 is 
a /ream ( electrons f ! filam5t to !    
plate 7on ! inside ( ! tube71 & f !     
plate back ?r\gh ! ,b-batt]y to !       
filam5t 7on ! outside ( ! tube74 ,! grid
 alt]nately assi/s & opposes t /ream4   
,:5 x assi/s1 ! electrons 9 ! plate     
circuit are mov$ at a fa/] rate4 ,:5 !  
grid becomes negative & opposes ! plate 
! /ream ( electrons is at a sl[] rate4  

,! /ream is always go+ 9 ! same         
direction b x v>ies 9 its rate dep5d+ ^u
 ! *anges 9 grid pot5tial4              
,illu/ration3 ,fig #ee  ,:5 ! grid is   
positive1 t is = half a cycle ( !       
alt]nat+ grid-voltage1 ! /ream is l>g]  
?an x w\ld be if ! plate curr5t dep5d$  
only on ! ,b-batt]y4 ,= ! o!r half ( a  
cycle x is less4 ,! "q ,i am rais+ is   
?is3 ,d m electrons move ar\nd ! plate  
circuit if !re is a signal com+ 9 ?an :5
 !re is no incom+ signal8 ,to answ] ?is 
we mu/ look at ! audion *>act]i/ic ( our
 p>ticul> tube & ?is *>act]i/ic mu/ h   
be5 tak5 ) ! same ,b-batt]y z we use :5 
we try to receive ! signals4  ,!re are j
 ?ree possible answ]s to ?is "q4 ,! fir/
 answ] is3 ,no1 !re is a small] numb] ( 
electrons pass+ ?r\gh ! plate circuit   
ea* second if ! grid is be+ affect$ by  
an incom+ signal4 ,! second is3 ,!      
signal doesn't make any diff]5ce 9 !    
total numb] ( electrons :i* move ea*    
second f filam5t to plate4 ,& ! ?ird    
answ] is3 ,yes1 !re is a great] total   

numb] ea* second4  ,illu/ration3 ,fig   
#ef  ,any "o ( ! ?ree answ]s may be "r4 
,x all dep5ds on ! *>act]i/ic ( ! tube z
 we are op]at+ x1 & t dep5ds n only ^u !
 type & design ( tube b also ^u :at     
voltages we are us+ 9 our batt]ies4     
,suppose ! v>iations 9 ! voltage ( !    
grid are z repres5t$ 9 ,fig4 #ee1 & t ! 
*>act]i/ic ( ! tube is z %[n 9 ! same   
figure4 ,!n obvi\sly ! fir/ answ] is    
correct4 ,y c see = y\rself t :5 ! grid 
becomes positive ! curr5t 9 ! plate     
circuit c't increase mu* anyway4 ,= !   
o!r half ( ! cycle1 t is1 :ile ! grid is
 negative1 ! curr5t 9 ! plate is v mu*  
decreas$4 ,! decrease 9 "o half-cycle is
 l>g] ?an ! increase dur+ ! o!r         
half-cycle1 s t on ! av]age ! curr5t is 
less :5 ! signal is com+ 94 ,! dott$ l9e
 %[s ! av]age curr5t4  ,suppose t we    
take ! same tube & use a ,b-batt]y ( l[]
 voltage4 ,! *>act]i/ic w h ! same %ape 
b !re w n be z mu* curr5t unless ! grid 
helps1 s t ! *>act]i/ic w be l t ( ,fig4
 #ef4 ,?is *>act]i/ic crosses ! axis (  

z]o volts at a small] numb] ( mil-amp]es
 ?an does ! o!r because ! ,b-batt]ies   
c't pull z h>d z !y did 9 ! o!r case4   
,illu/ration3 ,fig #eg  ,y c see !      
result4 ,:5 ! grid becomes positive x   
helps & increases ! plate curr5t4 ,:5 x 
becomes negative x opposes & decreases !
 plate curr5t4 ,b ! increase j balances 
! decrease1 s t on ! av]age ! curr5t is 
un*ang$1 z %[n by ! dott$ l9e4  ,on !   
o!r h&1 if we use a /ill small] voltage 
( ,b-batt]y we get a *>act]i/ic :i* %[s 
a /ill small] curr5t :5 ! grid is at z]o
 pot5tial4 ,= ?is case1 z %[n 9 ,fig4   
#eg1 ! plate curr5t is l>g] on ! av]age 
:5 !re is an incom+ signal4  ,if we want
 to "k :e!r or n !re is any incom+      
signal we w n use ! tube 9 ! second     
condition1 t ( ,fig4 #ef1 because x     
won't tell u any?+4 ,on ! o!r h& :y use 
! tube "u ! fir/ conditions :]e we ne$ a
 l>ge plate batt]y8 ,if we c get ! same 
result1 t is an indication :5 ! o!r     
/ation is signall+1 by us+ a small      
batt]y let's d x t way = batt]ies co/   

money4 ,= t reason we %all conf9e       
ourselves to ! /udy ( :at takes place "u
 ! conditions ( ,fig4 #eg4  ,we n[ "k t 
:5 a signal is be+ s5t by ! di/ant      
/ation ! curr5t 9 ! plate circuit ( our 
audion at ! receiv+ /ation is great]1 on
 ! av]age4 ,we are ready to see :at     
effect ?is has on ! telephone receiv]4  
,& to d ?is requires a little /udy ( h[ 
! telephone receiv] works & :y4         
,illu/ration3 ,fig #eh  ,i %all n /op n[
 to tell y mu* ab\t ! telephone receiv] 
= x des]ves a :ole lett] all to itself4 
,y "k t a magnet attracts iron4 ,suppose
 y w9d a coil ( insulat$ wire ar\nd a b>
 magnet or put ! magnet inside su* a    
coil z 9 ,fig4 #eh4 ,s5d a /ream (      
electrons ?r\gh ! turns ( ! coil--a     
/eady /ream su* z comes f ! batt]y %[n 9
 ! figure4 ,! /r5g? ( ! magnet is alt]$4
 ,= "o direction ( ! electron /ream     
?r\gh ! coil ! magnet is /rong]4 ,= !   
opposite direction ( curr5t ! magnet w  
be weak]4  ,illu/ration3 ,fig #ei  ,fig4
 #ei %[s a simple design ( telephone    

receiv]4 ,x is =m$ by a b> magnet1 a    
coil ab\t x ?r\gh :i* a curr5t c fl[1 & 
a ?9 disc ( iron4 ,! iron disc1 or      
diaphragm1 is held at its edges s t x _c
 move z a :ole t[>d ! magnet4 ,! c5t] c 
move1 h[ev]1 & s ! diaphragm is b[$ out 
9 ! =m %[n 9 ! small] sket*4  ,n[       
connect a batt]y to ! receiv] w9d+ &    
all[ a /eady /ream ( electrons to fl[4  
,! magnet w be ei!r /r5g!n$ or weak5$4  
,suppose ! /ream ( electrons is 9 !     
direction to make x /rong]--,i'll give y
 ! rule lat]4 ,!n ! diaphragm is b[$ out
 /ill m4 ,if we op5 ! batt]y circuit & s
 /op ! /ream ( electrons ! diaphragm w  
fly back to its orig9al position1 = x is
 ela/ic4 ,! effect is v mu* t ( pu%+ 9 !
 bottom ( a t9 pan & lett+ x fly back :5
 y remove y\r h&4  ,next rev]se !       
batt]y4 ,! magnet does n pull z h>d z x 
w\ld if !re 7 no curr5t4 ,! diaphragm is
 !re=e n b[$ out s mu*4  ,suppose t     
in/ead ( rev]s+ ! curr5t by rev]s+ !    
batt]y we arrange to s5d an alt]nat+    
curr5t ?r\gh ! coil4 ,t w h ! same      

effect4 ,= "o direction ( curr5t fl[1 ! 
diaphragm is attract$ /ill m by ! magnet
 b = ! o!r direction x is n attract$ z  
mu*4 ,! result is t ! c5t] ( ! diaphragm
 moves back & =? dur+ "o complete cycle 
( ! alt]nat+ curr5t 9 ! coil4  ,!       
diaphragm vibrates back & =? 9 tune ) ! 
alt]nat+ curr5t 9 ! receiv] w9d+4 ,z x  
moves away f ! magnet x pu%es ahead ( x 
! neighbor+ molecules ( air4 ,^!        
molecules !n cr[d & pu% ! molecules (   
air :i* are j a little fur!r away f !   
diaphragm4 ,^! 9 turn pu% aga9/ ^?      
beyond !m & s a pu% or %ove is s5t out  
by ! diaphragm f molecule to molecule   
until p]haps x rea*es y\r e>4 ,:5 !     
molecules ( air next y\r e> receive !   
pu% !y 9 turn pu% aga9/ y\r e>drum4  ,9 
! meantime :at has happ5$8 ,! curr5t 9 !
 telephone receiv] has rev]s$ its       
direction4 ,! diaphragm is n[ pull$ t[>d
 ! magnet & ! adjac5t molecules ( air h 
ev5 m room ?an !y _h be=e4 ,s !y /op    
cr[d+ ea* o!r & foll[ ! diaphragm 9 !   
o!r direction4 ,! molecules ( air j     

beyond ^!1 on ! way t[>d y\r e>1 ne$    
cr[d no long] & !y also move back4 ,(   
c\rse1 !y g ev5 f>!r ?an _! old         
positions = !re is n[ m room on ! o!r   
side4 ,t same ?+ happ5s all along ! l9e 
until ! air molecules next y\r e> />t   
back & give y\r e>drum a *ance to exp&  
outw>d4 ,z !y move away !y make a little
 vacuum !re & ! e>drum puffs out4  ,t   
goes on ov] & ov] aga9 j z (t5 z !      
alt]nat+ curr5t passes ?r\gh "o cycle ( 
values4 ,& y1 unless y are ?9k+         
p>ticul>ly ( ! sci5tific explanations1  
say t y he> a musical note4 ,z a matt] (
 fact if we increase ! frequ5cy ( !     
alt]nat+ curr5t y w say t ! pit* ( !    
note has be5 increas$ or t y he> a note 
high] 9 ! musical scale4  ,if we />t$ ) 
a v l[-frequ5cy alt]nat+ curr5t1 say "o 
( fifte5 or tw5ty cycles p] second1 y   
w\ldn't say y he>d a note at all4 ,y    
w\ld he> a sort ( a rumble4 ,if we %\ld 
gradually increase ! frequ5cy ( !       
alt]nat+ curr5t y w\ld f9d t ab\t sixty 
or p]haps a hundr$ cycles a second w\ld 

give y ! impression ( a musical note4 ,z
 ! frequ5cy is made /ill l>g] y h m]ely 
! impression ( a high]-pit*$ note until 
we get up into ! ?\s&s ( cycles a       
second4 ,!n1 p]haps ab\t tw5ty-?\s&     
cycles a second1 y f9d y he> only a     
little s\nd l w9d or l /eam escap+ sl[ly
 f a jet or ?r\gh a leak4 ,a few ?\s&   
cycles m ea* second & y don't he> any?+ 
at all4  ,y "k t = radio-transmitt+     
/ations we use audion oscillators :i*   
are produc+ alt]nat+ curr5ts )          
frequ5cies ( sev]al hundr$-?\s& cycles  
p] second4 ,x c]ta9ly w\ldn't d any good
 to connect a telephone receiv] 9 !     
ant5na circuit at ! receiv+ /ation z 9  
,fig4 #fj4 ,we c\ldn't he> s high pit*$ 
a note4  ,illu/ration3 ,fig #fj  ,ev5 if
 we c\ld1 !re are sev]al reasons :y !   
telephone receiv] w\ldn't "w at su* high
 frequ5cies4 ,! fir/ is t ! diaphragm   
c't be mov$ s fa/4 ,x has "s in]tia1 y  
"k1 t is1 "s unwill+ness to get />t$4   
,if y try to />t x 9 "o direction &1    
be=e y really get x go+1 *ange y\r m9d &

 try to make x g 9 ! o!r direction1 x   
simply isn't go+ to g at all4 ,s ev5 if 
!re is an alt]nat+ curr5t 9 ! coil ar\nd
 ! magnet !re w n be any correspond+    
vibration ( ! diaphragm if ! frequ5cy is
 v high1 c]ta9ly n if x is above ab\t   
#bj1jjj cycles a second4  ,! o!r reason 
is t !re w only be a v feeble curr5t 9 !
 coil anyway1 no matt] :at y d1 if !    
frequ5cy is high4 ,y rememb] t !        
electrons 9 a coil are sort ( b&$ toge!r
 & ea* has an effect on all ! o!rs :i* c
 move 9 p>allel pa?s4 ,! result is t !y 
h a great unwill+ness to get />t$ & an  
equal unwill+ness to /op4 ,_!           
unwill+ness is mu* m ?an if ! wire 0    
long & /raight4 ,x is also made v mu*   
great] by ! pres5ce ( ! iron core4 ,an  
alt]nat+ e4 m4 f4 ( high frequ5cy h>dly 
gets ! electrons />t$ at all be=e x's "t
 to get !m go+ 9 ! opposite direction4  
,!re is v little movem5t to ! electrons 
& h5ce only a v small curr5t 9 ! coil if
 ! frequ5cy is high4  ,if y want a rule 
= x y c rememb] t ! high] ! frequ5cy (  

an alt]nat+ e4 m4 f4 ! small] ! electron
 /ream :i* x c set oscillat+ 9 a giv5   
coil4 ,( c\rse1 we might make ! e4 m4 f4
 /rong]1 t is pull & %ove ! electrons   
h>d]1 b unless ! coil has a v small     
inductance or unless ! frequ5cy is v l[ 
we %\ld h to use an e4 m4 f4 ( enorm\s  
/r5g? to get any appreciable curr5t4    
,cond5s]s are j ! o!r way 9 _! action4  
,if !re is a cond5s] 9 a circuit1 :]e an
 alt]nat+ e4 m4 f4 is active1 !re is    
lots ( tr\ble if ! frequ5cy is l[4 ,if1 
h[ev]1 ! frequ5cy is high ! same-siz$   
curr5t c be ma9ta9$ by a small] e4 m4 f4
 ?an if ! frequ5cy is l[4 ,y see1 :5 !  
frequ5cy is high ! electrons h>dly get  
into ! wait+-room ( ! cond5s] be=e x is 
"t = !m to turn ar\nd & g t[>d ! o!r    
room4 ,unless !re is a l>ge curr5t1 !re 
are n en\gh electrons cr[d$ toge!r 9 !  
wait+-room to pu% back v h>d on ! next  
"o to be s5t along by ! e4 m4 f4        
,because ! electrons d n pu% back v h>d 
a small e4 m4 f4 c drive !m back & =?4  
,ord9>ily we say t a cond5s] imp$es an  

alt]nat+ curr5t less & less ! high] is !
 frequ5cy ( ! curr5t4 ,& z to           
inductances1 we say t an inductance     
imp$es an alt]nat+ curr5t m & m ! high] 
is ! frequ5cy4  ,n[ we are ready to /udy
 ! receiv+ circuit ( ,fig4 #ed4 ,i %[$ y
 9 ,fig4 #eg h[ ! curr5t ?r\gh1 ! tube w
 v>y z "t goes on4 ,x increases &       
decreases ) ! frequ5cy ( ! curr5t 9 !   
ant5na ( ! di/ant transmitt+ /ation4 ,we
 h a picture1 or graph1 z we say1 ( h[  
?is plate curr5t v>ies4 ,x w be necess>y
 to /udy t c>efully & to resolve x into 
its compon5ts1 t is to sep>ate x into   
p>ts1 :i*1 add$ toge!r aga9 w give !    
:ole4 ,to %[ y :at ,i mean ,i am go+ to 
treat fir/ a v simple case involv+      
money4  ,suppose a boy 0 />t$ by 8 fa!r 
) #ej c5ts ( sp5d+ money4 ,he sp5ds t & 
runs #ej c5ts 9 debt4 ,! next day 8 fa!r
 gives him a doll>4 ,half ( ?is he has  
to sp5d to pay up 8 ye/]day's           
indebt$ness4 ,?is he does at once & t   
leaves him #ej c5ts ahead4 ,b aga9 he   
buys some?+ = a doll> & s runs #ej c5ts 

9 debt4 ,day aft] day ?is cycle is      
repeat$4 ,we c %[ :at happ5s by ! curve 
( ,fig4 #fa;a4  ,illu/ration3 ,fig #fa;a
  ,on ! o!r h&1 suppose he already _h   
#fj c5ts :i*1 he 0 sav+ = "s special    
purpose4 ,?is he doesn't t\*1 pref]r+ to
 run into debt ea* day & to pay up !    
next1 z %[n 9 ,fig4 #fa;a4 ,!n we w\ld  
repres5t ! /ory ( ?is #fj c5ts by !     
graph ( ,fig4 #fa;b4  ,illu/ration3 ,fig
 #fa;b  ,n[ suppose t in/ead ( go+ 9    
debt ea* day he uses "p ( ?is #fj c5ts4 
,ea* day aft] ! fir/ 8 fa!r gives him a 
doll>1 j z be=e4 ,he />ts !n ) #fj c5ts 
z %[n 9 ,fig4 #fa;c1 increases 9 weal?  
to #a4aj1 !n sp5ds #a4jj1 br++ 8 funds  
d[n to #aj c5ts4 ,!n he receives #a4jj f
 8 fa!r & ! process is repeat$          
cyclically4  ,illu/ration3 ,fig #fa;c   
,if y saw ! graph ( ,fig4 #fa;c y w\ld  
be able to say t1 :atev] he actually    
did1 ! effect 0 ! same z if he _h two   
pockets1 9 "o ( :i* he kept #fj c5ts all
 ! "t z %[n 9 ,fig4 #fa;b4 ,9 8 o!r     
pocket he ei!r _h money or he 0 9 debt z

 %[n 9 ,fig4 #fa;a4 ,if y did t y w\ld  
be resolv+ ! money *anges ( ,fig4 #fa;c 
into ! two compon5ts ( ,figs4 #fa;a & b4
  ,t is :at ,i want y to d ) ! curve (  
,fig4 #eg :i* ,i am reproduc+ "h1 r$rawn
 z ,fig4 #fb;a4 ,y see x is really !    
result ( add+ toge!r ! two curves (     
,figs4 #fb;b & c1 :i* are %[n on !      
foll[+ page4  ,illu/ration3 ,fig #fb;a  
,we c ?9k1 !re=e1 ( ! curr5t 9 ! plate  
circuit z if x 7 two curr5ts add$       
toge!r1 t is1 two electron /reams pass+ 
?r\gh ! same wire4 ,"o /ream is /eady & 
! o!r alt]nates4  ,illu/ration3 ,fig    
#fb;b  ,n[ look aga9 at ! diagram ( our 
receiv+ set :i* ,i am reproduc+ z ,fig4 
#fc4 ,:5 ! signal is incom+ !re fl[ 9 ! 
plate circuit two /reams ( electrons1 "o
 /eady & ( a value 9 mil-amp]es         
correspond+ to t ( ! graph 9 ,fig4      
#fb;b1 & ! o!r alt]nat+ z %[n 9 ,fig4   
#fb;c4  ,! /eady /ream ( electrons w h  
no m difficulty 9 gett+ ?r\gh ! coil$   
wire ( ! receiv] ?an x w\ld ?r\gh ! same
 am\nt ( /raight wire4 ,on ! o!r h& x _c

 pass ! gap ( ! cond5s]4  ,!            
alt]nat+-curr5t compon5t c't get along 9
 ! coil because its frequ5cy is s high t
 ! coil imp$es ! motion ( ! electrons s 
mu* z practically to /op !m4 ,on ! o!r  
h& ^! electrons c easily run into !     
wait+-room (f]$ by ! cond5s] & !n run   
out aga9 z soon z x is "t4              
,illu/ration3 ,fig #fb;c  ,:5 ! curr5t 9
 ! plate circuit is l>ge all ! electrons
 :i* ar5't ne$$ = ! /eady /ream ?r\gh ! 
telephone receiv] run into "o plate ( ! 
cond5s]4 ,( c\rse1 at t same in/ant an  
equal numb] leave ! o!r plate & />t (f  
t[>d ! ,b-batt]y & ! filam5t4 ,an in/ant
 lat]1 :5 ! curr5t 9 ! plate circuit is 
small1 electrons />t to come out ( !    
plate & to jo9 ! /ream ?r\gh ! receiv] s
 t ?is /ream is kept /eady4             
,illu/ration3 ,fig #fc  ,?is /eady /ream
 ( electrons1 :i* is pass+ ?r\gh !      
receiv] w9d+1 is l>g] ?an x w\ld be if  
!re 0 no incom+ radio signal4 ,! result 
is a /rong] pull on ! diaphragm ( !     
receiv]4 ,! mom5t ! signal />ts ?is     

diaphragm is pull$ ov] t[>d ! magnet & x
 /ays pull$ ov] z long z ! signal la/s4 
,:5 ! signal ceases x flies back4 ,we   
w\ld he> !n a click :5 ! signal />t$ &  
ano!r :5 x /opp$4  ,if we want$ to      
di/+ui% dots f da%es ?is w\ldn't be at  
all satisfactory4 ,s 9 ! next lett]     
,i'll %[ y :at sort ( *anges we c make 9
 ! app>atus4 ,to und]/& :at effect ^!   
*anges w h y ne$1 h[ev]1 to und]/&      
pretty well mo/ ( ?is lett]4     ,,lett]
 #ae  ,,radio-,,telephony   ,,de> ,,lad3
  ,be=e we />t on ! important subject   
matt] ( ?is lett] let u make a %ort     
review ( ! prec$+ two lett]s4  ,an      
oscillat+ audion at ! transmitt+ /ation 
produces an effect on ! plate curr5t ( !
 detector audion at ! receiv+ /ation4   
,!re is impress$ ^u ! grid ( ! detector 
an alt]nat+ e4 m4 f4 :i* has ! same     
frequ5cy z ! alt]nat+ curr5t :i* is be+ 
produc$ at ! s5d+ /ation4 ,:ile ?is e4  
m4 f4 is active1 & ( c\rse x is active  
only :ile ! s5d+ key is held d[n1 !re is
 m curr5t ?r\gh ! w9d+ ( ! telephone    

receiv] & its diaphragm is consequ5tly  
pull$ clos] to its magnet4  ,:at w happ5
 if ! e4 m4 f4 :i* is active on ! grid (
 ! detector is made /rong] or weak]8 ,! 
pull on ! receiv] diaphragm w be /rong] 
or weak] & ! diaphragm w h to move      
accord+ly4 ,if ! pull is weak] !        
ela/icity ( ! iron w move ! diaphragm   
away f ! magnet1 b if ! pull is /rong] !
 diaphragm w be mov$ t[>d ! magnet4  ,e 
"t ! diaphragm moves x affects ! air 9 !
 imm$iate neighborhood ( itself & t air 
9 turn affects ! air f>!r away & s ! e> 
( ! li/5]4 ,!re=e if !re are *anges 9 ! 
int5sity or /r5g? ( ! incom+ signal !re 
are go+ to be correspond+ motions ( !   
receiv] diaphragm4 ,& some?+ to li/51   
too1 if ^! *anges are frequ5t en\gh b n 
s frequ5t t ! receiv] diaphragm has     
difficulty 9 foll[+ !m4  ,!re are _m    
ways ( affect+ ! /r5g? ( ! incom+       
signal4 ,suppose1 = example1 t we       
arrange to decrease ! curr5t 9 ! ant5na 
( ! transmitt+ /ation4 ,t w mean a weak]
 signal & a small] increase 9 curr5t    

?r\gh ! w9d+ ( ! telephone receiv] at ! 
o!r /ation4 ,on ! o!r h& if ! signal    
/r5g? is increas$ !re is m curr5t 9 ?is 
w9d+4  ,illu/ration3 ,fig #fd  ,suppose 
we connect a f9e wire 9 ! ant5na circuit
 z 9 ,fig4 #fd & h a slid+ contact z    
%[n4 ,suppose t :5 we depress ! swit* 9 
! oscillator circuit & s />t !          
oscillations t ! slid+ contact is at o z
 %[n4 ,correspond+ to t /r5g? ( signal  
!re is a c]ta9 value ( curr5t ?r\gh !   
receiv] w9d+ at ! o!r /ation4 ,n[ let u 
move ! slid]1 fir/ to a & !n back to b &
 s on1 back & =?4 ,y see :at w happ54   
,we alt]nately make ! curr5t 9 ! ant5na 
l>g] & small] ?an x orig9ally 04 ,:5 !  
slid] is at b !re is m ( ! f9e wire 9   
s]ies ) ! ant5na1 h5ce m resi/ance to ! 
oscillations ( ! electrons1 & h5ce a    
small] oscillat+ /ream ( electrons4 ,t  
means a weak] outgo+ signal4 ,:5 ! slid]
 is at a !re is less resi/ance 9 !      
ant5na circuit & a l>g] alt]nat+ curr5t4
  ,illu/ration3 ,fig #fe  ,illu/ration3 
,fig #ff  ,a picture ( :at happ5s w\ld  

be l t ( ,fig4 #fe4 ,! signal v>ies 9   
int5sity1 !re=e1 becom+ l>g] & small]   
alt]nately4 ,t means ! voltage impress$ 
on ! grid ( ! detector is alt]nately    
l>g] & small]4 ,& h5ce ! /ream (        
electrons ?r\gh ! w9d+ ( ! telephone    
receiv] is alt]nately l>g] & small]4 ,& 
t means t ! diaphragm moves back & =? 9 
j ! "t x takes to move ! slid] back &   
=?4  ,in/ead ( ! slid] we might use a   
little cup almo/ full ( gra9s ( c>bon4  
,! c>bon gra9s lie betwe5 two flat discs
 ( c>bon4 ,"o ( ^! discs is held fix$4  
,! o!r is connect$ to ! c5t] ( a ?9     
diaphragm ( /eel & moves back & =? z ?is
 diaphragm is mov$4 ,! :ole ?+ makes a  
telephone transmitt] su* z y h (t5 talk$
 to4  ,illu/ration3 ,fig #fg;a  ,wires  
connect to ! c>bon discs z %[n 9 ,fig4  
#ff4 ,a /ream ( electrons c fl[ ?r\gh ! 
wires & f gra9 to gra9 ?r\gh ! c>bon    
button1 z we call x4 ,! electrons h less
 difficulty if ! gra9s are compress$1 t 
is ! button !n (f]s less resi/ance to ! 
fl[ ( curr5t4 ,if ! diaphragm moves     

back1 all[+ ! gra9s to h m room1 !      
electron /ream is small] & we say !     
button is (f]+ m resi/ance to ! curr5t4 
,illu/ration3 ,fig #fg;b  ,y c see :at  
happ5s4 ,suppose "s "o talks into !     
transmitt] & makes its diaphragm g back 
& =? z %[n 9 ,fig4 #fg;a4 ,!n ! curr5t 9
 ! ant5na v>ies1 be+ great] or less1    
dep5d+ ^u :e!r ! button (f]s less or m  
resi/ance4 ,! correspond+ v>iations 9 ! 
ant5na curr5t are %[n 9 ,fig4 #fg;b4  ,9
 ! ant5na at ! receiv+ /ation !re are   
correspond+ v>iations 9 ! /r5g? ( !     
signal & h5ce correspond+ v>iations 9 ! 
/r5g? ( ! curr5t ?r\gh ! telephone      
receiv]4 ,i %all %[ graphically :at     
happ5s 9 ,fig4 #fh4 ,y see t ! telephone
 receiv] diaphragm does j ! same motions
 z does ! transmitt] diaphragm4 ,t means
 t ! molecules ( air ne> ! receiv]      
diaphragm are go+ ?r\gh j ! same k9d (  
motions z are ^? ne> ! transmitt]       
diaphragm4 ,:5 ^! air molecules affect  
y\r e>drum y he> j :at y w\ld h he>d if 
y _h be5 "r !re beside ! transmitt]4    

,t's "o way ( mak+ a radio-telephone4 ,x
 is n a v effici5t me?od b x has be5 us$
 9 ! pa/4 ,be=e we look at any ( ! m    
rec5t me?ods we c draw "s g5]al ideas f 
?is me?od & le>n "s words t are us$     
almo/ always 9 speak+ (                 
radio-telephones4  ,9 any sy/em (       
radio-telephony y w always f9d t !re is 
produc$ at ! transmitt+ /ation a        
high-frequ5cy alt]nat+ curr5t & t ?is   
curr5t fl[s 9 a tun$ circuit "o "p ( :i*
 is ! cond5s] =m$ by ! ant5na & ! gr\nd 
7or some?+ :i* acts l a gr\nd74 ,?is    
high-frequ5cy curr5t1 or radio-curr5t1 z
 we usually say1 is v>i$ 9 its /r5g?4 ,x
 is v>i$ 9 con=mity ) ! human voice4 ,if
 ! human voice speak+ into ! transmitt] 
is l[ pit*$ !re are sl[ v>iations 9 !   
int5sity ( ! radio curr5t4 ,if ! voice  
is high pit*$ !re are m rapid v>iations 
9 ! /r5g? ( ! radio-frequ5cy curr5t4 ,t 
is :y we say ! radio-curr5t is modulat$ 
by ! human voice4  ,illu/ration3 ,fig   
#fh  ,! signal :i* radiates out f !     
transmitt+ ant5na c>ries all ! little   

v>iations 9 pit* & l\dness ( ! human    
voice4 ,:5 ?is signal rea*es ! di/ant   
ant5na x e/abli%es 9 t ant5na circuit a 
curr5t ( high frequ5cy :i* has j ! same 
v>iations z did ! curr5t 9 ! ant5na at !
 s5d+ /ation4 ,! human voice isn't !re4 
,x is n transmitt$4 ,x did its "w at !  
s5d+ /ation by modulat+ ! radio-signal1 
modulat+ ! c>ri] curr5t1 z we sometimes 
say4 ,b !re is spee* significance hidd5 
9 ! v>iations 9 /r5g? ( ! receiv$       
signal4  ,if a telephone-receiv]        
diaphragm c be made to vibrate 9        
accordance ) ! v>iations 9 signal       
int5sity !n ! air adjac5t to t diaphragm
 w be set into vibration & ^! vibrations
 w be j l ^? :i* ! human voice set up 9 
! air molecules ne> ! m\? ( ! speak]4   
,all ! diff]5t sy/ems ( receiv+         
radio-telephone signals are m]ely       
diff]5t me?ods ( gett+ a curr5t :i* w   
affect ! telephone receiv] 9 con=mity ) 
! v>iations 9 signal /r5g?4 ,gett+ su* a
 curr5t is call$ detect+4 ,!re are _m   
diff]5t k9ds ( detectors b ! vacuum tube

 is mu* to be pref]r$4  ,! *eape/       
detector1 b n ! mo/ s5sitive1 is !      
cry/al4 ,if y und]/& h[ ! audion works z
 a detector y w h no difficulty 9       
und]/&+ ! cry/al detector4  ,! cry/al   
detector consi/s ( "s m9]al cry/al & a  
f9e-wire po9t1 usually plat9um4 ,cry/als
 are peculi> ?+s4 ,l ev]y?+ else !y are 
made ( molecules & ^! molecules ( atoms4
 ,! atoms are made ( electrons gr\p$    
ar\nd nuclei :i*1 9 turn1 are =m$ by    
close gr\p+s ( protons & electrons4 ,!  
great diff]5ce betwe5 cry/als &         
sub/ances :i* are n cry/all9e1 t is1    
sub/ances :i* don't h a special natural 
%ape1 is ?is3 ,9 cry/als ! molecules &  
atoms are all arrang$ 9 "s ord]ly mann]4
 ,9 o!r sub/ances1 sub/ances )\t special
 =m1 amorph\s sub/ances1 z we call !m1 !
 molecules are j gr\p$ toge!r 9 a       
haphaz>d way4  ,illu/ration3 ,fig #fi   
,= "s cry/als we "k v closely inde$ h[  
_! molecules or r _! individual atoms   
are arrang$4 ,sometime y may wi% to read
 h[ ?is 0 f\nd out by ! use ( ,x-rays4#f

 ,take ! cry/al ( common salt = example4
 ,t is well kn[n4 ,ea* molecule ( salt  
is =m$ by an atom ( sodium & "o (       
*lor9e4 ,9 a cry/al ( salt ! molecules  
are gr\p$ toge!r s t a sodium atom      
always has *lor9e atoms on e side ( x1 &
 ! o!r way ar\nd1 ( c\rse4  ,suppose y  
took a lot ( wood dumb-bells & pa9t$ "o 
( ! balls ( ea* dumb-bell black to /& = 
a sodium atom1 leav+ ! o!r unpa9t$ to /&
 = a *lor9e atom4 ,n[ try to pile !m up 
s t above & bel[ ea* black ball1 to ! "r
 & left ( x1 & also 9 front & beh9d x1  
!re %all be a :ite ball4 ,! pile :i* y  
w\ld probably get w\ld look l t ( ,fig4 
#fi4 ,i h omitt$ ! gripp+ "p ( ea*      
dumbell because ,i don't believe x is   
!re4 ,9 my picture ea* circle repres5ts 
! nucleus ( an atom4 ,i hav5't attempt$ 
to %[ ! planet>y electrons4 ,o!r cry/als
 h m complex arrangem5ts = pil+ up _!   
molecules4  ,n[ suppose we put two      
diff]5t k9ds ( sub/ances close toge!r1 t
 is1 make contact betwe5 !m4 ,h[ _!     
electrons w behave w dep5d entirely ^u  

:at ! atoms are & h[ !y are pil$ up4 ,"s
 v curi\s effects c be obta9$4          
,illu/ration3 ,fig #gj  ,! "o :i*       
int]e/s u at pres5t is t across !       
contact po9ts ( "s comb9ations (        
sub/ances x is easi] to get a /ream (   
electrons to fl[ "o way ?an ! o!r4 ,!   
contact doesn't h ! same resi/ance 9 !  
two directions4 ,usually also !         
resi/ance dep5ds ^u :at voltage we are  
apply+ to =ce ! electron /ream across ! 
po9t ( contact4  ,! "o way to f9d out is
 to take ! voltage-curr5t *>act]i/ic ( !
 comb9ation4 ,to d s we use ! same g5]al
 me?od z we did = ! audion4 ,& :5 we get
 ?r\gh we plot ano!r curve & call x1 =  
example1 a plat9um-gal5a *>act]i/ic4    
,fig4 #gj %[s ! set-up = mak+ !         
measurem5ts4 ,!re is a gr\p ( batt]ies  
arrang$ s t we c v>y ! e4 m4 f4 appli$  
across ! contact po9t ( ! cry/al &      
plat9um4 ,a voltmet] %[s ! value ( ?is  
e4 m4 f4 & an ammet] tells ! /r5g? ( !  
electron /ream4 ,ea* "t we move ! slid] 
we get a new pair ( values = volts &    

amp]es4 ,z a matt] ( fact we don't get  
amp]es or ev5 mil-amp]es2 we get        
million?s ( an amp]e or microamp]es1 z  
we say4 ,we c plot ! pairs ( values :i* 
we measure & make a curve l t ( ,fig4   
#ga4  ,illu/ration3 ,fig #ga  ,:5 !     
voltage across ! contact is rev]s$1 (   
c\rse1 ! curr5t rev]ses4 ,"p ( ! curve  
looks some?+ l ! l[] "p ( an audion     
*>act]i/ic4  ,illu/ration3 ,fig #gb  ,n[
 connect ?is cry/al 9 a receiv+ circuit 
z 9 ,fig4 #gb4 ,we use an ant5na j z we 
did = ! audion & we tune ! ant5na       
circuit to ! frequ5cy ( ! incom+ signal4
 ,! receiv+ circuit is c\pl$ to ! ant5na
 circuit & is tun$ to ! same frequ5cy4  
,:atev] voltage !re may be across !     
cond5s] ( ?is circuit is appli$ to !    
cry/al detector4 ,we hav5't put !       
telephone receiv] 9 ! circuit yet4 ,i   
want to wait until y h se5 :at ! cry/al 
does :5 an alt]nat+ voltage is appli$ to
 x4  ,illu/ration3 ,fig #gc  ,we c draw 
a famili> =m ( sket* z 9 ,fig4 #gc to %[
 h[ ! curr5t 9 ! cry/al v>ies4 ,y see t 

!re fl[s ?r\gh ! cry/al a curr5t v mu* l
 t ( ,fig4 #fb;a4 ,& y "k t su* a curr5t
 is really equival5t to two electron    
/reams1 "o /eady & ! o!r alt]nat+4 ,!   
cry/al detector gives u mu* ! same sort 
( a curr5t z does ! vacuum tube detector
 ( ,fig4 #ed4 ,! curr5t isn't any:]e ne>
 z l>ge1 h[ev]1 = x is microamp]es      
in/ead ( mil-amp]es4  ,our cry/al       
detector produces ! same results s f> z 
giv+ u a /eady compon5t ( curr5t to s5d 
?r\gh a telephone receiv]4 ,s we c      
connect a receiv] 9 s]ies ) ! cry/al z  
%[n 9 ,fig4 #gd4 ,because ! receiv] w\ld
 (f] a l>ge imp$ance to ! high-frequ5cy 
curr5t1 t is1 s]i\sly imp$e & s r$uce ! 
high-frequ5cy curr5t1 we connect a      
cond5s] ar\nd ! receiv]4  ,illu/ration3 
,fig #gd  ,!re is a simple cry/al       
detector circuit4 ,if ! signal int5sity 
v>ies !n ! curr5t :i* passes ?r\gh !    
receiv] w v>y4 ,if ^! v>iations are     
caus$ by a human voice at ! s5d+ /ation 
! cry/al w p]mit "o to he> f ! telephone
 receiv] :at ! speak] is say+4 ,t is j  

:at ! audion detector does v _m times   
bett]4  ,9 ! lett] on h[ to exp]im5t    
y'll f9d details z to ! con/ruction ( a 
cry/al-detector set4 ,excell5t          
in/ructions = an inexp5sive set are     
conta9$ 9 ,bull4 ,no4 #abj ( ! ,bureau (
 ,/&>ds4 ,a copy c be obta9$ by s5d+ t5 
c5ts to ! ,commission] ( ,public        
,docum5ts1 ,wa%+ton1 ,d4 ,c4  ,footnote 
#f3 ,cf4 ,)9 ! ,atom1 ,*apt] ,x4        
,,lett] #af  ,,! ,,human ,,voice   ,,de>
 ,,sir3  ,! radio-telephone does n      
transmit ! human voice4 ,x reproduces   
ne> ! e>s ( ! li/5] simil> motions ( !  
air molecules & h5ce causes 9 ! e>s ( ! 
li/5] ! same s5sations ( s\nd z if he 7 
li/5+ directly to ! speak]4 ,?is        
reproduction takes place almo/          
in/antane\sly s great is ! spe$ ) :i* ! 
electrical effects travel outw>d f !    
s5d+ ant5na4 ,if y wi% to und]/&        
radio-telephony y mu/ "k some?+ ( !     
me*anism by :i* ! voice is produc$ &    
some?+ ( ! peculi> or *>act]i/ic        
prop]ties ( voice s\nds4  ,illu/ration3 

,fig #ge  ,! human voice is produc$ by a
 sort ( organ pipe4 ,imag9e a long pipe 
connect$ at "o end to a pair (          
fire-bell[s1 & clos$ at ! o!r end by two
 /ret*$ %eets ( rubb]4 ,fig4 #ge is a   
sket* ( :at ,i mean4 ,correspond+ to !  
bell[s !re is ! human diaphragm1 !      
muscul> membrane sep>at+ ! ?orax &      
abdom51 :i* exp&s or contracts z "o     
brea!s4 ,correspond+ to ! pipe is !     
w9dpipe4 ,correspond+ to ! two /ret*$   
pieces ( rubb] are ! vocal cords1 ,l &  
,r1 %[n 9 cross section 9 ,fig4 #gg4 ,!y
 are "p ( ! l>ynx & d n %[ 9 ,fig4 #gf  
7,pl4 viii7 :i* %[s ! w9d pipe & an     
outside view ( ! l>ynx4  ,illu/ration3  
,fig #gg  ,:5 ! sides ( ! bell[s are    
squeez$ toge!r ! air molecules )9 are   
cr[d$ clos] toge!r & ! air is compress$4
 ,! great] ! compression ! great]1 (    
c\rse1 is ! pressure ) :i* ! enclos$ air
 seeks to escape4 ,t x c d only by lift+
 up1 t is by bl[+ out1 ! two ela/ic     
/rips :i* close ! end ( ! pipe4  ,! air 
pressure1 !re=e1 rises until x is       

suffici5t to pu% aside ! ela/ic         
membranes or vocal cords & ?us to p]mit 
"s ( ! air to escape4 ,x doesn't =ce !  
membranes f> ap>t1 j en\gh to let "s air
 out4 ,b ! mom5t "s air has escap$ !re  
isn't s mu* inside & ! pressure is r$uc$
 j z 9 ! case ( an automobile tire f :i*
 y let ! air escape4 ,:at is ! result8  
,! membranes fly back aga9 & close !    
op5+ ( ! pipe4 ,:at got out1 !n1 0 j a  
little puff ( air4  ,! bell[s are work+ 
all ! :ile1 h[ev]1 & s ! space available
 = ! rema9+ air soon aga9 becomes s     
cr[d$ ) air molecules t ! pressure is   
aga9 suffici5t to op5 ! membranes4      
,ano!r puff ( air escapes4  ,?is happ5s 
ov] & ov] aga9 :ile "o is speak+ or s++4
 ,hundr$s ( times a second ! vocal cords
 vibrate back & =?4 ,! frequ5cy ) :i* !y
 d s det]m9es ! note or pit* ( !        
speak]'s voice4  ,:at det]m9es !        
significance ( ! s\nds :i* he utt]s8    
,?is is a mo/ int]e/+ "q & "o des]v+ (  
mu* m "t ?an ,i propose to devote to x4 
,to give y en\gh ( an answ] = y\r /udy (

 radio-telephony ,i am go+ to tell y    
fir/ ab\t vibrat+ /r+s = !y are easi] to
 picture ?an membranes l ! vocal cords4 
,suppose y h a /ret*$ /r+1 a piece (    
rubb] b& or a wire w d4 ,y pluck x1 t is
 pull x to "o side4 ,:5 y let g x flies 
back4 ,because x has in]tia#g x doesn't 
/op :5 x gets to its old position b goes
 on ?r\gh until x b[s out almo/ z f> on 
! o!r side4  ,illu/ration3 ,pl4         
,,vii4--,photographs ( ,vibrat+ ,/r+s4  
,x took "s "w to pluck ?is /r+1 n mu*   
p]haps2 b all ! "w :i* y did 9 de=m+ x1 
goes to ! /r+ & becomes its en]gy1 its  
ability to d "w4 ,?is "w x does 9 pu%+ !
 air molecules ahead ( x z x vibrates4  
,9 ?is way x uses up its en]gy & s      
f9ally comes aga9 to re/4 ,its          
vibrations damp out1 z we say1 t is die 
d[n4 ,ea* sw+ c>ries x a small] di/ance 
away f its orig9al position4 ,we say t !
 amplitude1 mean+ ! size1 ( its         
vibration decreases4 ,! frequ5cy does n4
 ,x takes j z long = a small-siz$       
vibration z = ! l>g]4 ,( c\rse1 = !     

vibration ( l>ge amplitude ! /r+ mu/    
move fa/] b x has to move f>!r s t ! "t 
requir$ = a vibration is n *ang$4  ,fir/
 ! /r+ cr[ds aga9/ ea* o!r ! air        
molecules :i* are 9 its way & s leads to
 cr[d+ fur!r away1 j z fa/ z ^!         
molecules c pass along ! %ove !y are    
receiv+4 ,t takes place at ! rate ( ab\t
 #aajj feet a second4 ,:5 ! /r+ sw+s    
back x pu%es away ! molecules :i* are   
beh9d x & s lets ^? t 7 be+ cr[d$ foll[ 
x4 ,y "k t !y w4 ,air molecules w always
 g :]e !re is ! lea/ cr[d+4 ,foll[+ !   
%ove1 !re=e1 !re is a *ance = !         
molecules to move back & ev5 to occupy m
 room ?an !y _h orig9ally4  ,! news (   
?is travels out f ! /r+ j z fa/ z did ! 
news ( ! cr[d+4 ,z fa/ z molecules are  
able !y move back & s make m room = _!  
neighbors :o are f>!r away2 & ^! 9 turn 
move back4  ,d y want a picture ( x8    
,imag9e a great cr[d ( p & at ! c5t] "s 
"o ) au?ority4 ,! cr[d is ! molecules ( 
air & ! "o ) au?ority is "o ( !         
molecules ( ! /r+ :i* has en]gy4 ,:atev]

 ?is molecule ( ! /r+ says is repeat$ by
 ea* memb] ( ! cr[d to 8 neighbor next  
f>!r away4 ,fir/ ! /r+ says3 ,g back &  
ea* molecule acts z soon z he gets ! ^w4
 ,& !n ! /r+ says3 ,come on & ea*       
molecule ( air obeys z soon z ! comm&   
rea*es him4 ,ov] & ov] ?is happ5s1 z _m 
times a second z ! /r+ makes complete   
vibrations4  ,illu/ration3 ,fig #gh  ,if
 we %\ld make a picture ( ! v>i\s       
positions ( "o ( ^! air molecules mu* z 
we pictur$ ,br[nie 9 ,lett] #i x w\ld   
appe> z 9 ,fig4 #gh;a :]e ! c5tral l9e  
repres5ts ! ord9>y position ( !         
molecule4  ,t's exactly ! picture also (
 ! successive positions ( an electron 9 
a circuit :i* is c>ry+ an alt]nat+      
curr5t4 ,fir/ x moves 9 "o direction    
along ! wire & !n back 9 ! opposite     
direction4 ,! electron next to x does ! 
same ?+ almo/ imm$iately = x does n take
 any:]e ne> z long = su* an effect to   
pass ?r\gh a cr[d ( electrons4 ,if we   
make ! /r+ vibrate twice z fa/1 t is1 h 
twice ! frequ5cy1 ! /ory ( an adjac5t   

p>ticle ( air w be z 9 ,fig4 #gh;b4     
,unless we tight5 ! /r+1 h[ev]1 we c't  
make x vibrate z a :ole & d x twice z   
fa/4 ,we c make x vibrate 9 two p>ts or 
ev5 9 m p>ts1 z %[n 9 ,fig4 #gi ( ,pl4  
,,vii4 ,:5 x vibrates z a :ole1 its     
frequ5cy is ! l[e/ possible1 !          
fundam5tal frequ5cy z we say4 ,:5 x     
vibrates 9 two p>ts ea* "p ( ! /r+ makes
 twice z _m vibrations ea* second4 ,s d 
! adjac5t molecules ( air & s does !    
e>drum ( a li/5]4  ,! result is t !     
li/5] he>s a note ( twice ! frequ5cy t  
he did :5 ! /r+ 0 vibrat+ z a :ole4 ,he 
says he he>s ! octave ( ! note he he>d  
fir/4 ,if ! /r+ vibrates 9 ?ree p>ts &  
gives a note ( ?ree times ! frequ5cy !  
li/5] he>s a note two octaves above !   
fundam5tal note ( :i* ! /r+ is capable4 
,x is entirely possible1 h[ev]1 = a /r+ 
to vibrate simultane\sly 9 a numb] (    
ways & s to give n only its fundam5tal  
note b sev]al o!rs at ! same "t4 ,!     
photographs#h ( ,fig4 #hj ( ,pl4 ,,vii  
illu/rate ?is possibility4  ,:at happ5s 

!n to ! molecules ( air :i* are adjac5t 
to ! vibrat+ /r+8 ,!y mu/ p]=m q complex
 vibrations = !y are call$ ^u to move   
back & =? j z if !re 7 sev]al /r+s all  
try+ to pu% !m ) diff]5t frequ5cies (   
vibration4 ,look aga9 at ! pictures1 (  
,fig4 #hj & see t ea* might j z well be 
! picture ( sev]al /r+s plac$ close     
toge!r1 ea* vibrat+ 9 a diff]5t way4    
,ea* ( ! /r+s has a diff]5t frequ5cy (  
vibration & a diff]5t maximum amplitude1
 t is1 greate/ size ( sw+ away f its    
/raight position4  ,illu/ration3 ,fig   
#ha  ,suppose in/ead ( a s+le /r+ act+  
^u ! adjac5t molecules we _h ?ree /r+s4 
,suppose ! fir/ w\ld make a ne>by       
molecule move z 9 ,fig4 #ha;,a1 ! second
 z 9 ,fig4 #ha;,b1 & ! ?ird z 9 ,fig4   
#ha;,c4 ,x is q evid5t t ! molecule c   
satisfy all ?ree if x w vibrate z 9     
,fig4 #ha;,d4  ,n[ take x ! o!r way     
ar\nd4 ,suppose we _h a picture ( !     
motion ( a molecule & t x 0 n simple l  
^? %[n 9 ,fig4 #gh b 0 complex l t (    
,fig4 #ha;,d4 ,we c\ld say t ?is complex

 motion 0 made up ( ?ree p>ts1 t is1 _h 
?ree compon5t simple motions1 ea*       
repres5t$ by "o ( ! ?ree o!r graphs (   
,fig4 #ha4 ,t means we c resolve any    
complex vibratory motion into compon5t  
motions :i* are simple4  ,x means m ?an 
t4 ,x means t ! vibrat+ /r+ :i* makes ! 
neighbor+ molecules ( air behave z %[n 9
 ,fig4 #ha;,d is really act+ l ?ree /r+s
 & is produc+ simultane\sly ?ree pure   
musical notes4  ,n[ suppose we _h two   
diff]5t /r+s1 say a piano /r+ 9 ! piano 
& a viol9 /r+ on its prop] m\nt+4       
,suppose we play$ bo? in/rum5ts & "s    
musician told u !y 7 9 tune4 ,:at w\ld  
he mean8 ,he w\ld mean t bo? /r+s       
vibrat$ ) ! same fundam5tal frequ5cy4   
,!y diff]1 h[ev]1 9 ! o!r notes :i* !y  
produce at ! same "t t !y produce _!    
fundam5tal notes4 ,t is1 !y diff] 9 !   
frequ5cies & amplitudes ( ^! o!r        
compon5t vibrations or ov]tones :i* are 
go+ on at ! same "t z _! fundam5tal     
vibrations4 ,x is ?is diff]5ce :i* lets 
u tell at once :i* in/rum5t is be+      

play$4  ,t br+s u to ! ma9 idea ab\t    
musical s\nds & ab\t human spee*4 ,!    
pit* ( any complex s\nd is ! pit* ( its 
fundam5tal or l[e/ s\nd2 b ! "* ( !     
complex s\nd dep5ds ^u all ! ov]tones or
 h>monics :i* are be+ produc$ & ^u _!   
relative frequ5cies & amplitudes4       
,illu/ration3 ,fig #hb  ,! organ pipe   
:i* ends 9 ! l>ynx produces a v complex 
s\nd4 ,i c't %[ y h[ complex b ,i'll %[ 
y 9 ,fig4 #hb ! complicat$ motion ( an  
air molecule :i* is vibrat+ z ! result (
 be+ ne> an organ pipe4 7,organ pipes   
diff]--?is is only "o case47 ,y c see t 
!re are a l>ge numb] ( pure notes (     
v>i\s int5sities1 t is1 /r5g?s1 :i* g to
 make up ! s\nd :i* a li/5] to ?is organ
 pipe w\ld he>4 ,! note f ! human pipe  
is mu* m complex4  ,:5 "o speaks !re are
 little puffs ( air escap+ f 8 l>ynx4 ,!
 vocal cords vibrate z ,i expla9$4 ,& ! 
molecules ( air ne> ! l>ynx are set into
 v complex vibrations4 ,^! transmit _!  
vibrations to o!r molecules until ^? 9 !
 m\? are rea*$4 ,9 ! m\?1 h[ev]1 some?+ 

v important happ5s4  ,did y ev] s+ or   
h[l d[n a ra9 b>rel or into a long pipe 
or hallway & he> ! s\nd8 ,x s\nds j ab\t
 ! same no matt] :o does x4 ,! reason is
 t ! long column ( air 9 ! pipe or b>rel
 is set into vibration & vibrates       
accord+ to its own ideas ( h[ fa/ to d  
x4 ,x has a natural frequ5cy ( its own4 
,if 9 y\r voice !re is a note ( j t     
frequ5cy x w respond beautifully4 ,9    
fact x resonates1 or s+s back1 :5 x he>s
 ?is note4  ,! net result is t x        
emphasizes ?is note s mu* t y don't he> 
any ( ! o!r compon5t notes ( y\r        
voice--all y he> is ! ra9 b>rel4 ,we say
 x re9=ces "o ( ! compon5t notes ( y\r  
voice & makes x l\d]4  ,t same ?+ happ5s
 9 ! m\? cavity ( a speak]4 ,! size &   
%ape ( ! column ( air 9 ! m\? c be v>i$ 
by ! tongue & lip positions & s !re are 
_m diff]5t possibilities ( resonance4   
,dep5d+ on lip & tongue1 diff]5t        
frequ5cies ( ! complex s\nd :i* comes f 
! l>ynx are re9=c$4 ,y c see t = y\rself
 f ,fig4 #hc :i* %[s ! tongue positions 

= ?ree diff]5t v[el s\nds4 ,y c see also
 f ,fig4 #hd1 :i* %[s ! m\? positions = 
! diff]5t v[els1 h[ ! size & %ape ( !   
m\? cavity is *ang$ to give diff]5t     
s\nds4 ,^! figures are 9 ,pl4 ,,viii4   
,! pit* ( ! note ne$ n *ange z e s+]    
kn[s4 ,y c try t also = y\rself by s++ !
 v[el s\nd ( ahh & !n *ang+ ! %ape ( y\r
 m\? s z to give ! s\nd ah--aw--ow--ou4 
,! pit* ( ! note w n *ange because !    
fundam5tal /ays ! same4 ,! spee*        
significance ( ! s\nd1 h[ev]1 *anges    
completely because ! m\? cavity         
resonates to diff]5t ones ( ! high]     
notes :i* come f ! l>ynx along ) !      
fundam5tal note4  ,n[ y c see :at is    
necess>y = telephonic transmission4 ,ea*
 & e compon5t note :i* ent]s into human 
spee* mu/ be transmitt$ & accurately    
reproduc$ by ! receiv]4 ,m ?an t1 all ! 
proportions mu/ be kept j ! same z 9 !  
orig9al spok5 s\nd4 ,we usually say t   
!re mu/ be reproduc$ 9 ! air at !       
receiv] exactly ! same wave =m z is     
pres5t 9 ! air at ! transmitt]4 ,if t   

isn't done ! spee* won't be natural & "o
 _c recognize voices al?\gh he may      
und]/& pretty well4 ,if !re is too mu*  
di/ortion ( ! wave =m1 t is if !        
relative int5sities ( ! compon5t notes (
 ! voice are too mu* alt]$1 !n !re may  
ev5 be a loss ( intelligibility s t !   
li/5] _c und]/& :at is be+ said4  ,:at  
p>ticul> notes are 9 ! human voice      
dep5ds p>tly on ! p]son :o is speak+4 ,y
 "k t ! fundam5tal ( a bass voice is l[]
 ?an t ( a soprano4 ,besides !          
fundam5tal1 h[ev]1 !re are a lot ( high]
 notes always pres5t4 ,?is is p>ticul>ly
 true :5 ! spok5 s\nd is a consonant1 l 
s or f or v4 ,! p>ticul> notes1 :i* are 
pres5t & are important1 dep5d ^u :at    
s\nd "o is say+4  ,usually1 h[ev]1 we   
f9d t if we c transmit & reproduce      
exactly all ! notes :i* lie betwe5 a    
frequ5cy ( ab\t #bjj cycles a second &  
"o ( ab\t #bjjj cycles a second !       
reproduc$ spee* w be q natural & v      
intelligible4 ,= s++ & = transmitt+     
in/rum5tal music x is necess>y to       

transmit & reproduce /ill high] notes4  
,:at y w h to look out =1 !re=e1 9 a    
receiv+ set is t x does n cut out "s ( !
 high notes :i* are necess>y to give !  
s\nd its naturalness4 ,y w also h to    
make sure t y\r app>atus does n di/ort1 
t is1 does n receive & reproduce "s     
notes or voice frequ5cies m effici5tly  
?an x does "s o!rs :i* are equally      
necess>y4 ,= t reason :5 y buy a        
trans=m] or a telephone receiv] x is    
well to ask = a *>act]i/ic curve ( !    
app>atus :i* w %[ h[ ! action v>ies z ! 
frequ5cy ( ! curr5t is v>i$4 ,! action  
or response %\ld1 ( c\rse1 be           
practically ! same at all ! frequ5cies  
)9 ! necess>y "p ( ! voice range4       
,footnote #g3 ,cf4 ,*ap4 ,,vi ( ,!      
,realities ( ,mod]n ,sci5ce4  ,footnote 
#h3 ,my ?anks are due to ,pr(essor ,d4  
,c4 ,mill] & to ! ,macmillan ,company = 
p]mission to reproduce ,figs4 #gi to #hc
 inclusive f t int]e/+ book1 ,! ,sci5ce 
( ,musical ,s\nds4     ,,lett] #ag      
,,grid ,,batt]ies ,,& ,,grid ,,cond5s]s 

,,= ,,detectors   ,,de> ,,son3  ,y      
rememb] ! audion *>act]i/ics :i* ,i us$ 
9 ,figs4 #ee1 #ef & #eg ( ,lett] #ad to 
%[ y h[ an incom+ signal w affect !     
curr5t 9 ! plate circuit4 ,look aga9 at 
^! figures & y w see t ^! *>act]i/ics   
all _h ! same g5]al %ape b t !y diff]$ 9
 _! positions ) ref]5ce to ! ma9 /reets 
( z]o volts on ! grid & z]o mil-amp]es 9
 ! plate circuit4 ,*ang+ ! voltage ( !  
,b-batt]y 9 ! plate circuit *ang$ !     
position ( ! *>act]i/ic4 ,we might say t
 *ang+ ! ,b-batt]y %ift$ ! curve )      
ref]5ce to ! axis ( z]o volts on ! grid4
  ,illu/ration3 ,fig #ef  ,illu/ration3 
,fig #fc  ,9 ! case ( ! ?ree *>act]i/ics
 :i* we are discuss+ ! %ift 0 made by   
*ang+ ! ,b-batt]y4 ,increas+ ,b-voltage 
%ifts *>act]i/ic to ! left4 ,x is       
possible1 h[ev]1 to produce su* a %ift  
by us+ a ,c-batt]y1 t is1 a batt]y 9 !  
grid circuit1 :i* makes ! grid p]man5tly
 negative 7or positive1 dep5d+ ^u h[ x  
is connect$74 ,?is batt]y ei!r helps or 
h9d]s ! plate batt]y1 & because ( !     

/rategic position ( ! grid "r ne> !     
filam5t "o volt appli$ to ! grid        
produces z l>ge an effect z w\ld sev]al 
volts 9 ! plate batt]y4 ,usually1 !re=e1
 we arrange to %ift ! *>act]i/ic by us+ 
a ,c-batt]y4  ,illu/ration3 ,fig #he    
,suppose = example t we _h an audion 9 !
 receiv+ circuit ( ,fig4 #fc & t its    
*>act]i/ic "u ^! conditions is giv5 by  
,fig4 #ef4 ,i've r$rawn ! figures to    
save y\r turn+ back4 ,! audion w n act z
 a detector because an incom+ signal w n
 *ange ! av]age value ( ! curr5t 9 !    
plate circuit4 ,if1 h[ev]1 we connect a 
,c-batt]y s z to make ! grid negative1  
we c %ift ?is *>act]i/ic s t ! incom+   
signal w be detect$4 ,we h only to make 
! grid suffici5tly negative to r$uce !  
plate curr5t to ! value %[n by ! l9e oa 
9 ,fig4 #he4 ,!n ! signal w be detect$  
because1 :ile x makes ! plate curr5t    
alt]nately l>g] & small] ?an ?is value  
oa1 x w result1 on ! av]age1 9 a high]  
value ( ! plate curr5t4  ,illu/ration3  
,fig #hf  ,y see t :at we h done is to  

arrange ! po9t on ! audion *>act]i/ic   
ab\t :i* ! tube is to "w by prop]ly     
*oos+ ! value ( ! grid voltage ,e,c4    
,!re is an important me?od ( us+ an     
audion = a detector :]e we arrange to h 
! grid voltage *ange /eadily1 gett+ m & 
m negative all ! "t ! signal is com+ 94 
,be=e ,i tell h[ x is done ,i want to %[
 y :at w happ54  ,suppose we />t ) an   
audion detector1 = :i* ! *>act]i/ic is t
 ( ,fig4 #ef1 b arrang$ z 9 ,fig4 #hf to
 give ! grid any pot5tial :i* we wi%4 ,!
 batt]ies & slide wire resi/ance :i* are
 connect$ 9 ! grid circuit are already  
famili> to y4  ,:5 ! slid] is set z %[n 
9 ,fig4 #hf ! grid is at z]o pot5tial & 
we are at ! po9t #a ( ! *>act]i/ic %[n 9
 ,fig4 #hg4 ,n[ imag9e an incom+ signal1
 z %[n 9 t same figure1 b suppose t z   
soon z ! signal has /opp$ mak+ ! grid   
positive we %ift ! slid] a little s t ! 
,c-batt]y makes ! grid slightly         
negative4 ,we h %ift$ ! po9t on !       
*>act]i/ic ab\t :i* ! tube is be+ work$ 
by ! incom+ signal f po9t #a to po9t #b4

  ,illu/ration3 ,fig #hg  ,e "t ! incom+
 signal makes "o complete cycle ( *anges
 we %ift ! slid] a little fur!r & make !
 grid p]man5tly m negative4 ,y c see :at
 happ5s4 ,z ! grid becomes m negative ! 
curr5t 9 ! plate circuit decreases on ! 
av]age4 ,f9ally1 ( c\rse1 ! grid w      
become s negative t ! curr5t 9 ! plate  
circuit w be r$uc$ to z]o4 ,"u ^!       
conditions an incom+ signal f9ally makes
 a l>ge *ange 9 ! plate curr5t & h5ce 9 
! curr5t ?r\gh ! telephone4  ,! me?od ( 
%ift+ a slid] along1 e "t ! incom+      
signal makes a complete cycle1 is       
impossible to accompli% by h& if !      
frequ5cy ( ! signal is high4 ,x c be    
done automatically1 h[ev]1 no matt] h[  
high ! frequ5cy if we use a cond5s] 9 ! 
grid circuit z %[n 9 ,fig4 #hh4         
,illu/ration3 ,fig #hh  ,:5 ! incom+    
signal />ts a /ream ( electrons ?r\gh ! 
coil ,l ( ,fig4 #hh & draws !m away f   
plate #a ( ! cond5s] ,c x is also draw+ 
electrons away f ! #a plate ( ! cond5s] 
,cg :i* is 9 s]ies ) ! grid4 ,z         

electrons leave plate #a ( ?is cond5s]  
o!rs ru% away f ! grid & ent] plate #b4 
,?is means t ! grid doesn't h its ord9>y
 numb] ( electrons & s is positive4  ,if
 ! grid is positive x w be pleas$ to get
 electrons2 & x c d s at once1 = !re are
 lots ( electrons /ream+ pa/ x on _! way
 to ! plate4 ,:ile ! grid is positive1  
!re=e1 !re is a /ream ( electrons to x f
 ! filam5t4 ,fig4 #hi %[s ?is curr5t4   
,all ?is takes place dur+ ! fir/        
half-cycle ( ! incom+ signal4 ,dur+ !   
next half-cycle electrons are s5t into  
plate #a ( ! cond5s] ,c & also into     
plate #a ( ! grid cond5s] ,cg4 ,z       
electrons are =c$ into plate #a ( ! grid
 cond5s] ^? 9 plate #b ( t cond5s] h to 
leave & g back to ! grid :]e !y came f4 
,t is all "r1 b :ile !y 7 away ! grid   
got "s electrons f ! filam5t to take _! 
places4 ,! result is t ! grid has n[ too
 _m electrons1 t is1 x is negatively    
*>g$4  ,illu/ration3 ,fig #hi  ,an      
in/ant lat] ! signal e4 m4 f4 rev]ses & 
calls electrons away f plate #a ( ! grid

 cond5s]4 ,aga9 electrons f ! grid ru%  
into plate #b & aga9 ! grid is left )\t 
its prop] numb] & s is positive4 ,aga9 x
 receives electrons f ! filam5t4 ,!     
result is /ill m electrons 9 ! "p ( !   
grid circuit :i* is =m$ by ! grid1 !    
plate #b ( ! grid cond5s] & ! connect+  
wire4 ,^! electrons c't get across ! gap
 ( ! cond5s] ,cg & !y c't g back to !   
filam5t any o!r way4 ,s !re !y are1     
trapp$4 ,f9ally !re are s _m ( ^! trapp$
 electrons t ! grid is s negative all ! 
"t z almo/ entirely to oppose ! ef=ts ( 
! plate to draw electrons away f !      
filam5t4  ,illu/ration3 ,pl4            
,,viii4--,to ,illu/rate ! ,me*anism = ! 
,production ( ! ,human ,voice4  ,!n !   
plate curr5t is r$uc$ practically to    
z]o4  ,t's ! way to arrange an audion s 
t ! incom+ signal makes ! l>ge/ possible
 *ange 9 plate curr5t4 ,we c tell if !re
 is an incom+ signal because x w block !
 tube1 z we say4 ,! plate-circuit curr5t
 w be *ang$ f its ord9>y value to almo/ 
z]o 9 ! %ort "t x takes = a few cycles (

 ! incom+ signal4  ,we c detect "o      
signal t way1 b only "o because ! fir/  
signal makes ! grid p]man5tly negative &
 blocks ! tube s t !re isn't any curr5t 
9 ! plate circuit & c't be any4 ,if we  
want to put ! tube 9 condition to       
receive ano!r signal we mu/ all[ ^!     
electrons1 :i* orig9ally came f !       
filam5t1 to get out ( _! trapp$ position
 & g back to ! filam5t4  ,illu/ration3  
,fig #ij  ,to d s we connect a v f9e    
wire betwe5 plates #a & #b ( ! grid     
cond5s]4 ,we call t wire a grid-cond5s] 
leak because x lets ! electrons slip    
ar\nd pa/ ! gap4 ,by us+ a v high       
resi/ance1 we c make x s h>d = !        
electrons to get ar\nd ! gap t n _m w d 
s :ile ! signal is com+ 94 ,9 t case we 
c leave ! leak p]man5tly across !       
cond5s] z %[n 9 ,fig4 #ij4 ,( c\rse1 !  
leak mu/ (f] s easy a pa? = ! electrons 
t all ! trapp$ electrons c get home     
betwe5 "o incom+ signal & ! next4  ,"o  
way ( mak+ a high resi/ance l ?is is to 
draw a heavy p5cil l9e on a piece (     

pap]1 or bett] a l9e ) ,india ink1 t is 
ink made ( f9e gr\nd p>ticles ( c>bon4  
,! leak %\ld h a v high resi/ance1      
usually "o or two million ohms if !     
cond5s] is ab\t #j4jjb micr(>ad4 ,if x  
has a million ohms we say x has a megohm
 ( resi/ance4  ,?is me?od ( detect+ ) a 
leaky grid-cond5s] & an audion is v     
effici5t s f> z tell+ ! li/5] :e!r or n 
a signal is com+ into 8 set4 ,x is      
widely us$ 9 receiv+ radio-telephone    
signals al?\gh x is be/ adapt$ to       
receiv+ ! telegraph signals f a sp>k    
set4  ,i don't propose to /op to tell y 
h[ a sp>k-set transmitt] works4 ,x is   
suffici5t to say t :5 ! key is depress$ 
! set s5ds out radio signals at ! rate  
usually ( #ajjj signals a second4 ,e "t 
a signal rea*es ! receiv+ /ation !      
curr5t 9 ! telephone receiv] is sudd5   
r$uc$2 & 9 ! "t betwe5 signals ! leak   
across ! grid cond5s] br+s ! tube back  
to a condition :]e x c receive ! next   
signal4 ,:ile ! s5d+ key is depress$ !  
curr5t 9 ! receiv] is decreas+ &        

increas+ once = e signal :i* is be+     
transmitt$4 ,= ea* decrease & increase 9
 curr5t ! diaphragm ( ! telephone       
receiv] makes "o vibration4 ,:at ! li/5]
 !n he>s is a musical note ) a frequ5cy 
correspond+ to t numb] ( vibrations a   
second1 t is1 a note ) a frequ5cy ( "o  
?\s& cycles p] second4 ,he he>s a note (
 frequ5cy ab\t t ( two octaves above    
middle ,c on ! piano4 ,!re are usually  
o!r notes pres5t at ! same "t & ! s\nd  
is n l t ( any musical in/rum5t4        
,illu/ration3 ,fig #ia  ,if ! key is    
held d[n a long "t = a da% ! li/5] he>s 
?is note = a correspond+ "t4 ,if x is   
depress$ only ab\t a ?ird ( t "t s z to 
s5d a dot1 ! li/5] he>s ! note = a %ort]
 "t & int]prets x to mean a dot4  ,9    
,fig4 #ia ,i h drawn a sket* to %[ ! e4 
m4 f4 :i* ! signals f a sp>k set impress
 on ! grid ( a detector & to %[ h[ !    
plate curr5t v>ies if !re is a cond5s] &
 leak 9 ! grid circuit4 ,i h only %[n   
?ree signals 9 succession4 ,if ! op]ator
 s5ds at ! rate ( ab\t tw5ty words a    

m9ute a dot is =m$ by ab\t sixty ( ^!   
signals 9 succession4  ,! frequ5cy ( !  
alt]nations 9 "o ( ! little signals w   
dep5d ^u ! wave l5g? :i* ! s5d+ op]ator 
is us+4 ,if he uses ! wave l5g? ( #fjj  
met]s1 z %ip /ations d1 he w s5d ) a    
radio frequ5cy ( #ejj1jjj cycles a      
second4 ,s9ce ! signals are at ! rate ( 
a ?\s& a second ea* "o is made up ( #ejj
 complete cycles ( ! curr5t 9 ! ant5na4 
,x w\ld be impracticable !re=e to %[ y a
 complete picture ( ! signal f a sp>k   
set4 ,i h1 h[ev]1 lett]$ ! figure q     
completely to cov] :at ,i h j told y4   
,if ! grid-cond5s] & its leak are s *os5
 z to "w well = signals f a #ejj-;cycle 
sp>k set !y w also "w well = ! notes 9  
human spee* :i* are ab\t #ajjj cycles a 
second 9 frequ5cy4 ,! detect+ circuit w 
n1 h[ev]1 "w s well = ! o!r notes :i*   
are 9 ! human voice & are necess>y to   
spee*4 ,= example1 if notes ( ab\t #bjjj
 cycles a second are involv$ 9 ! spee*  
:i* is be+ transmitt$1 ! leak across !  
cond5s] w n "w fa/ en\gh4 ,on ! o!r h&1 

= ! v l[e/ notes 9 ! voice ! leak w "w  
too fa/ & su* v>iations 9 ! signal      
curr5t w n be detect$ z effici5tly z are
 ^? ( #ajjj cycles a second4  ,y c see t
 !re is always a little favoritism on ! 
"p ( ! grid-cond5s] detector4 ,x doesn't
 exactly reproduce ! v>iations 9        
int5sity ( ! radio signal :i* 7 made at 
! s5d+ /ation4 ,x di/orts a little4 ,z  
amateurs we usually =give x t di/ortion 
because x is s effici5t4 ,x makes s l>ge
 a *ange 9 ! curr5t ?r\gh ! telephone :5
 x receives a signal t we c use x to    
receive mu* weak] signals1 t is1 signals
 f small] or m di/ant s5d+ /ations1 ?an 
we c receive ) ! arrangem5t describ$ 9  
,lett] #ad4     ,,lett] #ah  ,,amplifi]s
 ,,& ,,! ,,reg5]ative ,,circuit   ,,my  
,,de> ,,receiv]3  ,!re is "o way ( mak+ 
an audion ev5 m effici5t z a detector   
?an ! me?od describ$ 9 ! la/ lett]4 ,& t
 is to make x talk to itself4  ,suppose 
we arrange a receiv+ circuit z 9 ,fig4  
#ib4 ,x is exactly l t ( ,fig4 #ij ( !  
previ\s lett] except = ! fact t ! curr5t

 9 ! plate circuit passes ?r\gh a little
 coil1 ,lt1 :i* is plac$ ne> ! coil ,l &
 s c induce 9 x an e4 m4 f4 :i* w       
correspond 9 int5sity & wave =m to !    
curr5t 9 ! plate circuit4  ,if we %\ld  
take out ! grid cond5s] & its leak ?is  
circuit w\ld be l t ( ,fig4 #ed 9 ,lett]
 #ac :i* we us$ = a g5]ator (           
high-frequ5cy alt]nat+ curr5ts4 ,y      
rememb] h[ t circuit op]ates4 ,a small  
effect 9 ! grid circuit produces a l>ge 
effect 9 ! plate circuit4 ,because !    
plate circuit is c\pl$ to ! grid circuit
 ! grid is aga9 affect$ & s !re is a    
/ill l>g] effect 9 ! plate circuit4 ,& s
 on1 until ! curr5t 9 ! plate circuit is
 sw++ f z]o to its maximum possible     
value4  ,:at happ5s dep5ds ^u h[ closely
 ! coils ,l & ,lt are c\pl$1 t is1 ^u h[
 mu* ! *ang+ curr5t 9 "o c affect ! o!r4
 ,if !y are turn$ at "r angles to ea*   
o!r1 s t !re is no possible mutual      
effect we say !re is z]o c\pl+4  ,/>t ) 
! coils at "r angles to ea* o!r & turn  
,lt s z to br+ its w9d+s m & m p>allel  

to ^? ( ,l4 ,if we want ,lt to h a l>ge 
effect on ,l its w9d+s %\ld be p>allel &
 also 9 ! same direction j z !y 7 9     
,fig4 #ed ( ,lett] #ac to :i* we j      
ref]r$4 ,z we approa* ne>] to t position
 ! curr5t 9 ,lt induces m & m e4 m4 f4 9
 coil ,l4 ,= "s position ( ! two coils1 
& ! actual position dep5ds on ! tube we 
are us+1 !re w be en\gh effect f ! plate
 circuit ^u ! grid circuit s t !re w be 
cont9u\s oscillations4  ,illu/ration3   
,fig #ib  ,we want to /op j %ort ( ?is  
position4 ,!re w !n be no cont9u\s      
oscillations2 b if any *anges d take    
place 9 ! plate curr5t !y w affect !    
grid4 ,& ^! *anges 9 ! grid voltage w   
result 9 /ill l>g] *anges 9 ! plate     
curr5t4  ,n[ suppose t !re is com+ into 
! detector circuit ( ,fig4 #ib a radio  
signal )1 spee* significance4 ,! curr5t 
9 ! plate circuit v>ies accord+ly4 ,s   
does ! curr5t 9 coil ,lt :i* is 9 !     
plate circuit4 ,b ?is curr5t induces an 
e4 m4 f4 9 coil ,l & ?is adds to ! e4 m4
 f4 ( ! incom+ signal s z to make a     

great] v>iation 9 ! plate curr5t4 ,?is  
goes on z long z !re is an incom+       
signal4 ,because ! plate circuit is     
c\pl$ to ! grid circuit ! result is a   
l>g] e4 m4 f4 9 ! grid circuit ?an !    
incom+ signal c\ld set up all by itself4
  ,y see n[ :y ,i said ! tube talk$ to  
itself4 ,x repeats to itself :atev] x   
receives4 ,x has a great] /r5g? ( signal
 to detect ?an if x didn't repeat4 ,(   
c\rse1 x detects also j z ,i told y 9 ! 
prec$+ lett]4  ,9 adju/+ ! c\pl+ ( ! two
 coils ( ,fig4 #ib we /opp$ %ort ( all[+
 ! tube circuit to oscillate & to g5]ate
 a high frequ5cy4 ,if we _h gone on     
increas+ ! c\pl+ we %\ld h rea*$ a      
position :]e /eady oscillations w\ld    
beg94 ,usually ?is is m>k$ by a little  
click 9 ! receiv]4 ,! reason is t :5 !  
tube oscillates ! av]age curr5t 9 !     
plate circuit is n ! same z ! /eady     
curr5t :i* ord9>ily fl[s betwe5 filam5t 
& plate4 ,!re is a sudd5 *ange1 !re=e1 9
 ! av]age curr5t 9 ! plate circuit :5 ! 
tube />ts to oscillate4 ,y rememb] t :at

 affects ! receiv] is ! av]age curr5t 9 
! plate circuit4 ,s ! receiv] diaphragm 
sudd5ly *anges position z ! tube />ts to
 oscillate & a li/5] he>s a little      
click4  ,! frequ5cy ( ! alt]nat+ curr5t 
:i* ! tube produces dep5ds ^u ! tun$    
circuit =m$ by ,l & ,c4 ,suppose t ?is  
frequ5cy is n ! same z t to :i* !       
receiv+ ant5na is tun$4 ,:at w happ58   
,!re w be impress$ on ! grid ( ! tube   
two alt]nat+ e4 m4 f4's1 "o due to !    
tube's own oscillations & ! o!r incom+ f
 ! di/ant transmitt+ /ation4 ,! two e4  
m4 f4 's are bo? active at once s t at  
ea* in/ant ! e4 m4 f4 ( ! grid is really
 ! sum ( ^! two e4 m4 f4's4 ,suppose at 
"s in/ant bo? e4 m4 f4's are act+ to    
make ! grid positive4 ,a little lat] "o 
( !m w be try+ to make ! grid negative  
:ile ! o!r is /ill try+ to make x       
positive4 ,& lat] /ill :5 ! fir/ e4 m4  
f4 is ready aga9 to make ! grid positive
 ! second w be try+ to make x negative4 
,x's l two m5 walk+ along toge!r b )    
diff]5t l5g?s ( /ep4 ,ev5 if !y />t     

toge!r ) _! left feet !y are soon s     
completely out ( /ep t "o is putt+ d[n 8
 "r foot :ile ! o!r is putt+ d[n 8 left4
 ,a little lat]1 b j = an in/ant1 !y are
 9 /ep aga94 ,& s x goes4 ,!y are 9 /ep 
= a mom5t & !n completely out ( /ep4    
,suppose "o ( !m makes t5 /eps 9 ! "t t 
! o!r makes n9e4 ,9 t "t !y w be once 9 
/ep & once completely out ( /ep4 ,if "o 
makes t5 /eps :ile ! o!r does eight ?is 
w happ5 twice4  ,! same ?+ happ5s 9 !   
audion detector circuit :5 two e4 m4    
f4's :i* diff] slightly 9 frequ5cy are  
simultane\sly impress$ on ! grid4 ,if "o
 e4 m4 f4 passes ?r\gh t5 complete      
cycles :ile ! o!r is mak+ eight cycles1 
!n dur+ t "t !y w twice be exactly 9    
/ep1 t is1 9 phase z we say4 ,twice 9 t 
"t !y w be exactly out ( /ep1 t is1     
exactly opposite 9 phase4 ,twice 9 t "t 
! two e4 m4 f4's w aid ea* o!r 9 _!     
effects on ! grid & twice !y w exactly  
oppose4 ,unless !y are equal 9 amplitude
 !re w /ill be a net e4 m4 f4 ev5 :5 !y 
are exactly oppos$4 ,! result ( all ?is 

is t ! av]age curr5t 9 ! plate circuit (
 ! detector w alt]nately increase &     
decrease twice dur+ ?is "t4  ,! li/5] w 
!n he> a note ( a frequ5cy equal to !   
diff]5ce betwe5 ! frequ5cies ( ! two e4 
m4 f4's :i* are be+ simultane\sly       
impress$ on ! grid ( ! detector4        
,suppose ! incom+ signal has a frequ5cy 
( #ajj1jjj cycles a second b t !        
detector tube is oscillat+ 9 its own    
circuit at ! rate ( #ii1jjj cycles p]   
second1 !n ! li/5] w he> a note ( #ajjj 
cycles p] second4 ,"o ?\s& times ea*    
second ! two e4 m4 f4's w be exactly 9  
phase & "o ?\s& times ea* second !y w be
 exactly opposite 9 phase4 ,! voltage   
appli$ to ! grid w be a maximum "o ?\s& 
times a second & alt]nately a m9imum4   
,we c ?9k ( x1 !n1 z if !re 7 impress$  
on ! grid ( ! detector a high-frequ5cy  
signal :i* v>i$ 9 int5sity "o ?\s& times
 a second4 ,?is we "k w produce a       
correspond+ v>iation 9 ! curr5t ?r\gh ! 
telephone receiv] & ?us give rise to a  
musical note ( ab\t two octaves above   

middle ,c on ! piano4  ,?is circuit (   
,fig4 #ib w let u detect signals :i* are
 n v>y+ 9 int5sity4 ,& consequ5tly ?is  
is ! me?od :i* we use to detect !       
telegraph signals :i* are s5t out by su*
 a cont9u\s wave transmitt] z ,i %[$ y  
at ! end ( ,lett] #ac4  ,:5 ! key ( a   
,c-,w transmitt] is depress$ !re is set 
up 9 ! di/ant receiv+-ant5na an alt]nat+
 curr5t4 ,?is curr5t doesn't v>y 9      
/r5g?4 ,x is !re z long z ! s5d] has 8  
key d[n4 ,because1 h[ev]1 ( ! effect :i*
 ,i describ$ above !re w be an audible  
note f ! telephone receiv] if ! detector
 tube is oscillat+ at a frequ5cy )9 two 
or ?ree ?\s& cycles ( t ( ! transmitt+  
/ation4  ,?is me?od ( receiv+ cont9u\s  
wave signals is call$ ! het]odyne me?od4
 ,! "n comes f two ,greek words1 dyne   
mean+ =ce & ! o!r "p mean+ diff]5t4 ,we 
receive by comb9+ two diff]5t           
electron-mov+-=ces1 "o produc$ by !     
di/ant s5d+-/ation & ! o!r produc$      
locally at ! receiv+ /ation4 ,nei!r by  
itself w produce any s\nd1 except a     

click :5 x />ts4 ,bo? toge!r produce a  
musical s\nd 9 ! telephone receiv]2 & ! 
frequ5cy ( t note is ! diff]5ce ( ! two 
frequ5cies4  ,!re are a numb] ( words   
us$ to describe ?is circuit ) "s ( :i* y
 %\ld be famili>4 ,x is sometimes call$ 
a fe$-back circuit because "p ( ! output
 ( ! audion is f$ back into its input   
side4 ,m g5]ally x is kn[n z !          
reg5]ative circuit because ! tube keeps 
on g5]at+ an alt]nat+ curr5t4 ,! little 
coil :i* is us$ to fe$ back into ! grid 
circuit "s ( ! effects f ! plate circuit
 is sometimes call$ a tickl] coil4  ,x  
is n necess>y to use a grid cond5s] 9 a 
fe$-back circuit b x is p]haps ! usual  
me?od ( detect+ :]e ! reg5]ative circuit
 is us$4 ,! :ole value ( ! reg5]ative   
circuit s f> z receiv+ is conc]n$ is 9 !
 high effici5cy :i* x p]mits4 ,"o tube c
 d ! "w ( two4  ,we c get j z l\d       
signals by us+ ano!r tube in/ead ( mak+ 
"o d all ! "w4 ,9 ! reg5]ative circuit !
 tube is p]=m+ two jobs at once4 ,x is  
detect+ b x is also amplify+4#i ,by     

amplify+ we mean mak+ an e4 m4 f4 l>g]  
?an x is )\t *ang+ ! %ape ( its picture1
 t is )\t *ang+ its wave =m4  ,to %[ j  
:at we mean by amplify+ we mu/ look aga9
 at ! audion & see h[ x acts4 ,y "k t a 
*ange 9 ! grid pot5tial makes a *ange 9 
! plate curr5t4 ,let u arrange an audion
 9 a circuit :i* w tell u a little m (  
:at happ5s4 ,fig4 #ic %[s ! circuit4    
,?is circuit is ! same z we us$ to f9d !
 audion *>act]i/ic except t !re is a    
clip = v>y+ ! numb] ( batt]ies 9 ! plate
 circuit & a voltmet] = measur+ _! e4 m4
 f4 ,we />t ) ! grid at z]o pot5tial & !
 usual numb] ( batt]ies 9 ! plate       
circuit4 ,! voltmet] tells u ! e4 m4 f4 
,we read ! ammet] 9 ! plate circuit &   
note :at t curr5t is4 ,!n we %ift !     
slid] 9 ! grid circuit s z to give !    
grid a small pot5tial4 ,! curr5t 9 !    
plate circuit *anges4 ,we c n[ move !   
clip on ! ,b-batt]ies s z to br+ !      
curr5t 9 ?is circuit back to its orig9al
 value4 ,( c\rse1 if we make ! grid     
positive we move ! clip s z to use few] 

cells ( ! ,b-batt]y4 ,on ! o!r h& if we 
make ! grid negative we %all ne$ m e4 m4
 f4 9 ! plate circuit4 ,9 ei!r case we  
%all f9d t we ne$ to make a v mu* l>g]  
*ange 9 ! voltage ( ! plate circuit ?an 
we h made 9 ! voltage ( ! grid circuit4 
,illu/ration3 ,fig #ic  ,usually we p]=m
 ! exp]im5t a little diff]5tly s z to   
get m accurate results4 ,we read !      
voltmet] 9 ! plate circuit & ! ammet] 9 
t circuit4 ,!n we *ange ! numb] (       
batt]ies :i* we are us+ 9 ! plate       
circuit4 ,t *anges ! plate curr5t4 ,!   
next /ep is to %ift ! slid] 9 ! grid    
circuit until we h aga9 ! orig9al value 
( curr5t 9 ! plate circuit4 ,suppose t !
 tube is ord9>ily run ) a plate voltage 
( #dj volts & we />t ) t e4 m4 f4 on !  
plate4 ,suppose t we n[ make x #ej volts
 & !n v>y ! position ( ! slid] 9 ! grid 
circuit until ! ammet] reads z x did at 
! />t4 ,next we read ! voltage impress$ 
on ! grid by read+ ! voltmet] 9 ! grid  
circuit4 ,suppose x reads #b volts4 ,:at
 does t mean8  ,illu/ration3 ,fig #id   

,x means t two volts 9 ! grid circuit h 
! same effect on ! plate curr5t z t5    
volts 9 ! plate circuit4 ,if we apply a 
volt to ! grid circuit we get five times
 z l>ge an effect 9 ! plate circuit z we
 w\ld if ! volt 7 appli$ !re4 ,we get a 
great] effect1 ! effect ( m volts1 by   
apply+ our voltage to ! grid4 ,we say t 
! tube acts z an amplifi] ( voltage     
because we c get a l>g] effect ?an !    
numb] ( volts :i* we apply w\ld ord9>ily
 entitle u to4  ,n[ let's take a simple 
case ( ! use ( an audion z an amplifi]4 
,suppose we h a receiv+ circuit ) :i* we
 f9d t ! signals are n easily und]/ood  
because !y are too weak4 ,let ?is be !  
receiv+ circuit ( ,fig4 #hh :i* ,i am   
reproduc+ "h z "p ( ,fig4 #id4  ,we h   
replac$ ! telephone receiv] by a        
trans=m]4 ,a trans=m] is two coils1 or  
w9d+s1 c\pl$ toge!r4 ,an alt]nat+ curr5t
 9 "o w give rise to an alt]nat+ curr5t 
9 ! o!r4 ,y are already famili> ) ! idea
 b ?is is our fir/ use ( ! ^w4 ,usually 
we call ! fir/ coil1 t is ! "o ?r\gh :i*

 ! alt]nat+ curr5t fl[s1 ! prim>y & !   
second coil1 9 :i* a curr5t is induc$1 !
 second>y4  ,! second>y ( ?is trans=m]  
is connect$ to ! grid circuit ( ano!r   
vacuum tube1 to ! plate circuit ( :i* is
 connect$ ano!r trans=m] & ! telephone  
receiv]4 ,! result is a detector & "o   
/age ( amplification4  ,! prim>y ( !    
fir/ trans=m]1 s we %all suppose1 has 9 
x ! same curr5t z w\ld h be5 9 !        
telephone4 ,?is alt]nat+ curr5t induces 
9 ! second>y an e4 m4 f4 :i* has ! same 
v>iations z ?is curr5t4 ,?is e4 m4 f4   
acts on ! grid ( ! second tube1 t is on 
! amplifi]4 ,because ! audion amplifies1
 ! e4 m4 f4 act+ on ! telephone receiv] 
is l>g] ?an x w\ld h be5 )\t ! use ( ?is
 audion4 ,& h5ce !re is a great]        
response on ! "p ( its diaphragm & a    
l\d] s\nd4  ,9 sett+ up su* a circuit z 
?is !re are sev]al ?+s to wat*4 ,= "s ( 
^! y w h to rely on ! deal] f :om y buy 
y\r supplies & = ! o!rs ^u y\rself4 ,b x
 w take ano!r lett] to tell y ( ! prop] 
precautions 9 us+ an audion z an        

amplifi]4  ,illu/ration3 ,fig #ie  ,9 ! 
circuit :i* ,i h j describ$ an audion is
 us$ to amplify ! curr5t :i* comes f !  
detector be=e x rea*es ! telephone      
receiv]4 ,sometimes we use an audion to 
amplify ! e4 m4 f4 ( ! signal be=e      
impress+ x ^u ! grid ( ! detector4 ,fig4
 #ie %[s a circuit = do+ t4 ,9 ! case ( 
,fig4 #id we are amplify+ !             
audio-frequ5cy curr5t4 ,9 t ( ,fig4 #ie 
x is ! radio-frequ5cy effect :i* is     
amplifi$4 ,! fe$-back or reg5]ative     
circuit ( ,fig4 #ib is a "o-tube circuit
 = do+ ! same ?+ z is done ) two tubes 9
 ,fig #ie4  ,footnote #i3 ,!re is always
 "s amplification tak+ place 9 an audion
 detector b ! reg5]ative circuit        
amplifies ov] & ov] aga9 until ! signal 
is z l>ge z ! tube c detect4     ,,lett]
 #ai  ,,! ,,audion ,,amplifi] ,,& ,,its 
,,connections   ,,de> ,,son3  ,9 our use
 ( ! audion we =m ?ree circuits4 ,! fir/
 or ,a-circuit includes ! filam5t4 ,!   
,b-circuit includes ! "p ( ! tube betwe5
 filam5t & plate4 ,! ,c-circuit includes

 ! "p betwe5 filam5t & grid4 ,we        
sometimes speak ( ! ,c-circuit z ! input
 circuit & ! ,b-circuit z ! output      
circuit ( ! tube4 ,?is is because we c  
put into ! grid-filam5t t]m9als an e4 m4
 f4 & obta9 f ! plate-filam5t circuit an
 effect 9 ! =m ( a *ange ( curr5t4      
,illu/ration3 ,fig #if  ,suppose we _h  
conceal$ 9 a box ! audion & circuit (   
,fig4 #if & t only ! t]m9als :i* are %[n
 came ?r\gh ! box4 ,we are giv5 a batt]y
 & an ammet] & ask$ to f9d out all we c 
z to :at is betwe5 ! t]m9als ,f & ,g4   
,we connect ! batt]y & ammet] 9 s]ies ) 
^! t]m9als4 ,no curr5t fl[s ?r\gh !     
circuit4 ,we rev]se ! batt]y b no curr5t
 fl[s 9 ! opposite direction4 ,!n we    
reason t !re is an op5-circuit betwe5 ,f
 & ,g4  ,z long z we d n use a high]    
voltage ?an t ( ! ,c-batt]y :i* is 9 !  
box no curr5t c fl[4 ,ev5 if we d use a 
high] voltage ?an ! negative ,c-batt]y (
 ! hidd5 grid-circuit !re w be a curr5t 
only :5 ! ext]nal batt]y is connect$ s z
 to make ! grid positive ) respect to ! 

filam5t4  ,n[ suppose we take sev]al    
cells ( batt]y & try 9 ! same way to f9d
 :at is hidd5 betwe5 ! t]m9als ,p & ,f4 
,we />t ) "o batt]y & ! ammet] z be=e & 
f9d t if ?is batt]y is connect$ s z to  
make ,p positive ) respect to ,f1 !re is
 a feeble curr5t4 ,we increase ! batt]y 
& f9d t ! curr5t is increas$4 ,two      
cells1 h[ev]1 d n give exactly twice !  
curr5t t "o cell does1 nor d ?ree give  
?ree times z mu*4 ,! curr5t does n      
increase proportionately to ! appli$    
voltage4 ,!re=e we reason t :atev] is   
betwe5 ,p & ,f acts l a resi/ance b n l 
a wire resi/ance4  ,!n1 we try ano!r    
exp]im5t ) ?is hidd5 audion4 ,we connect
 a batt]y to ,g & ,f1 & note :at effect 
x has on ! curr5t :i* our o!r batt]y is 
s5d+ ?r\gh ! box betwe5 ,p & ,f4 ,!re is
 a *ange ( curr5t 9 ?is circuit1 j z if 
our act ( connect+ a batt]y to ,g-,f _h 
result$ 9 connect+ a batt]y 9 s]ies ) ! 
,p-,f circuit4 ,! effect is exactly z if
 !re is inside ! box a batt]y :i* is    
connect$ into ! hidd5 "p ( ! circuit    

,p-,f4 ,?is conceal$ batt]y1 :i* n[ />ts
 to act1 appe>s to be sev]al times      
/rong] ?an ! batt]y :i* is connect$ to  
,g-,f4  ,sometimes ?is hidd5 batt]y     
helps ! ,b-batt]y :i* is on ! outside2 &
 sometimes x seems to oppose1 = ! curr5t
 9 ! ,p-,f circuit ei!r increases or    
decreases1 dep5d+ ^u h[ we connect !    
batt]y to ,g & ,f4 ,! hidd5 batt]y is   
always l>g] ?an our batt]y connect$ to  
,g & ,f4 ,if we arrange rapidly to      
rev]se ! batt]y connect$ to ,g-,f x     
appe>s z if !re is inside ! box 9 !     
,p-,f circuit an alt]nator1 t is1 some?+
 :i* c produce an alt]nat+ e4 m4 f4     
,all ?is1 ( c\rse1 is m]ely a review    
/atem5t ( :at we already "k4 ,^!        
exp]im5ts are int]e/+1 h[ev]1 because !y
 foll[ some:at ^? :i* 7 p]=m$ 9 /udy+ ! 
audion & f9d+ out h[ to make x d all !  
wond]ful ?+s :i* x n[ c4  ,z f> z we h  
c>ri$ our s]ies ( exp]im5ts ! box might 
conta9 two sep>ate circuits4 ,"o betwe5 
,g & ,f appe>s to be an op5 circuit4 ,! 
o!r appe>s to h 9 x a resi/ance & a     

batt]y 7or else an alt]nator74 ,! e4 m4 
f4 ( ! batt]y1 or alt]nator1 z ! case   
may be1 dep5ds on :at s\rce ( e4 m4 f4  
is connect$ to ,g-,f4 ,:atev] t e4 m4 f4
 is1 !re is a correspond+ k9d ( e4 m4 f4
 inside ! box b "o sev]al times l>g]4   
,illu/ration3 ,fig #ig  ,we might1      
!re=e1 pay no fur!r att5tion to :at is  
actually inside ! box or h[ all ^!      
effects are br\ght ab\t4 ,we might treat
 ! entire box z if x 0 =m$ by two       
sep>ate circuits z %[n 9 ,fig4 #ig4 ,if 
we d s1 we are replac+ ! box by some?+  
:i* is equival5t s f> z effects are     
conc]n$1 t is we are replac+ an actual  
audion by two circuits :i* toge!r are   
equival5t to x4  ,! m5 :o fir/ p]=m$ su*
 exp]im5ts want$ "s conv5i5t way ( say+ 
t if an alt]nator1 :i* has an e4 m4 f4 (
 ,v volts1 is connect$ to ,f & ,g1 !    
effect is ! same z if a mu* /rong]      
alt]nator is connect$ betwe5 ,f & ,p4   
,h[ mu* /rong] ?is imag9>y alt]nator is 
dep5ds ^u ! design ( ! audion4 ,= "s    
audions x might be five times z /rong1 =

 o!r designs #f4e or almo/ any o!r      
numb]1 al?\gh usually a numb] ( times   
less ?an #dj4 ,!y us$ a little ,greek   
lett] call$ mu to /& = ?is numb] :i*    
dep5ds on ! design ( ! tube4 ,!n !y said
 t ! hidd5 alt]nator 9 ! output circuit 
0 mu times z /rong z ! actual alt]nator 
:i* 0 appli$ betwe5 ! grid & ! filam5t4 
,( c\rse1 in/ead ( writ+ ! s\nd & "n ( !
 lett] !y us$ ! lett] ,greek3 m itself4 
,& t is :at ,i h done 9 ! sket* ( ,fig4 
#ig4  ,n[ we are ready to talk ab\t !   
audion z an amplifi]4 ,! fir/ ?+ to     
notice is ! fact t we h an op5 circuit  
betwe5 ,f & ,g4 ,?is is true z long z we
 don't apply an e4 m4 f4 l>ge en\gh to  
ov]come ! ,c-batt]y ( ,fig4 #if & ?us   
let ! grid become positive & attract    
electrons f ! filam5t4 ,we ne$ !n sp5d  
no fur!r "t ?9k+ ab\t :at w happ5 9 !   
circuit ,g-,f1 = !re w be no curr5t4  ,z
 to ! circuit ,f-,p1 we c treat x z a   
resi/ance 9 s]ies ) :i* !re is a g5]ator
 ,greek3 m times z /rong z t :i* is     
connect$ to ,f & ,g4 ,! next problem is 

h[ to get ! mo/ out ( ?is hidd5 g5]ator4
 ,we call ! resi/ance :i* ! tube (f]s to
 ! passage ( electrons betwe5 ,p & ,f ! 
int]nal resi/ance ( ! plate circuit ( ! 
tube4 ,h[ l>ge x is dep5ds ^u ! design (
 tube4 ,9 "s tubes x may be five or six 
?\s& ohms1 & 9 o!rs sev]al times z high4
 ,9 ! l>ge tubes us$ 9 high-p[]$        
transmitt+ sets x is mu* less4 ,s9ce x w
 be diff]5t 9 diff]5t cases we %all use 
a symbol = x & say t x is ,rp ohms4  ,!n
 "o rule = us+ an audion z an amplifi]  
is ?is3 ,to get ! mo/ out ( an audion   
see t ! telephone1 or :atev] circuit or 
piece ( app>atus is connect$ to ! output
 t]m9als1 %all h a resi/ance ( ,rp ohms4
 ,:5 ! resi/ance ( ! circuit1 :i* an    
audion is supply+ ) curr5t1 is ! same z 
! int]nal resi/ance ( ! output side ( ! 
tube1 !n ! audion gives its greate/     
output4 ,t is ! condition = ! greate/   
am\nt ( en]gy ea* second1 or ! greate/  
p[] z we say4  ,t rule is :y we always  
select ! telephone receiv]s :i* we use )
 an audion & always ask c>efully z to _!

 resi/ance :5 we buy4 ,sometimes1 h[ev]1
 x is n practicable to use receiv]s ( j 
! "r resi/ance4 ,:]e we connect ! output
 side ( an audion to "s o!r circuit1 z  
:]e we let "o audion supply ano!r1 x is 
usually impossible to foll[ ?is rule )\t
 add+ "s special app>atus4  ,?is leads  
to ! next rule3 ,if ! telephone receiv]1
 or ! circuit1 :i* we wi% to connect to 
! output ( an audion1 does n h q ne>ly a
 resi/ance ( ,rp ohms we use a prop]ly  
design$ trans=m] z we h already done 9  
,figs4 #id & #ie4  ,a trans=m] is two   
sep>ate coils c\pl$ toge!r s t an       
alt]nat+ curr5t 9 ! prim>y w induce an  
alt]nat+ curr5t 9 ! second>y4 ,( c\rse1 
if ! second>y is op5-circuit$ !n no     
curr5t c fl[ b !re w be induc$ 9 x an e4
 m4 f4 :i* is ready to act if ! circuit 
is clos$4 ,trans=m]s h an int]e/+       
ability to make a l>ge resi/ance look   
small or vice v]sa4 ,to %[ y :y1 ,i %all
 h to develop "s rules = trans=m]s4     
,suppose y h an alt]nat+ e4 m4 f4 ( t5  
volts appli$ to ! prim>y ( an iron-cor$ 

trans=m] :i* has t5 turns4 ,!re is "o   
volt appli$ to ea* turn4 ,n[1 suppose ! 
second>y has only "o turn4 ,t "o turn   
has induc$ 9 x an alt]nat+ e4 m4 f4 ( "o
 volt4 ,if !re are m turns ( wire =m+ ! 
second>y1 !n ea* turn has induc$ 9 x j  
"o volt4 ,b ! e4 m4 f4's ( all ^! turns 
add toge!r4 ,if ! second>y has tw5ty    
turns1 !re is induc$ 9 x a total ( tw5ty
 volts4 ,s ! fir/ rule is ?is3 ,9 a     
trans=m] ! numb] ( volts 9 ea* turn (   
wire is j ! same 9 ! second>y z 9 !     
prim>y4  ,if we want a high-voltage     
alt]nat+ e4 m4 f4 all we h to d is to   
s5d an alt]nat+ curr5t ?r\gh ! prim>y ( 
a trans=m] :i* has 9 ! second>y1 _m     
times m turns ( wire ?an x has 9 !      
prim>y4 ,f ! second>y we obta9 a high]  
voltage ?an we impress on ! prim>y4  ,y 
c see "o application ( ?is rule at once4
 ,:5 we use an audion z an amplifi] ( an
 alt]nat+ curr5t we s5d ! curr5t :i* is 
to be amplifi$ ?r\gh ! prim>y ( a       
trans=m]1 z 9 ,fig4 #id4 ,we use a      
trans=m] ) _m times m turns on !        

second>y ?an on ! prim>y s z to apply a 
l>ge e4 m4 f4 to ! grid ( ! amplify+    
tube4 ,t w mean a l>ge effect 9 ! plate 
circuit ( ! amplifi]4  ,y rememb] t !   
grid circuit ( an audion ) a prop] value
 ( negative ,c-batt]y is really         
op5-circuit$ & no curr5t w fl[ 9 x4 ,= t
 case we get a real ga9 by us+ a /ep-up 
trans=m]1 t is1 "o ) m turns 9 !        
second>y ?an 9 ! prim>y4  ,x looks at   
fir/ z if a trans=m] w\ld always give a 
ga94 ,if we mean a ga9 9 en]gy x w n    
al?\gh we may use x1 z we %all see 9 a  
m9ute1 to p]mit a vacuum tube to "w into
 an output circuit m effici5tly ?an x   
c\ld )\t ! trans=m]4 ,we _c h any m     
en]gy 9 ! second>y circuit ( a trans=m] 
?an we give to ! prim>y4  ,suppose we h 
a trans=m] ) twice z _m turns on !      
second>y z on ! prim>y4 ,to ! prim>y we 
apply an alt]nat+ e4 m4 f4 ( a c]ta9    
numb] ( volts4 ,9 ! second>y !re w be   
twice z _m volts because x has twice z  
_m turns4 ,! curr5t 9 ! second>y1 h[ev]1
 w be only half z l>ge z is ! curr5t 9 !

 prim>y4 ,we h twice ! =ce 9 ! second>y 
b only half ! electron /ream4  ,x is    
some?+ l ?is3 ,y are out coa/+ & two    
y\ng/]s ask y to pull !m & _! sl$s up   
hill4 ,y pull "o ( !m all ! way & d a   
c]ta9 am\nt ( "w4 ,on ! o!r h& suppose y
 pull !m bo? at once b only half way up4
 ,y pull twice z h>d b only half z f> & 
y d ! same am\nt ( "w z be=e4           
,illu/ration3 ,fig #ih  ,we c't get m "w
 out ( ! second>y ( a trans=m] ?an we d 
9 ! prim>y4 ,if we design ! trans=m] s t
 !re is a great] pull 7e4 m4 f47 9 !    
second>y ! electron /ream 9 ! second>y w
 be correspond+ly small]4  ,y rememb] h[
 we measure resi/ance4 ,we divide ! e4  
m4 f4 7numb] ( volts7 by ! curr5t 7numb]
 ( amp]es7 to f9d ! resi/ance 7numb] (  
ohms74 ,suppose we d t = ! prim>y & = ! 
second>y ( ! trans=m] ( ,fig4 #ih :i* we
 are discuss+4 ,see :at happ5s 9 !      
second>y4 ,!re is only half z mu*       
voltage b twice z mu* curr5t4 ,x looks z
 ?\gh ! second>y _h "o-f\r? z mu*       
resi/ance z ! prim>y4 ,& s x has1 b we  

usually call x imp$ance in/ead (        
resi/ance because /raight wires resi/ b 
coils or cond5s]s imp$e alt]nat+ e4 m4  
f4's4  ,illu/ration3 ,fig #ii  ,be=e we 
return to ! "q ( us+ a trans=m] 9 an    
audion circuit let u turn ?is trans=m]  
ar\nd z 9 ,fig4 #ii & s5d ! curr5t ?r\gh
 ! side ) ! l>g] numb] ( w9d+s4 ,let's  
talk ( prim>y & second>y j z be=e b1 (  
c\rse1 rememb] t n[ ! prim>y has twice !
 turns ( ! second>y4 ,on ! second>y side
 we %all h only half ! curr5t1 b !re w  
be twice ! e4 m4 f4 ,! resi/ance ( !    
second>y !n is f\r times t ( ! prim>y4  
,n[ return to ! amplifi] ( ,fig4 #id &  
see :at sort ( a trans=m] %\ld be betwe5
 ! plate circuit ( ! tube & ! telephone 
receiv]s4 ,suppose ! int]nal resi/ance (
 ! tube is #ab1jjj ohms & ! resi/ance ( 
! telephones is #c1jjj ohms4 ,suppose   
also t ! resi/ance 7really imp$ance7 ( !
 prim>y side ( ! trans=m] :i* we j      
consid]$ is #ab1jjj ohms4 ,! imp$ance ( 
its second>y w be a qu>t] ( ?is or      
#c1jjj ohms4 ,if we connect su* a       

trans=m] 9 ! circuit1 z %[n1 we %all    
obta9 ! greate/ output f ! tube4  ,9 !  
fir/ place ! prim>y ( ! trans=m] has a  
numb] ( ohms j equal to ! int]nal       
resi/ance ( ! tube4 ,! tube1 !re=e1 w   
give its be/ to t trans=m]4 ,9 ! second 
place ! second>y ( ! trans=m] has a     
resi/ance j equal to ! telephone        
receiv]s s x c give its be/ to !m4 ,!   
effect ( ! trans=m] is to make !        
telephones act z if !y _h f\r times z   
mu* resi/ance & s 7 exactly suit$ to be 
connect$ to ! audion4  ,?is :ole matt] (
 ! prop] use ( trans=m]s is q simple b v
 important 9 sett+ up vacuum-tube       
circuits4 ,to ov]look x 9 build+ or buy+
 y\r radio set w mean poor effici5cy4   
,:5ev] y h two p>ts ( a vacuum-tube     
circuit to connect toge!r be sure & buy 
only a trans=m] :i* is design$ to "w    
betwe5 ! two imp$ances 7or resi/ances7  
:i* y wi% to connect toge!r4  ,!re is "o
 m precaution 9 connection ) ! pur*ase (
 trans=m]s4 ,!y %\ld d ! same ?+ = all !
 important frequ5cies :i* !y are to     

transmit4 ,if !y d n1 ! spee* or signals
 w be di/ort$ & may be un9telligible4   
,if y take ! precautions :i* ,i h       
m5tion$ y\r radio receiv+ set =m$ by a  
detector & "o amplifi] w look l t (     
,fig4 #id4 ,t is only "o possible s*eme 
( connections4 ,y c use any detector    
circuit :i* y wi%1#aj "o ) a grid       
cond5s] & leak1 or "o arrang$ = fe$-back
 ,9 ei!r case y\r amplifi] may well be z
 %[n 9 ! figure4  ,illu/ration3 ,fig    
#ajj  ,! circuit ,i h describ$ uses an  
audion to amplify ! audio-frequ5cy      
curr5ts :i* come f ! detector & are     
capable ( op]at+ ! telephones4 ,9 "s    
cases x is desirable to amplify ! radio 
signals be=e apply+ !m to ! detector4   
,?is is especially true :]e a loop      
ant5na is be+ us$4 ,loop ant5nas are    
small] & m conv5i5t ?an arials & !y also
 h c]ta9 abilities to select ! signals  
:i* !y are to receive because !y receive
 be/ f /ations :i* lie along a l9e drawn
 p>allel to _! turns4 ,un=tunately1     
h[ev]1 !y are mu* less effici5t & s     

require ! use ( amplifi]s4  ,) a small  
loop made by t5 turns ( wire sep>at$ by 
ab\t a qu>t] ( an in* & w\nd on a squ>e 
m\nt+1 ab\t ?ree feet on a side1 y w    
usually require two amplifi]s4 ,"o ( ^! 
might be us$ to amplify ! radio signals 
be=e detection & ! o!r to amplify aft]  
detection4 ,to tune ! loop = broadca/s a
 cond5s] ( ab\t #j4jjje mf4 w be ne$$4  
,! diagram ( ,fig4 #ajj %[s ! complete  
circuit ( a set ) ?ree /ages (          
radio-amplification & none ( audio4     
,footnote #aj3 ,except = pat5t$         
circuits4 ,see p4 #bbd4     ,,lett] #bj 
,,telephone ,,receiv]s ,,& ,,o!r        
,,electromagnetic ,,devices   ,,de>     
,,son3  ,9 an e>li] lett] :5 we fir/    
introduc$ a telephone receiv] into a    
circuit ,i told y some?+ ( h[ x op]ates4
 ,i want n[ to tell :y & also ( "s o!r  
important devices :i* op]ate = ! same   
reason4  ,y rememb] t a /ream (         
electrons :i* is />t+ or /opp+ c induce 
! electrons ( a neighbor+ p>allel       
circuit to />t (f 9 p>allel pa?s4 ,we d 

n "k ! explanation ( ?is4 ,nor d we "k !
 explanation ( ano!r fact :i* seems to  
be relat$ to ?is fact ( induction & is !
 basis = our explanations ( magnetism4  
,illu/ration3 ,fig #aja  ,if two p>allel
 wires are c>ry+ /eady electron /reams 9
 ! same g5]al direction ! wires attract 
ea* o!r4 ,if ! /reams are oppositely    
direct$ ! wires repel ea* o!r4 ,fig4    
#aja illu/rates ?is fact4 ,if ! /reams  
are n at all 9 ! same direction1 t is1  
if !y are at "r angles1 !y h no effect  
on ea* o!r4  ,illu/ration3 ,fig #ajb    
,^! facts1 ( ! attraction ( electron    
/reams :i* are 9 ! same direction &     
repulsion ( /reams 9 opposite           
directions1 are all t "o ne$ rememb] to 
figure out = himself :at w happ5 "u     
v>i\s conditions4 ,= example1 if two    
coils ( wire are c>ry+ curr5ts :at w    
happ5 is easily se54 ,fig4 #ajb %[s !   
two coils & a section ?r\gh !m4         
,illu/ration3 ,fig #ajc  ,look+ at ?is  
cross section we seem to h f\r wires1 #a
 & #b ( coil ,a & #c & #d ( coil ,b4 ,y 

see at once t if ! coils are free to    
move !y w move into ! dott$ positions   
%[n 9 ,fig #ajb1 because wire #a        
attracts wire #c & repels wire #d1 :ile 
wire #b attracts wire #d & repels wire  
#c4 ,if necess>y1 & if !y are free to   
move1 ! coils w turn completely ar\nd to
 get to ?is position4 ,i h %[n su* a    
case 9 ,fig4 #ajc4  ,wires :i* are n    
c>ry+ curr5ts d n behave 9 ?is way4 ,!  
action is due1 b h[ we don't yet "k1 to 
! motions ( ! electrons4 ,z f> z we c   
expla9 x to-day1 ! attraction ( two     
wires :i* are c>ry+ curr5ts is due to ! 
attraction ( ! two /reams ( electrons4  
,( c\rse ^! electrons are "p ( ! wires4 
,!y c't get f> away f ! /ay-at-home     
electrons & ! nuclei ( ! atoms :i* =m ! 
wires4 ,9 fact x is ^! nuclei :i* keep !
 w&]+ electrons )9 ! wires4 ,! result is
 t if ! /reams ( electrons are to move  
t[>d ea* o!r ! wires mu/ g along ) !m4  
,if ! wires are held firmly ! electron  
/reams _c approa* "o ano!r = !y mu/ /ay 
9 ! wires4 ,wires1 !re=e1 p]=m !        

important s]vice ( act+ z pa?s =        
electrons :i* are travel+ z electric    
curr5ts4 ,!re are o!r ways 9 :i*        
electrons c be kept 9 a pa?1 & o!r means
 beside batt]ies = keep+ !m go+4 ,x     
doesn't make any diff]5ce s f> z !      
attraction or ! repulsion is conc]n$ :y 
!y are foll[+ a c]ta9 pa? or :y !y /ay 9
 x4 ,s f> z we "k two /reams (          
electrons1 foll[+ p>allel pa?s1 w       
always1 behave j l ! two /reams ( ,fig4 
#aja4  ,illu/ration3 ,fig #ajd          
,suppose1 = example1 !re 7 two atoms :i*
 7 ea* =m$ by a nucleus & a numb] (     
electrons sw++ ar\nd ab\t ! nucleus z   
pictur$ 9 ,fig4 #ajd4 ,! electrons are  
go+ ( _! own accord & ! nucleus keeps !m
 f fly+ (f at a tang5t1 ! way mud flies 
f ! :eel ( an automobile4 ,suppose ^!   
two atoms are free to turn b n to move  
f> f _! pres5t positions4 ,!y w turn s z
 to make _! electron pa?s p>allel j z   
did ! loops ( ,fig4 #ajb4  ,illu/ration3
 ,fig #aje  ,n[1 ,i don't say t !re are 
any atoms at all l ! ones ,i h pictur$4 

,!re is /ill a great deal to be le>n$   
ab\t h[ electrons act inside diff]5t    
k9ds ( atoms4 ,we d "k1 h[ev]1 t ! atoms
 ( iron act j z if !y 7 t9y loops )     
electron /reams4  ,illu/ration3 ,fig    
#ajf  ,suppose we _h sev]al loops & t !y
 7 l9$ up l ! ?ree loops 9 ,fig4 #aje4  
,y c see t !y w\ld all attract ! o!r    
loop1 on ! "r 9 ! figure4 ,on ! o!r h&  
if !y 7 gr\p$ 9 ! triangle ( ,fig4 #ajf 
!y w\ld b>ely affect ! loop because !y  
w\ld be pull+ at cross purposes4 ,if a  
lot ( ! t9y loops ( ! iron atoms are l9$
 up s z to act toge!r & attract o!r     
loops1 z 9 ! fir/ figure1 we say ! iron 
is magnetiz$ & is a magnet4 ,9 an ord9>y
 piece ( iron1 h[ev]1 ! atoms are s     
gr\p$ t !y don't pull toge!r b l ! loops
 ( our second figure pull 9 diff]5t     
directions & neutralize ea* o!r's ef=ts 
s t !re is no net effect4  ,illu/ration3
 ,pl4 ,,ix4--,we/]n ,electric ,l\d      
,speak+ ,receiv]4 ,cry/al ,detector ,set
 ( ! ,g5]al ,electric ,co4 ,audibility  
,met] ( ,g5]al ,radio ,co4  ,& l ! loops

 ( ,fig4 #ajf ! atoms 9 an unmagnetiz$  
piece ( iron are pretty well satisfi$ to
 /ay z !y are )\t all l9+ up to pull    
toge!r4 ,to magnetize ! iron we mu/ =ce 
"s ( ^! atomic loops to turn "p way     
ar\nd4 ,t c be done by br++ ne> !m a    
/rong magnet or a coil ( wire :i* is    
c>ry+ a curr5t4 ,!n ! atoms are =c$ to  
turn & if en\gh turn s t !re is an      
appreciable effect !n ! iron is         
magnetiz$4 ,! m t are prop]ly turn$ !   
/rong] is ! magnet4 ,"o end or pole we  
call nor?-seek+ & ! o!r s\?-seek+1      
because a magnetiz$ b> ( iron acts l a  
compass ne$le4  ,illu/ration3 ,fig #ajg 
,a coil ( wire1 c>ry+ a curr5t1 acts j l
 a magnet because its l>g] loops are all
 ready to pull toge!r4 ,i h m>k$ ! coil 
( ,fig4 #ajg ) ,n & ,s = nor? & s\?4 ,if
 ! electron /ream 9 x is rev]s$ !       
pol>ity is rev]s$4 ,!re is a simple rule
 = ?is4 ,p>tially close y\r left h& s t 
! f+]s =m loops4 ,let ! ?umb /ick out at
 "r angles to ^! loops4 ,if ! electron  
/reams are fl[+ ar\nd ! loops ( a coil 9

 ! same direction z y\r f+]s po9t !n y\r
 ?umb is ! ,n pole & ! coil w repel !   
nor? poles ( o!r loops or magnets 9 !   
direction 9 :i* y\r ?umb po9ts4 ,if y "k
 ! pol>ity already !re is a simple rule 
= ! repulsion or attraction4 ,l poles   
repel1 unlike poles attract4  ,f :at has
 be5 said ab\t magnetism y c n[ und]/&  
:y 9 a telephone receiv] ! curr5t 9 !   
w9d+ c make ! magnet /rong]4 ,x does s  
because x makes m ( ! atomic loops ( !  
iron turn ar\nd & help pull4 ,on ! o!r  
h& if ! curr5t 9 ! w9d+ is rev]s$ x w   
turn "s ( ! loops :i* are already help+ 
into o!r positions :]e !y don't help &  
may h9d]4 ,if ! curr5t 9 ! coil is to   
help1 ! electron /ream 9 x mu/ be s     
direct$ t ! nor? pole ( ! coil is at !  
same end z ! nor? pole ( ! magnet4  ,?is
 idea ( ! attraction or repulsion (     
electron /reams1 :e!r 9 coils ( wire or 
9 atoms ( iron & o!r magnetizable       
sub/ances1 is ! fundam5tal idea ( mo/   
=ms ( telephone receiv]s1 ( electric    
motors1 & ( a lot ( o!r devices :i* we  

call electromagnetic4  ,! ammet]s &     
voltmet]s :i* we use = ! measurem5t (   
audion *>act]i/ics & ! l are usually    
electromagnetic in/rum5ts4 ,ammet]s &   
voltmet]s are alike 9 _! design4 ,bo?   
are s5sitive curr5t-measur+ in/rum5ts4  
,9 ! case ( ! voltmet]1 z y "k1 we h a  
l>ge resi/ance 9 s]ies ) !              
curr5t-measur+ "p = ! reason ( :i* ,i   
told 9 ,lett] #h4 ,9 ! case ( ammet]s we
 sometimes let all ! curr5t g ?r\gh !   
curr5t-measur+ "p b g5]ally we let only 
a c]ta9 fraction ( x d s4 ,to pass ! re/
 ( ! curr5t we connect a small resi/ance
 9 p>allel ) ! measur+ "p4 ,9 bo? types 
( in/rum5ts ! resi/ances are sometimes  
hidd5 away "u ! cov]4 ,bo? in/rum5ts    
mu/1 ( c\rse1 be calibrat$ z ,i h       
expla9$ be=e4  ,9 ! electromagnetic     
in/rum5ts !re are sev]al ways ( mak+ !  
curr5t-measur+ "p4 ,! simple/ is to let 
! curr5t1 or "p ( x1 fl[ ?r\gh a coil   
:i* is pivot$ betwe5 ! ,n & ,s poles ( a
 /rong p]man5t magnet4 ,a spr+ keeps !  
coil 9 its z]o position & if ! curr5t   

makes ! coil turn x mu/ d s aga9/ ?is   
spr+4 ,! /rong] ! curr5t 9 ! coil !     
great] ! int]action ( ! loops ( ! coil &
 ^? ( ! iron atoms & h5ce ! fur!r ! coil
 w turn4 ,a po9t] atta*$ to ! coil      
indicates h[ f>2 & ! numb] ( volts or   
amp]es is read (f f ! calibrat$ scale4  
,su* in/rum5ts measure direct-curr5ts1 t
 is1 /eady /reams ( electrons 9 "o      
direction4 ,to measure an alt]nat+      
curr5t or voltage we c use a hot-wire   
in/rum5t or "o ( sev]al diff]5t types ( 
electromagnetic in/rum5ts4 ,p]haps !    
simple/ ( ^! is ! s-call$ plung] type4  
,! alt]nat+ curr5t fl[s 9 a coil2 & a   
piece ( s(t iron is s pivot$ t x c be   
attract$ & mov$ into ! coil4 ,s(t iron  
does n make a good p]man5t magnet4 ,if y
 put a piece ( x inside a coil :i* is   
c>ry+ a /eady curr5t x becomes a magnet 
b ab\t z soon z y int]rupt ! curr5t !   
atomic loops ( ! iron /op pull+ toge!r4 
,almo/ imm$iately !y turn into all sorts
 ( positions & =m little self-satisfi$  
gr\ps :i* don't take any int]e/ 9 !     

outside _w4 7,t isn't true ( /eel1 :]e !
 atomic loops are h>d] to turn & to l9e 
up1 b are mu* m likely to /ay 9 _! new  
positions47  ,because ! plung] 9 an     
alt]nat+-curr5t ammet] is s(t iron its  
loops l9e up ) ^? ( ! coil no matt] :i* 
way ! electron /ream happ5s to be go+ 9 
! coil4 ,! atomic magnets 9 ! iron turn 
ar\nd ea* "t ! curr5t rev]ses & !y are  
always1 !re=e1 l9$ up s t ! plung] is   
attract$4 ,if ! plung] has mu* in]tia or
 if ! oscillations ( ! curr5t are       
reasonably frequ5t ! plung] w n move    
back & =? ) ea* rev]sal ( ! curr5t b w  
take an av]age position4 ,! /rong] ! a-c
 7alt]nat+ curr5t7 ! f>!r inside ! coil 
w be ?is position ( ! plung]4 ,!        
position ( ! plung] becomes !n a measure
 ( ! /r5g? ( ! alt]nat+ curr5t4         
,in/rum5ts = measur+ alt]nat+ e4 m4 f4's
 & curr5ts1 read 9 volts & 9 amp]es4 ,s 
f> ,i hav5't /opp$ to tell :at we mean  
by "o amp]e ( alt]nat+ curr5t4 ,y "k f  
,lett] #g :at we mean by an amp]e ( d-c 
7direct curr5t74 ,x wasn't necess>y to  

expla9 be=e because ,i told y only (    
hot-wire in/rum5ts & !y w read ! same = 
ei!r d-c or a-c4  ,:5 !re is an alt]nat+
 curr5t 9 a wire ! electrons />t1 ru%   
ahead1 /op1 ru% back1 /op1 & d x all ov]
 aga9 & aga94 ,t heats ! wire 9 :i* x   
happ5s4 ,if an alt]nat+ /ream (         
electrons1 :i* are do+ ?is sort ( ?+1   
heats a wire j exactly z mu* z w\ld a   
d-c ( "o amp]e1 !n we say t ! a-c has an
 effective value ( "o amp]e4 ,( c\rse "p
 ( ! "t ( ea* cycle ! /ream is l>g] ?an 
an amp]e b = "p x is less4 ,if ! av]age 
heat+ effect is ! same ! a-c is said to 
be "o amp]e4  ,9 ! same way1 if a /eady 
e4 m4 f4 7a d-c e4 m4 f47 ( "o volt w   
heat a wire to :i* x is appli$ a c]ta9  
am\nt & if an alt]nat+ e4 m4 f4 w h !   
same heat+ effect 9 ! same "t1 !n ! a-c 
e4 m4 f4 is said to be "o volt4  ,ano!r 
electromagnetic in/rum5t :i* we h       
discuss$ b ( :i* m %\ld be said is !    
iron-cor$ trans=m]4 ,we consid] fir/ :at
 happ5s 9 "o ( ! coils ( ! trans=m]4  ,!
 inductance ( a coil is v mu* high] if x

 has an iron core4 ,! reason is t !n !  
coil acts z if x _h an enorm\sly l>g]   
numb] ( turns4 ,all ! atomic loops ( !  
core add _! effects to ! loops ( ! coil4
 ,:5 ! curr5t />ts x mu/ l9e up a lot ( 
^! atomic loops4 ,:5 ! curr5t /ops & ^! 
loops turn back into "s ( _! old        
self-satisfi$ gr\p+s1 !y affect !       
electrons 9 ! coil4 ,:]e fir/ !y oppos$ 
! motion ( ^! electrons1 n[ !y insi/ on 
its be+ cont9u$ = a mom5t long]4 ,i'll  
prove t by describ+ two simple          
exp]im5ts2 & !n we'll h ! basis =       
und]/&+ ! effect ( an iron core 9 a     
trans=m]4  ,illu/ration3 ,fig #cc  ,look
 aga9 at ,fig4 #cc ( ,lett] #i :i* ,i am
 reproduc+ = conv5i5ce4 ,we consid]$    
only :at w\ld happ5 9 coil cd if a      
curr5t 0 />t$ 9 coil ab4 ,suppose in/ead
 ( plac+ ! coils z %[n 9 t figure !y are
 plac$ z 9 ,fig4 #ajh4 ,because !y are  
at "r angles !re w be no effect 9 cd :5 
! curr5t is />t$ 9 ab4 ,let ! curr5t fl[
 /eadily ?r\gh ab & !n sudd5ly turn !   
coils s t !y are aga9 p>allel z %[n by !

 dott$ positions4 ,we get ! same        
tempor>y curr5t 9 cd z we w\ld if we    
%\ld place ! coils p>allel & !n />t !   
curr5t 9 ab4  ,illu/ration3 ,fig #ajh   
,! o!r exp]im5t is ?is3 ,/>t+ ) ! coils 
l9$ up z 9 ! dott$ position ( ,fig4 #ajh
 & ! curr5t /eadily fl[+ 9 ab1 we       
sudd5ly turn !m into positions at "r    
angles to ea* o!r4 ,!re is ! same       
mom5t>y curr5t 9 cd z if we _h left !m  
l9$ up & _h op5$ ! swit* 9 ! circuit (  
ab4  ,illu/ration3 ,fig #aji  ,n[ we "k 
t ! atomic loops ( iron behave 9 ! same 
g5]al way z d loops ( wire :i* are c>ry+
 curr5ts4 ,let u replace ! coil ab by a 
magnet z %[n 9 ,fig4 #aji4 ,fir/ we />t 
) ! magnet at "r angles to ! coil cd4   
,sudd5ly we turn x into ! dott$ position
 ( t figure4 ,!re is ! same mom5t>y     
curr5t 9 cd z if we 7 /ill us+ ! coil ab
 in/ead ( a magnet4 ,if n[ we turn !    
magnet back to a position at "r angles  
to cd1 we obs]ve ! opposite direction ( 
curr5t 9 cd4 ,^! effects are m          
noticeable ! m rapidly we turn ! magnet4

 ,! same is true ( turn+ ! coil4  ,!    
exp]im5t ( turn+ ! magnet illu/rates j  
:at happ5s 9 ! case ( a trans=m] )1 an  
iron core except t in/ead ( turn+ !     
entire magnet ! little atomic loops d ! 
turn+ inside ! core4 ,9 ! second>y ( an 
iron-cor$ trans=m] ! induc$ curr5t is ! 
sum ( two curr5ts bo? 9 ! same direction
 at ea* in/ant4 ,"o curr5t is caus$ by !
 />t+ or /opp+ ( ! curr5t 9 ! prim>y4 ,!
 o!r curr5t is due to ! turn+ ( ! atomic
 loops ( ! iron atoms s t m ( !m l9e up 
) ! turns ( ! prim>y4 ,^! atomic loops1 
( c\rse1 are turn$ by ! curr5t 9 !      
prim>y4 ,!re are s _m ( !m1 h[ev]1 t !  
curr5t due to _! turn+ is usually ! m   
important "p ( ! total curr5t4  ,9 all  
trans=m]s ! effect is great] ! m rapidly
 ! curr5t *anges direction & ! atomic   
loops turn ar\nd4 ,= ! same size (      
electron /ream 9 ! prim>y1 !re=e1 !re is
 induc$ 9 ! second>y a great] e4 m4 f4 !
 great] is ! frequ5cy ) :i* ! prim>y    
curr5t alt]nates4  ,:]e high frequ5cies 
are dealt ) x isn't necess>y to h iron  

cores because ! effect is l>ge en\gh )\t
 ! help ( ! atomic loops4 ,& ev5 if we  
want$ _! help x w\ldn't be easy to      
obta91 = !y dislike to turn s fa/ & x   
takes a lot ( p[] to make !m d s4 ,we "k
 t fact because we "k t an iron core    
increases ! inductance & s *okes !      
curr5t4 ,= l[ frequ5cies1 h[ev]1 t is ^?
 frequ5cies 9 ! audio range1 x is       
usually necess>y to h iron cores s z to 
get en\gh effect )\t too _m turns (     
wire4  ,! fact t iron decreases !       
inductance & s s]i\sly imp$es alt]nat+  
curr5ts leads u to use iron-core coils  
:]e we want high inductance4 ,su* coils 
are usually call$ *oke coils or ret>d   
coils4 ,( _! use we %all see m 9 a lat] 
lett] :]e we /udy radio-telephone       
transmitt]s4     ,,lett] #ba  ,,y\r     
,,receiv+ ,,set ,,& ,,h[ ,,to ,,exp]im5t
   ,,my ,,de> ,,/ud5t3  ,9 ?is lett] ,i 
want to tell y h[ to exp]im5t ) radio   
app>atus4 ,! fir/ rule is ?is3 ,/>t ) a 
simple circuit1 nev] add any?+ to x     
until y "k j :y y are do+ s1 & d n box x

 up 9 a cab9et until y "k h[ x is work+ 
& :y4  ,y\r ant5na at ! />t _h bett] be 
a s+le wire ab\t #be feet high & ab\t   
#ge feet long4 ,?is ant5na w h capacity 
( ab\t #j4jjja m4 f4 ,if y want an      
ant5na ( two wires spac$ ab\t ?ree feet 
ap>t ,i w\ld make x ab\t #ge feet long4 
,br+ d[n a lead f ea* wire1 twi/+ !m    
into a pigtail to act l "o wire except  
ne> ! horizontal "p ( ! ant5na4         
,illu/ration3 ,fig #aaj  ,y\r gr\nd     
connection c g to a wat] pipe4 ,to      
protect ! h\se & y\r app>atus f lightn+ 
ins]t a fuse & a little c>bon block     
lightn+ arre/] su* z are us$ by !       
telephone company 9 _! in/allations (   
h\se phones4 ,y c also use a s-call$    
vacuum lightn+ arre/]4 ,9 ei!r case !   
connections w be z %[n 9 ,fig4 #aaa4 ,if
 y use a loop ant5na1 ( c\rse1 no arre/]
 is ne$$4  ,at fir/ ,i w\ld plan to     
receive signals betwe5 #aej met]s & #cfj
 met]s4 ,?is w include ! amateurs :o "w 
betwe5 #afj & #bjj m41 ! special        
amateurs :o s5d ,c-,w telegraph at #bge 

m41 & ! broadca/+ /ations :i* op]ate at 
#cfj m4 ,?is range w give y pl5ty to    
li/5 to :ile y are exp]im5t+4 ,9        
addition y w get "s %ip signals at #cjj 
m4  ,illu/ration3 ,fig #aaa  ,to tune ! 
ant5na to any ( ! wave l5g?s 9 ?is range
 y c use a coil ( #ge turns w\nd on a   
c>dbo>d tube ( ?ree & a half in*es 9    
diamet]4 ,y c w9d ?is coil ( b>e wire if
 y are c>eful1 w9d+ a ?read along ) !   
wire s z to keep ! successive turns     
sep>at$4 ,9 t case y w ne$ to con/ruct a
 slid+ contact = x4 ,t is ! simple/ =m (
 tun]4  ,on ! o!r h& y c w9d ) s+le silk
 cov]$ wire & br+ out taps at ! #j1 #b1 
#d1 #f1 #h1 #aj1 #ad1 #bj1 #bh1 #cf1    
#dd1 #ef1 #ff1 & #ge? turns4 ,to make a 
tap drill a small hole ?r\gh ! tube1 b5d
 ! wire into a loop ab\t a foot long &  
pull ?is loop ?r\gh ! hole z %[n 9 ,fig4
 #aaj4 ,!n give ! wire a twi/1 z %[n1 s 
t x c't pull out1 & proce$ ) y\r w9d+4  
,use #bf s4 s4 c4 wire4 ,y w ne$ ab\t   
#hj feet & might buy #bjj to h en\gh = !
 second>y coil4 ,make contacts to ! taps

 by two rot>y swit*es z %[n 9 ,fig4     
#aab4 ,y c buy swit* arms & contacts    
/uds or a complete swit* m\nt$ on a     
small panel ( "s insulat+ comp\nd4 ,let 
swit* s#a make ! contacts = taps betwe5 
#ad & #ge turns1 & let swit* s#b make ! 
o!r contacts4  ,= ! second>y coil use ! 
same size ( wire & ( core4 ,w9d #fj     
turns1 br++ out a tap at ! middle4 ,to  
tune ! second>y circuit y w ne$ a       
v>iable cond5s]4 ,y c buy "o ( ! small  
ones ) a maximum capacity ( ab\t #j4jjjc
 mf41 "o ( ! l>g] ones ) a maximum      
capacity ( #j4jjje mf41 or ev5 ! l>g]   
size :i* has a maximum capacity ( #j4jja
 mf4 ,i %\ld pref] ! "o ( #j4jjje mf4   
,y w ne$ a cry/al detector--,i %\ld try 
gal5a fir/--& a s-call$ cat's :isk] )   
:i* to make contact ) ! gal5a4 ,= ^!    
p>ts & = ! swit* m5tion$ above y c %op  
ar\nd to advantage4 ,= telephone        
receiv]s ,i w\ld buy a really good pair 
) a resi/ance ( ab\t #bejj ohms4 ,buy   
also a small mica cond5s] ( #j4jjb mf4 =
 a block+ cond5s]4 ,y\r entire outfit w 

!n look z 9 ,fig4 #aab4 ,! swit* ,s is a
 small knife swit*4  ,to op]ate1 leave !
 swit* ,s op51 place ! prim>y & second>y
 coils ne> toge!r z 9 ! figure & li/54  
,! tun+ is v>i$1 :ile y li/51 by mov+ ! 
slid] ( ! slide-wire tun] or by mov+ !  
swit*es if y h connect$ y\r coil = t    
me?od4 ,make l>ge *anges 9 ! tun+ by    
v>y+ ! swit* s#a & !n turn sl[ly ?r\gh  
all positions ( s#b1 li/5+ at ea*       
position4  ,illu/ration3 ,fig #aab  ,:5 
a signal is he>d adju/ to ! position (  
s#a & s#b :i* gives ! l\de/ signal & !n 
clos+ ,s />t to tune ! second>y circuit4
 ,to d ?is1 v>y ! capacity ( ! cond5s] 9
 ! second>y circuit4 ,don't *ange !     
prim>y tun+ until y h tun$ ! second>y & 
c get ! signal ) good volume1 t is l\d4 
,y w want to v>y ! position ( ! prim>y &
 second>y coils1 t is1 v>y _! c\pl+1 = y
 w get %>p] tun+ z !y are drawn f>!r    
ap>t4 ,%>p] tun+ means less int]f]5ce f 
o!r /ations :i* are s5d+ on wave l5g?s  
ne> t :i* y wi% to receive4 ,r$uce !    
c\pl+1 !re=e1 & !n readju/ ! tun+4 ,x w 

usually be necess>y to make a slight    
*ange 9 bo? circuits1 9 "o case ) swit* 
s#a & 9 ! o!r ) ! v>iable cond5s]4  ,z  
soon z y c id5tify any /ation :i* y he> 
s5d+ make a note ( ! position ( !       
swit*es s#a & s#b1 & ( ! sett+ ( !      
cond5s] 9 ! second>y circuit4 ,9 t way y
 w acquire in=mation z to ! prop]       
adju/m5ts to receive c]ta9 wave-l5g?s4  
,?is is calibrat+ y\r set by ! kn[n     
wave-l5g?s ( di/ant /ations4  ,aft]     
le>n+ to receive ) ?is simple set ,i    
%\ld recomm5d buy+ a good audion tube4  
,ask ! sell] to supply y ) a blue pr9t (
 ! *>act]i/ic#aa ( ! tube tak5 "u !     
conditions ( filam5t curr5t & plate     
voltage :i* he recomm5ds = its use4 ,buy
 a /orage batt]y & a small slide-wire   
rheo/at1 t is v>iable resi/ance1 to use 
9 ! filam5t circuit4 ,buy also a bank ( 
dry batt]ies ( ! prop] voltage = ! plate
 circuit ( ! tube4 ,at ! same "t y %\ld 
buy ! prop] design ( trans=m] to g      
betwe5 ! plate circuit ( y\r tube & !   
pair ( receiv]s :i* y h4 ,x w usually be

 advisable to ask ! deal] to %[ y a     
*>act]i/ic curve = ! trans=m]1 :i* w    
indicate h[ well ! trans=m] op]ates at !
 diff]5t frequ5cies 9 ! audio range4 ,x 
%\ld op]ate v ne>ly ! same = all        
frequ5cies betwe5 #bjj & #bejj cycles4  
,! next /ep is to le>n to use ! tube z a
 detector4 ,connect x into y\r second>y 
circuit in/ead ( ! cry/al detector4 ,use
 ! prop] value ( ,c-batt]y z det]m9$ f  
y\r /udy ( ! *>act]i/ic ( ! tube4 ,"o or
 two small dry cells1 :i* h b9d+-po/    
t]m9als are conv5i5t ,c-batt]ies4 ,if y 
?9k y w ne$ a voltage mu* diff]5t f t   
obta9$ ) a :ole numb] ( batt]ies y c    
arrange to supply ! grid z we did 9     
,fig4 #hf ( ,lett] #ah4 ,9 t case y c   
use a few feet ( #cj ,g]man-silv] wire &
 make connections to x ) a susp5d] clip4
 ,le>n to receive ) ! tube & be         
p>ticul>ly c>eful n to let ! filam5t h  
too mu* curr5t & burn out4  ,n[ buy "s m
 app>atus4 ,y w ne$ a grid cond5s] (    
ab\t #j4jjjb mf4 ,! grid leaks to g ) x 
y c make = y\rself4 ,i w\ld use a piece 

( br[n wrapp+ pap] & two little metal   
eyelets4 ,! eyelets c be pun*$ into !   
pap]4 ,betwe5 !m coat ! pap] ) c>bon    
ink1 or ) lead p5cil m>ks4 ,a l9e ab\t  
an in* long ought to s]ve nicely4 ,y w  
probably wi% to make sev]al grid leaks  
to try4 ,:5 y get satisfactory op]ation 
9 receiv+ by ! grid-cond5s] me?od ! leak
 w probably be some:]e betwe5 a megohm &
 two megohms4  ,= ?is me?od y w n want a
 ,c-batt]y1 b y w wi% to op]ate !       
detector ) ab\t z high a voltage z !    
manufactur]s w recomm5d = ! plate       
circuit4 ,9 ?is way ! incom+ signal1 :i*
 decreases ! plate curr5t1 c produce !  
l>ge/ decrease4 ,x is also possible to  
/>t ) ! grid slightly positive in/ead ( 
be+ z negative z x is :5 connect$ to !  
negative t]m9al ( ! ,a-batt]y4 ,!re w !n
 be possible a great] *ange 9 grid      
voltage4 ,to d s connect ! grid z 9     
,fig4 #aae to ! positive t]m9al ( !     
,a-batt]y4  ,illu/ration3 ,fig #aac     
,ab\t ?is "t ,i w\ld %op ar\nd = two or 
?ree small d\ble-pole d\ble-?r[ swit*es4

 ,^? ( ! #e-;amp]e size w d4 ,) ^! y c  
arrange to make comp>isons betwe5       
diff]5t me?ods ( receiv+4 ,suppose1 =   
example1 y connect ! swit*es z %[n 9    
,fig4 #aac s t by ?r[+ !m to ! left y   
are us+ ! audion & to ! "r ! cry/al z a 
detector4 ,y c li/5 = a m9ute 9 "o      
position & !n swit* & li/5 = a m9ute 9 !
 o!r position1 & s on back & =?4 ,t way 
y c tell :e!r or n y really are gett+   
bett] results4  ,if y want a r\gh       
measure ( h[ mu* bett] ! audion is ?an !
 cry/al y might see1 :ile y are li/5+ to
 ! audion1 h[ mu* y c rob ! telephone   
receiv] ( its curr5t & /ill he> z well z
 y d :5 y swit* back to ! cry/al4 ,!    
easie/ way to d ?is is to put a v>iable 
resi/ance across ! receiv] z %[n 9 ,fig4
 #aac4 ,adju/ ?is resi/ance until !     
int5sity ( ! signal :5 detect$ by !     
audion is ! same z = ! cry/al4 ,y adju/ 
?is v>iable resi/ance until x by-passes 
s mu* ( ! curr5t1 :i* =m]ly w5t ?r\gh ! 
receiv]1 t ! audibility ( ! signal is   
r$uc$ until x is ! same z = ! cry/al    

detector4 ,c>efully made resi/ances =   
su* a purpose are sold "u ! "n (        
audibility met]s4 ,y c assemble a       
resi/ance :i* w d fairly well if y w buy
 a small rheo/at :i* w give a resi/ance 
v>y+ by /eps ( t5 ohms f z]o to "o      
hundr$ ohms4 ,at ! same "t y c buy f\r  
resi/ance spools ( "o hundr$ ohms ea* & 
p]haps "o ( #ejj ohms4 ,! spools ne$ n  
be v exp5sive = y d n ne$ c>efully      
adju/$ resi/ances4 ,assemble !m s z to  
make a rheo/at ) a range ( #j-ajjj ohms 
by /eps ( #aj ohms4 ,! *eape/ way to    
m\nt is ) ,fahne/ock clips z illu/rat$ 9
 ,fig4 #aad4 ,aft] a :ile1 h[ev]1 y w   
probably wi% to m\nt !m 9 a box ) a     
rot>y swit* on top4  ,illu/ration3 ,fig 
#aad  ,to /udy ! effect ( ! grid cond5s]
 y c arrange swit*es s z to ins]t ?is   
cond5s] & its leak & at ! same "t to cut
 out ! ,c-batt]y4 ,fig4 #aae %[s h[4 ,y 
c measure ! ga9 9 audibility at ! same  
"t4  ,illu/ration3 ,pl4                 
,x4--,audio-frequ5cy ,trans=m] &        
,bank$-w\nd ,coil4 7,c\rtesy ( ,pac5t   

,electric ,co47  ,illu/ration3 ,fig #aae
  ,aft] le>n+ to use ! audion z a       
detector1 bo? by virtue ( its curv$     
*>act]i/ic & by ! grid-cond5s] me?od1 ,i
 w\ld sugge/ /udy+ ! same tube z an     
amplifi]4 ,fir/ ,i w\ld le>n to use x z 
an audio-frequ5cy amplifi]4 ,set up !   
cry/al detector circuit4 ,use y\r       
audio-frequ5cy trans=m] ! o!r way ar\nd 
s z to /ep up to ! grid4 ,put !         
telephone 9 ! plate circuit4 ,*oose y\r 
,c-batt]y = amplification & n detection 
& try to receive4  ,y w get bett]       
results if y c af=d ano!r iron-core     
trans=m]4 ,if y c1 buy "o :i* w "w      
betwe5 ! plate circuit ( "o vacuum tube 
& ! grid circuit ( ano!r simil> tube4   
,!n y w h ! "r equipm5t :5 y come to    
make a two-/age audio-frequ5cy amplifi]4
 ,if y buy su* a trans=m] use ! o!r     
trans=m] betwe5 plate & telephones z y  
did be=e & ins]t ! new "o z %[n 9 ,fig4 
#aaf4 ,?is circuit also %[s h[ y c      
connect ! swit*es s z to see h[ mu* !   
audion is amplify+4  ,illu/ration3 ,fig 

#aaf  ,! next /ep is to use ! audion z  
an amplifi] ( ! radio-signal be=e its   
detection4 ,use ! prop] ,c-batt]y = an  
amplifi]1 z det]m9$ f ! blue pr9t ( !   
tube *>act]i/ic4 ,connect ! tube z %[n 9
 ,fig4 #aag4 ,y w see t 9 ?is circuit we
 are us+ a *oke coil to keep !          
radio-frequ5cy curr5t out ( ! batt]y "p 
( ! plate circuit & a small cond5s]1    
ano!r "o ( #j4jjb mf41 to keep ! batt]y 
curr5t f ! cry/al detector4 ,y c see f !
 same figure h[ y c arrange ! swit*es s 
z to f9d :e!r or n y are gett+ any ga9 f
 ! amplifi]4  ,n[ y are ready to receive
 ^? ,c-,w s5d]s at #bge met]s4 ,y w ne$ 
to w9d ano!r coil l ! second>y coil y   
already h4 ,"h is :]e y buy ano!r       
cond5s]4 ,y w ne$ x lat]4 ,if be=e y    
b\ght ! #j4jjje size1 ?is "t buy !      
#j4jja size or vice v]sa4 ,w9d also a   
small coil = a tickl]4 ,ab\t #bj turns (
 #bf wire on a core ( #c-a_/b 94 diamet]
 w d4 ,connect ! tickl] 9 ! plate       
circuit ( ! audion4 ,connect to ! grid  
y\r new coil & cond5s] & set ! audion   

circuit s t x w induce a curr5t 9 !     
second>y circuit :i* supplies ! cry/al4 
,fig4 #aah %[s ! hook-up4  ,illu/ration3
 ,fig #aag  ,y w see t y are n[ supply+ 
! cry/al ) curr5t f two s\rces1 namely !
 di/ant s\rce ( ! incom+ signals & !    
local oscillator :i* y h =m$4 ,! cry/al 
w detect ! beat note betwe5 ^! two      
curr5ts4  ,to receive ! #bge met]s      
signals y w ne$ to make sev]al adju/m5ts
 at ! same "t4 ,9 ! fir/ place ,i w\ld  
set ! tun+ ( ! ant5na circuit & ( !     
cry/al circuit ab\t :]e y ?9k "r because
 ( y\r k ( ! sett+s = o!r wave l5g?s4   
,!n ,i w\ld get ! local oscillator go+4 
,y c tell :e!r or n x is go+ if y       
sudd5ly increase or decrease ! c\pl+    
betwe5 ! tickl] coil & ! input circuit (
 ! audion4 ,if ?is motion is accompani$ 
by a click 9 ! receiv]s ! tube is       
oscillat+4  ,illu/ration3 ,fig #aah  ,n[
 y mu/ *ange ! frequ5cy at :i* x is     
oscillat+ by sl[ly *ang+ ! capacity 9 ! 
tun$ input circuit ( ! tube4 ,unless !  
ant5na circuit is prop]ly tun$ to ! #bge

 met] signal y w get no results4 ,if x  
is1 y w he> an int]mitt5t musical note =
 "s tune ( y\r local oscillator4 ,?is   
note w h ! duration ( dots & da%es4  ,y 
w h to keep *ang+ ! tun+ ( y\r detector 
circuit & ( ! ant5na4 ,= ea* new sett+ v
 sl[ly sw+ ! cond5s] plates 9 !         
oscillator circuit & see if y get a     
signal4 ,x w probably be easi] to use ! 
/&-by position1 :i* ,i h describ$1 )    
swit* ,s op5 9 ! second>y circuit (     
,fig4 #aah4 ,9 t case y h only to tune  
y\r ant5na to #bge met]s & !n y w pick  
up a note :5 y\r local oscillator is 9  
tune4 ,aft] y h done s y c tune !       
second>y circuit :i* supplies ! cry/al4 
,if y adopt ?is me?od y w want a close  
c\pl+ betwe5 ! ant5na & ! cry/al        
circuit4 ,y w always want a v weak c\pl+
 betwe5 ! oscillator circuit & !        
detector circuit4 ,y w also probably    
want a weak] c\pl+ betwe5 tickl] & tube 
input ?an y are at fir/ incl9$ to       
believe w be en\gh4 ,pati5ce & "s skill 
9 manipulation is always requir$ = ?is  

sort ( exp]im5t4  ,:5 y h complet$ ?is  
exp]im5t 9 het]odyne receiv+1 us+ a     
local oscillator1 y are ready to try !  
reg5]ative circuit4 ,?is has be5        
illu/rat$ 9 ,fig4 #ib ( ,lett] #ah &    
ne$s no fur!r description4 ,y w h !     
advantage :5 y come to ?is ( kn[+ v     
closely ! prop] sett+s ( ! ant5na       
circuit & ! second>y tun$ circuit4 ,y w 
ne$ !n only to adju/ ! c\pl+ ( ! tickl] 
& make f9] adju/m5ts 9 y\r tun+4  ,aft] 
y h complet$ ?is s]ies ( exp]im5ts y w  
be some?+ ( an adept at radio & are 9 a 
position to plan y\r f9al set4 ,= ?is   
set y w ne$ to pur*ase c]ta9 p>ts       
complete f reputable deal]s because _m (
 ! circuits :i* ,i h describ$ are pat5t$
 & %\ld n be us$ except z rights to use 
are obta9$ by ! pur*ase ( lic5s$        
app>atus :i* embodies ! pat5t$ circuits4
 ,kn[+ h[ radio receiv]s op]ate & :y1 y 
are n[ 9 a good condition to discuss )  
deal]s ! relative m]its & co/s ( receiv+
 sets4  ,illu/ration3 ,fig #aai  ,be=e y
 actually buy a complet$ set y may want 

to increase ! range ( frequ5cy ov] :i* y
 are c>ry+ out y\r exp]im5ts4 ,to       
receive at long] wave-l5g?s y w ne$ to  
increase ! inductance ( y\r ant5na s t x
 w be tun$ to a l[] frequ5cy4 ,?is is   
usually call$ load+ & c be done by      
ins]t+ a coil 9 ! ant5na4 ,to obta9     
small] wave-l5g?s decrease ! effective  
capacity ( ! ant5na circuit by putt+    
ano!r cond5s] 9 s]ies ) ! ant5na4       
,usually1 !re=e1 "o connects into 8     
ant5na circuit bo? a cond5s] & a load+  
coil4 ,by us+ a v>iable cond5s] !       
effective capacity ( ! ant5na sy/em may 
be easily *ang$4 ,! result is t ?is     
s]ies cond5s] me?od becomes ! easie/    
me?od ( tun+ & ! slide wire tun] is n   
ne$$4 ,fig4 #aai %[s ! circuit4  ,= q a 
range ( wave-l5g?s we may use ! same    
load+ coil & tune ! ant5na circuit      
entirely by ?is s]ies cond5s]4 ,= "s o!r
 range ( wave-l5g?s we %all !n ne$ a    
diff]5t load+ coil4 ,9 a well-design$   
set ! wave-l5g? ranges ov]lap4 ,!       
calculation ( ! size ( load+ coil is q  

easy b requires m ari?metic ?an ,i c>e  
to impose on y at pres5t4 ,i %all !re=e 
m]ely give y illu/rations bas$ on !     
assumption t y\r ant5na has a capacity (
 #j4jjja or ( #j4jjjb mf4 & t ! cond5s]s
 :i* y h b\ght are #j4jjje & #j4jja = _!
 maxima4  ,9 ,table ,i !re is giv51 =   
ea* ( sev]al values ( ! inductance ( !  
prim>y coil1 ! %orte/ & ! longe/        
wave-l5g?s :i* y c expect to receive4 ,!
 table is 9 two p>ts1 ! fir/ = an ant5na
 ( capacity #j4jjja mf4 & ! second = "o 
( #j4jjjb mf4 ,y\rs w be some:]e betwe5 
^! two limits4 ,! %orte/ wave-l5g?      
dep5ds ^u ! ant5na & n ^u ! cond5s] :i* 
y use 9 s]ies ) x = tun+4 ,x also dep5ds
 ^u h[ mu* inductance !re is 9 ! coil   
:i* y h 9 ! ant5na circuit4 ,! table    
gives values ( inductance 9 ! fir/      
column1 & ( m9imum wave-l5g? 9 ! second4
 ,! ?ird column %[s :at is ! greate/    
wave-l5g? y may expect if y use a tun+  
cond5s] ( #j4jjje mf42 & ! f\r? column !
 slightly l>ge wave-l5g? :i* is possible
 ) ! l>g] cond5s]4                      

,,table ,i         ,"p #a4 7,= ant5na ( 
#j4jjja mf47      ,inductance 9  ,%orte/
 wave-l5g?   ,longe/ wave-l5g? 9 met]s  
mil-h5ries4        9 met]s4       )     
#j4jjje mf4   ) #j4jja mf4              
#j4aj               #ajc                
#afi             #agi         #j4bj     
#adf                  #bch              
#bec         #j4dj               #bjg   
#ccg             #ceh         #j4he     
#cjj                  #dij              
#eae         #a4hj               #djj   
#gjj             #gfj         #b4jj     
#dbj                  #gej              
#hjj         #d4jj               #fjj   
#ajhj            #aacj         #e4jj    
#ffj                 #abjj              
#abfj        #aj4jj               #ijj  
#agjj            #agij        #cj4jj    
#afjj                 #bijj             
#cajj                     ,"p #b4 7,=   
ant5na ( #j4jjjb mf47          #j4aj    
#afi                  #bbe              
#bdj         #j4af               #baj   
#bhe             #cje         #j4bj     

#bdj                  #cbj              
#cdj         #j4be               #bgj   
#cee             #chj         #j4dj     
#cdj                  #dej              
#dhj         #j4fj               #dbj   
#eej             #eij         #j4hj     
#dhj                  #fcj              
#fhj         #a4bj               #ehe   
#gge             #hdj         #a4hj     
#gbj                  #iej              
#ajbj         #c4jj               #icj  
#abbj            #acbj         #e4jj    
#abjj                 #afjj             
#agjj         #h4jj              #aejj  
#bjjj            #baej        #ab4jj    
#ahej                 #bdjj             
#bfej        #af4jj              #baej  
#bhjj            #cjej  ,f ,table ,i y c
 f9d h[ mu* inductance y w ne$ 9 !      
prim>y circuit4 ,a c]ta9 am\nt y w ne$  
to c\ple ! ant5na & ! second>y circuit4 
,! coil :i* y w\nd at ! beg9n+ ( y\r    
exp]im5ts w d well = t4 ,any?+ m 9 ! way
 ( inductance1 :i* ! ant5na circuit     
requires to give a desir$ wave-l5g?1 y  

may consid] z load+4 ,9 ,table ,,ii are 
"s data z to w9d+ coils on /raight cores
 to obta9 v>i\s values ( inductance4    
,y\r #bf s4 s4 c4 wire w w9d ab\t #ed   
turns to ! in*4 ,i h assum$ t y w h ?is 
numb] ( turns p] in* on y\r coils &     
calculat$ ! inductance :i* y %\ld get = 
v>i\s numb]s ( total turns4 ,! fir/ "p (
 ! table is = a core ( #c4e in*es 9     
diamet] & ! second "p = "o ( #e in*es4  
,! fir/ column gives ! inductance 9     
mil-h5ries4 ,! second gives numb] (     
turns4 ,! ?ird & f\r? are m]ely =       
conv5i5ce & give ! approximate l5g? 9   
in*es ( ! coil & ! approximate total    
l5g? ( wire :i* is requir$ to w9d x4 ,i 
h all[$ = br++ out taps4 ,9 o!r words   
#eej feet ( ! wire w w9d a coil ( #aj4b 
in*es ) an inductance ( #h4jj           
mil-h5ries1 & p]mit y to br+ out taps at
 all ! l[] values ( inductance :i* are  
giv5 9 ! table4                         
,table ,,ii                     ,"p #a4 
7,= a core ( #c4e 94 diam47             
,inductance 9       ,numb]              

,l5g?         ,feet ( wire              
mil-h5ries4       ( turns4           9  
in*es4       requir$4         #j4aj     
#be                 #j4df               
#be         #j4af               #cd     
#j4fc             #cf         #j4bj     
#ci                 #j4gb               
#db         #j4be               #dd     
#j4ha             #di         #j4dj     
#eh                 #a4jg               
#fc         #j4fj               #ge     
#a4ch             #hj         #j4hj     
#ib                 #a4gj               
#ajj         #j4he               #if    
#a4gh            #ajd         #a4jj     
#ajh                 #b4jj              
#aah         #a4bj              #abc    
#b4bh            #acc         #a4hj     
#afd                 #c4jc              
#agf         #b4jj              #ahj    
#c4cc            #aij         #c4jj     
#bdb                 #d4dh              
#bej         #d4jj              #cjd    
#e4fb            #caj         #e4jj     
#cff                 #f4gg              

#cgj         #h4jj              #eej    
#aj4bj            #eej                  
,"p #b4 7,= core ( #e4j 94 diam47       
#b4jj              #abj                 
#b4bb            #afj         #c4jj     
#aeh                 #b4ic              
#bae         #d4jj              #aid    
#c4eh            #bfe         #e4jj     
#bbh                 #d4bb              
#caj         #h4jj              #cbd    
#f4jj            #dej        #aj4jj     
#chd                 #g4aj              
#ecj        #ab4jj              #dej    
#h4cj            #fbe  ,! coil :i* y    
w\nd at ! beg9n+ ( y\r exp]im5t _h only 
#ge turns & 0 tapp$ s t y c\ld1 by      
manipulat+ ! two swit*es ( ,fig4 #aab1  
get small v>iations 9 inductance4 ,9    
,table ,,iii is giv5 ! values ( !       
inductance :i* is controll$ by ! swit*es
 ( t figure1 ! correspond+ numb] (      
turns1 & ! wave-l5g? to :i* ! ant5na    
%\ld !n be tun$4 ,i am giv+ ?is = two   
values ( ant5na capacity1 z ,i h done   
be=e4 ,by ! aid ( ^! ?ree tables y %\ld 

h small difficulty 9 tak+ c>e ( matt]s (
 tun+ = all wave-l5g?s bel[ ab\t #cjjj  
met]s4 ,if y want to get long] waves ?an
 t y _h bett] buy a few bank$-w\nd      
coils4 ,^! are coils 9 :i* ! turns are  
w\nd ov] ea* o!r b 9 su* a way z to     
avoid 9 l>ge "p ! capacity effects :i*  
usually accompany su* w9d+4 ,y c try    
w9d+ !m = y\rself b ,i d\bt if !        
exp]i5ce has mu* value until y h gone   
f>!r 9 ! /udy ( ! ma!matical !ory (     
radio ?an ?is s]ies ( lett]s w c>ry y4  
,,table ,,iii                           
,circuit ( ,fig4 #aab     ,numb]        
,inductance 9     ,wave l5g? ) ant5na ( 
( turns4    mil-h5ries4      #j4jjja mf4
      #j4jjjb mf4       #ad             
#j4jd            #abj              #agj 
#bj            #j4jg            #afj    
#bbj       #bh            #j4ab         
#baj              #bij       #cf        
#j4ah            #bej              #cfj 
#dd            #j4be            #cjj    
#dbj       #ef            #j4ch         
#cgj              #ebj       #ge        

#j4fj            #dfj              #fej 
,9 ! second>y circuit !re is only "o    
capacity1 t ( ! v>iable cond5s]4 ,if x  
has a range ( values f ab\t #j4jjjje mf4
 to #j4jjje mf4 y\r coil ( #fj turns &  
#j4db mf4 p]mits a range ( wave-l5g?s f 
#bgj to #hfj m4 ,us+ half ! coil ! range
 is #aej to #dhj m4 ,) ! l>g] cond5s] ! 
ranges are respectively #bgj to #abbj & 
#bgj to #fgj4 ,= long] wave-l5g?s load )
 inductance4 ,f\r times ! inductance w  
tune to d\ble ^! wave-l5g?s4  ,footnote 
#aa3 ,if y c af=d to buy1 or if y c     
borr[1 ammet]s & voltmet]s ( ! prop]    
range y %\ld take ! *>act]i/ic y\rself4 
,,lett] #bb  ,,high-,,p[]$              
,,radio-,,telephone ,,transmitt]s   ,,my
 ,,de> ,,exp]im5t]3  ,?is lett] is to   
summ>ize ! op]ations :i* mu/ be p]=m$ 9 
radio-telephone transmission &          
reception2 & also to describe ! circuit 
( an important comm]cial sy/em4  ,to    
transmit spee* by radio ?ree op]ations  
are necess>y4 ,fir/1 !re mu/ be g5]at$ a
 high-frequ5cy alt]nat+ curr5t2 second1 

?is curr5t mu/ be modulat$1 t is1 v>i$ 9
 int5sity 9 accordance ) ! human voice2 
& ?ird1 ! modulat$ curr5t mu/ be suppli$
 to an ant5na4 ,= effici5t op]ation1 (  
c\rse1 ! ant5na mu/ be tun$ to !        
frequ5cy :i* is to be transmitt$4 ,!re  
is also a f\r? op]ation :i* is usually  
p]=m$ & t is amplification4 ,:]ev] !    
electrical effect is small] ?an desir$1 
or requir$ = satisfactory transmission1 
vacuum tubes are us$ z amplifi]s4 ,( ?is
 ,i %all give y an illu/ration lat]4    
,?ree op]ations are also ess5tial 9     
receiv+4 ,fir/1 an ant5na mu/ be s      
arrang$ & tun$ z to receive en]gy f !   
di/ant transmitt+ /ation4 ,!re is !n 9 !
 receiv+ ant5na a curr5t simil> 9 wave  
=m to t 9 ! transmitt+ ant5na4 ,second1 
! spee* significance ( ?is curr5t mu/ be
 detect$1 t is1 ! modulat$ curr5t mu/ be
 demodulat$4 ,a curr5t is !n obta9$ :i* 
has ! same wave =m z ! human voice :i* 0
 ! cause ( ! modulation at ! di/ant     
/ation4 ,! ?ird op]ation is p]=m$ by a  
telephone receiv] :i* makes ! molecules 

( air 9 its neighborhood move back & =? 
9 accordance ) ! detect$ curr5t4 ,z y   
already "k a f\r? op]ation may be c>ri$ 
on by amplifi]s :i* give on _! output   
sides curr5ts ( great] /r5g? b ( ! same 
=ms z !y receive at _! input t]m9als4   
,9 transmitt+ & 9 receiv+ equipm5t two  
or m ( ^! op]ations may be p]=m$ by !   
same vacuum tube z y w rememb] f our    
discussion ( ! reg5]ative circuit =     
receiv+4 ,= example1 also1 9 any receiv+
 set ! vacuum tube :i* detects is       
usually amplify+4 ,9 ! reg5]ative       
circuit = receiv+ cont9u\s waves by !   
het]odyne me?od ! vacuum tube functions 
z a g5]ator ( high-frequ5cy curr5t & z a
 detector ( ! v>iations 9 curr5t :i*    
occur because ! locally-g5]at$ curr5t   
does n keep 9 /ep ) t g5]at$ at !       
transmitt+ /ation4  ,ano!r example ( a  
vacuum tube p]=m+ simultane\sly two     
diff]5t functions is illu/rat$ 9 ,fig4  
#abj :i* %[s a simple radio-telephone   
transmitt]4 ,! s+le tube p]=ms 9 itself 
bo? ! g5]ation ( ! radio-frequ5cy curr5t

 & its modulation 9 accordance ) !      
output ( ! c>bon-button transmitt]4 ,?is
 audion is 9 a fe$-back circuit1 !      
oscillation frequ5cy ( :i* dep5ds ^u !  
cond5s] ,c & ! inductance ,l4 ,! voice  
drives ! diaphragm ( ! transmitt] & ?us 
v>ies ! resi/ance ( ! c>bon button4 ,?is
 v>ies ! curr5t f ! batt]y1 ,ba1 ?r\gh !
 prim>y1 ,t#a1 ( ! trans=m] ,t4 ,!      
result is a v>y+ voltage appli$ to !    
grid by ! second>y ,t#b4 ,! oscillat+   
curr5t 9 ! plate circuit ( ! audion     
v>ies accord+ly because x is dep5d5t ^u 
! grid voltage4 ,! cond5s] ,cr (f]s a l[
 imp$ance to ! radio-frequ5cy curr5t to 
:i* ! w9d+ ,t#b ( audio-frequ5cy        
trans=m] (f]s too mu*4  ,illu/ration3   
,fig #abj  ,9 ?is case ! tube is bo?    
g5]ator & modulator4 ,9 "s cases ^!     
op]ations are sep>ately p]=m$ by diff]5t
 tubes4 ,?is 0 true ( ! transmitt+ set  
us$ 9 #aiae :5 ! eng9e]s ( ! ,bell      
,telephone ,sy/em talk$ by radio f      
,arl+ton1 ne> ,wa%+ton1 ,d4 ,c41 to     
,p>is & ,honolulu4 ,i %all n draw out   

completely ! circuit ( _! app>atus b ,i 
%all describe x by us+ little squ>es to 
repres5t ! p>ts responsible = ea* ( !   
sev]al op]ations4  ,fir/ !re 0 a vacuum 
tube oscillator :i* g5]at$ a small      
curr5t ( ! desir$ frequ5cy4 ,!n !re 0 a 
telephone transmitt] :i* made v>iations 
9 a direct-curr5t fl[+ ?r\gh ! prim>y ( 
a trans=m]4 ,! e4 m4 f4 f ! second>y (  
?is trans=m] & ! e4 m4 f4 f !           
radio-frequ5cy oscillator 7 bo? impress$
 ^u ! grid ( an audion :i* act$ z a     
modulator4 ,! output ( ?is audion 0 a   
radio-frequ5cy curr5t modulat$ by !     
voice4 ,! output 0 amplifi$ by a        
two-/age audion amplifi] & suppli$ ?r\gh
 a c\pl+ coil to ! l>ge ant5na ( ! ,u4  
,s4 ,navy ,/ation at ,arl+ton4 ,fig4    
#aba %[s ! sy/em4  ,illu/ration3 ,fig   
#aba  ,! audion amplifi]s ea* consi/$ ( 
a numb] ( tubes op]at+ 9 p>allel4 ,:5   
tubes are op]at$ 9 p>allel !y are       
connect$ z %[n 9 ,fig4 #abb s t ! same  
e4 m4 f4 is impress$ on all ! grids & ! 
same plate-batt]y voltage on all !      

plates4 ,z ! grids v>y 9 voltage !re is 
a correspond+ v>iation ( curr5t 9 !     
plate circuit ( ea* tube4 ,! total *ange
 ( ! curr5t 9 ! plate-batt]y circuit is1
 !n1 ! sum ( ! *anges 9 all !           
plate-filam5t circuits ( ! tubes4 ,?is  
s*eme ( connections gives a result      
equival5t to t ( a s+le tube ) a        
correspond+ly l>g] plate & filam5t4     
,illu/ration3 ,fig #abb  ,p>allel       
connection is necess>y because a s+le   
tube w\ld be ov]heat$ 9 deliv]+ to !    
ant5na ! desir$ am\nt ( p[]4 ,y rememb] 
t :5 ! audion is op]at$ z an amplifi] ! 
resi/ance to :i* x supplies curr5t is   
made equal to its own int]nal resi/ance 
( ,rp4 ,t means t !re is 9 ! plate      
circuit j z mu* resi/ance inside ! tube 
z outside4 ,h5ce !re is ! same am\nt (  
"w done ea* second 9 =c+ ! curr5t ?r\gh 
! tube z ?r\gh ! ant5na circuit1 if t is
 :at ! tube supplies4 ,"w p] second is  
p[]2 ! plate batt]y is sp5d+ en]gy 9 !  
tube at ! same rate z x is supply+ x to 
! ant5na :]e x is useful = radiation4   

,illu/ration3 ,pl4 ,,xi4--,broadca/+    
,equipm5t1 ,develop$ by ! ,am]ican      
,telephone & ,telegraph ,company & !    
,we/]n ,electric ,company4  ,all ! en]gy
 exp5d$ 9 ! tube appe>s z heat4 ,x is   
due to ! bl[s :i* ! electrons /rike     
aga9/ ! plate :5 !y are drawn across f !
 filam5t4 ,^! impacts set into m rapid  
motion ! molecules ( ! plate2 & !       
temp]ature ( ! tube rises4 ,!re is a    
limit to ! am\nt ! temp]ature c rise )\t
 de/roy+ ! tube4 ,= t reason ! heat     
produc$ inside x mu/ n exce$ a c]ta9    
limit dep5d+ ^u ! design ( ! tube & !   
me?od ( cool+ x z x is op]at$4 ,9 !     
,arl+ton exp]im5ts1 :i* ,i m5tion$ a    
mom5t ago1 ! tubes 7 cool$ by bl[+ air  
on !m f fans4  ,we c f9d ! p[] exp5d$ 9 
! plate circuit ( a tube by multiply+ ! 
numb] ( volts 9 its batt]y by ! numb] ( 
amp]es :i* fl[s4 ,suppose ! batt]y is   
#bej volts & ! curr5t #j4jb amp]es1 !n !
 p[] is #e watts4 ,! watt is ! unit =   
measur+ p[]4 ,tubes are rat$ by ! numb] 
( watts :i* c be safely exp5d$ 9 !m4 ,y 

might ask1 :5 y buy an audion1 :at is a 
safe rat+ = x4 ,! "q w n be an important
 "o1 h[ev]1 unless y are to set up a    
transmitt+ set s9ce a detector is       
usually op]at$ ) su* small plate-voltage
 z n to h exp5d$ 9 x an am\nt ( p[]     
dang]\s to its life4  ,9 rec5t          
transmitt+ sets ! tubes are us$ 9       
p>allel = ! reasons ,i h j told1 b a    
diff]5t me?od ( modulation is us$4 ,!   
g5]ation ( ! radio-frequ5cy curr5t is by
 l>ge-p[]$ tubes :i* are op]at$ ) high  
voltages 9 _! plate circuits4 ,! output 
( ^! oscillators is suppli$ to ! ant5na4
 ,! int5sity ( ! oscillations ( ! curr5t
 9 ^! tubes is controll$ by *ang+ !     
voltage appli$ 9 _! plate circuits4 ,y c
 see f ,fig4 #abc t if ! plate voltage  
is *ang$ ! /r5g? ( ! alt]nat+ curr5t is 
*ang$ accord+ly4 ,x is ! me?od us$ 9    
*ang+ ! voltage :i* is p>ticul>ly       
int]e/+4  ,illu/ration3 ,fig #abc  ,!   
high voltages :i* are us$ 9 ! plate     
circuits ( ^! high-p[]$ audions are     
obta9$ f g5]ators in/ead ( batt]ies4 ,y 

rememb] f ,lett] #bj t an e4 m4 f4 is   
induc$ 9 a coil :5 ! coil & a magnet are
 sudd5ly *ang$ 9 _! positions1 "o be+   
turn$ ) ref]5ce to ! o!r4 ,a g5]ator is 
a ma*9e = turn+ a coil s t a magnet is  
always induc+ an e4 m4 f4 9 x4 ,x is =m$
 by an armature c>ry+ coils & by /rong  
electromagnets4 ,! ma*9e c be driv5 by a
 /eam or gas eng9e1 by a wat] :eel1 or  
by an electric motor4 ,g5]ators are     
design$ ei!r to give /eady /reams (     
electrons1 t is = d-c curr5ts1 or to act
 z alt]nators4  ,illu/ration3 ,fig #abd 
,suppose we h1 z %[n 9 ,fig4 #abd1 a d-c
 g5]ator supply+ curr5t to a vacuum tube
 oscillator4 ,! curr5t f ! g5]ator      
passes ?r\gh an iron-cor$ *oke coil1    
m>k$ ,la 9 ! figure4 ,betwe5 ?is coil & 
! plate circuit we connect across ! l9e 
a telephone transmitt]4 ,to make a sy/em
 :i* w "w effici5tly we %all h to       
suppose t ?is transmitt] has a high     
resi/ance1 say ab\t ! same z ! int]nal  
resi/ance1 ,rp1 ( ! tube & also t x c   
c>ry z l>ge a curr5t4  ,( ! curr5t :i*  

comes f ! g5]ator ab\t "o-half goes to !
 tube & ! re/ to ! transmitt]4 ,if !    
resi/ance ( ! transmitt] is increas$ x  
c't take z mu* curr5t4 ,! coil1 ,la1    
h[ev]1 because ( its inductance1 t5ds to
 keep ! same am\nt ( curr5t fl[+ ?r\gh  
itself4 ,= j an in/ant !n ! curr5t 9 ,la
 keeps /eady ev5 ?\gh ! transmitt]      
doesn't take its %>e4 ,! result is m    
curr5t = ! oscillat+ tube4 ,on ! o!r h& 
if ! transmitt] takes m curr5t1 because 
its resi/ance is decreas$1 ! *oke coil1 
,la1 w mom5t>ily t5d to keep ! curr5t   
/eady s t :at ! transmitt] takes mu/ be 
at ! exp5se ( ! oscillat+ tube4  ,t's "o
 way ( look+ at :at happ5s4 ,we "k1     
h[ev]1 f ,fig4 #abc t to get an increase
 9 ! amplitude ( ! curr5t 9 ! oscillat+ 
tube we mu/ apply an increas$ voltage to
 its plate circuit4 ,t is :at really    
happ5s :5 ! transmitt] increases 9      
resi/ance & s doesn't take its full %>e 
( ! curr5t4 ,! reason is ?is3 ,:5 !     
transmitt] resi/ance is increas$ !      
curr5t 9 ! transmitt] decreases4 ,j = a 

mom5t x looks z ?\gh ! curr5t 9 ,la is  
go+ to decrease4 ,t's ! way x looks to !
 electrons2 & y "k :at electrons d 9 an 
inductive circuit :5 !y ?9k !y %all h to
 /op4 ,!y induce ea* o!r to keep on = a 
mom5t4 ,= a mom5t !y act j z if !re 0 "s
 extra e4 m4 f4 :i* 0 act+ to keep !m   
go+4 ,we say1 !re=e1 t !re is an extra  
e4 m4 f41 & we call ?is an e4 m4 f4 (   
self-induction4 ,all ?is "t !re has be5 
active on ! plate circuit ( ! tube ! e4 
m4 f4 ( ! g5]ator4 ,to ?is !re is add$  
at ! in/ant :5 ! transmitt] resi/ance   
increases1 ! e4 m4 f4 ( self-induction 9
 ! coil1 ,la & s ! total e4 m4 f4 appli$
 to ! tube is mom5t>ily increas$4 ,?is  
increas$ e4 m4 f41 ( c\rse1 results 9 an
 increas$ amplitude = ! alt]nat+ curr5t 
:i* ! oscillator is supply+ to !        
transmitt+ ant5na4  ,:5 ! transmitt]    
resi/ance is decreas$1 & a l>g] curr5t  
%\ld fl[ ?r\gh ! *oke coil1 ! electrons 
are ask$ to spe$ up 9 go+ ?r\gh ! coil4 
,at fir/ !y object & dur+ t in/ant !y   
express _! objection by an e4 m4 f4 (   

self-induction :i* opposes ! g5]ator    
voltage4 ,= an in/ant1 !n1 ! voltage ( !
 oscillat+ tube is l[]$ & its           
alt]nat+-curr5t output is small]4       
,illu/ration3 ,fig #abe  ,= ! purpose ( 
br++ ab\t su* ?reat5$ *anges 9 curr5t1 &
 h5ce su* e4 m4 f4's ( self-induction1 !
 c>bon transmitt] is n suitable because 
x has too small a resi/ance & too small 
a curr5t c>ry+ ability4 ,! plate circuit
 ( a vacuum tube w s]ve admirably4 ,y "k
 f ! audion *>act]i/ic t )\t *ang+ !    
plate voltage we c1 by apply+ a voltage 
to ! grid1 *ange ! curr5t ?r\gh ! plate 
circuit4 ,n[ if x 0 a wire resi/ance )  
:i* we 7 deal+ & we %\ld be able to     
obta9 a *ange 9 curr5t )\t *ang+ !      
voltage act+ on ?is wire we w\ld say t  
we _h *ang$ ! resi/ance4 ,we c say1     
!re=e1 t ! int]nal resi/ance ( ! plate  
circuit ( a vacuum tube c be *ang$ by   
:at we d to ! grid4  ,9 ,fig4 #abe ,i h 
sub/itut$ ! plate circuit ( an audion = 
! transmitt] ( ,fig4 #abd & arrang$ to  
v>y its resi/ance by *ang+ ! pot5tial ( 

! grid4 ,?is we d by impress+ ^u ! grid 
! e4 m4 f4 develop$ 9 ! second>y ( a    
trans=m]1 to ! prim>y ( :i* is connect$ 
a batt]y & a c>bon transmitt]4 ,! curr5t
 ?r\gh ! prim>y v>ies 9 accordance ) !  
s\nds spok5 into ! transmitt]4 ,& = all 
! reasons :i* we h j f9i%$ /udy+ !re are
 simil> v>iations 9 ! output curr5t ( ! 
oscillat+ tube 9 ! transmitt+ set (     
,fig4 #abe4  ,9 ?is latt] figure y w    
notice a small air-core coil1 ,l,r1     
betwe5 ! oscillator & ! modulator tube4 
,?is coil has a small inductance b x is 
en\gh to (f] a l>ge imp$ance to         
radio-frequ5cy curr5ts4 ,! result is1 x 
does n let ! alt]nat+ curr5ts ( !       
oscillat+ tube fl[ into ! modulator4 ,^!
 curr5ts are conf9$ to _! own circuit1  
:]e !y are useful 9 e/abli%+ simil>     
curr5ts 9 ! ant5na4 ,on ! o!r h&1 ! coil
 ,l,r doesn't s]i\sly imp$e l[-frequ5cy 
curr5ts & !re=e x does n prev5t         
v>iations 9 ! curr5t :i* are at         
audio-frequ5cy4 ,x does n int]f]e ) !   
*anges 9 curr5t :i* accompany !         

v>iations 9 ! resi/ance ( ! plate       
circuit ( ! modulator4 ,t is1 x has too 
little imp$ance to act l ,la & s x      
p]mits ! modulator to v>y ! output ( !  
oscillator4  ,illu/ration3 ,fig #abf  ,!
 oscillat+ circuit ( ,fig4 #abe includes
 "p ( ! ant5na4 ,x diff]s also f ! o!rs 
,i h %[n 9 ! mann] 9 :i* grid & plate   
circuits are c\pl$4 ,i'll expla9 by     
,fig4 #abf4  ,! transmitt+ set :i* ,i h 
j describ$ involves _m ( ! pr9ciples ( !
 mo/ mod]n sets4 ,if y und]/& its       
op]ation y c probably reason out =      
y\rself any ( ! o!r sets ( :i* y w he> f
 "t to "t4     ,,lett] #bc              
,,amplification ,,at ,,int]m$iate       
,,frequ5cies   ,,de> ,,son3  ,9 ! matt] 
( receiv+ ,i h already cov]$ all !      
important pr9ciples4 ,!re is "o m sy/em1
 h[ev]1 :i* y w ne$ to "k4 ,?is is spok5
 ( ei!r z ! sup]-het]odyne or z !       
int]m$iate-frequ5cy amplification me?od 
( reception4  ,! sy/em has two important
 advantages4 ,fir/1 x p]mits %>p] tun+ &
 s r$uces int]f]5ce f o!r radio signals4

 ,second1 x p]mits m amplification ( !  
incom+ signal ?an is usually            
practicable4  ,fir/ z to amplification3 
,we h se5 t amplification c be          
accompli%$ ei!r by amplify+ !           
radio-frequ5cy curr5t be=e detection or 
by amplify+ ! audio-frequ5cy curr5t :i* 
results f detection4 ,!re are practical 
limitations to ! am\nt ( amplification  
:i* c be obta9$ 9 ei!r case4 ,an        
effici5t multi-/age amplifi] =          
radio-frequ5cies is difficult to build  
because ( :at we call capacity effects4 
,consid] = example ! portion ( circuit  
%[n 9 ,fig4 #abg4 ,! wires a & b act l  
small plates ( cond5s]s4 ,:at we really 
h1 is a lot ( t9y cond5s]s :i* ,i h %[n 
9 ! figure by ! light dott$-l9es4 ,if ! 
wires are transmitt+ high-frequ5cy      
curr5ts ^! cond5s]s (f] t9y wait+-rooms 
:]e ! electrons c run 9 & out )\t hav+  
to g on to ! grid ( ! next tube4 ,!re   
are o!r difficulties 9 high-frequ5cy    
amplifi]s4 ,?is "o ( capacity effects   
betwe5 p>allel wires is en\gh = !       

pres5t4 ,x is p]haps ! mo/ int]e/+      
because x is always m or less tr\blesome
 :5ev] a pair ( wires is us$ to transmit
 an alt]nat+ curr5t4  ,illu/ration3 ,fig
 #abg  ,9 ! case ( a multi-/age amplifi]
 ( audio-frequ5cy curr5t !re is always !
 possibility ( ! amplification ( any    
small v>iations 9 curr5t :i* may        
naturally occur 9 ! action ( ! batt]ies4
 ,!re are always small v>iations 9 !    
curr5ts f batt]ies1 due to impurities 9 
! mat]ials ( ! plates1 air bubbles1 &   
o!r causes4 ,ord9>ily we don't obs]ve ^!
 *anges because !y are too small to make
 an audible s\nd 9 ! telephone receiv]s4
 ,suppose1 h[ev]1 t !y take place 9 !   
batt]y ( ! fir/ tube ( a s]ies (        
amplifi]s4 ,any t9y *ange ( curr5t is   
amplifi$ _m times & results 9 a         
tr\blesome noise 9 ! telephone receiv]  
:i* is connect$ to ! la/ tube4  ,9 bo?  
types ( amplifi]s !re is1 ( c\rse1      
always ! *ance t ! output circuit ( "o  
tube may be c\pl$ to & induce "s effect 
9 ! input circuit ( "o ( ! e>li] tubes (

 ! s]ies4 ,?is w be amplifi$ & result 9 
a great] induction4 ,9 o!r words1 9 a   
circuit :]e !re is l>ge amplification1  
!re is always ! difficulty ( avoid+ a   
fe$-back ( en]gy f "o tube to ano!r s t 
! entire gr\p acts l an oscillat+       
circuit1 t is reg5]atively4 ,mu* ( ?is  
difficulty c be avoid$ aft] exp]i5ce4   
,if a multi-/age amplifi] is to be built
 = a curr5t :i* does n h too high a     
frequ5cy ! capacity effects & ! o!r     
difficulties due to high-frequ5cy ne$ n 
be s]i\sly tr\blesome4 ,if ! frequ5cy is
 n too high1 b is /ill well above !     
audible limit1 ! noises due to v>iations
 9 batt]y curr5ts ne$ n bo!r = !y are ( 
q l[ frequ5cy4 ,curr5ts f #bj1jjj to    
#fj1jjj cycles a second are1 !re=e1 !   
mo/ satisfactory to amplify4  ,suppose1 
h[ev]1 "o wi%es to amplify ! signals f a
 radio-broadca/+ /ation4 ,! wave-l5g? is
 #cfj met]s & ! frequ5cy is ab\t        
#hcd1jjj cycles a second4 ,! sy/em (    
int]m$iate-frequ5cy amplification solves
 ! difficulty & we %all see h[ x does s4

  ,illu/ration3 ,fig #abh  ,at ! receiv+
 /ation a local oscillator is us$4 ,?is 
g5]ates a frequ5cy :i* is ab\t #cj1jjj  
cycles less ?an t ( ! incom+ signal4    
,bo? curr5ts are impress$ on ! grid ( a 
detector4 ,! result is1 9 ! output ( !  
detector1 a curr5t :i* has a frequ5cy ( 
#cj1jjj cycles a second4 ,! int5sity (  
?is detect$ curr5t dep5ds ^u ! int5sity 
( ! incom+ signal4 ,! beat note curr5t (
 #cj1jjj cycles v>ies1 !re=e1 9         
accordance ) ! voice :i* is modulat+ at 
! di/ant s5d+ /ation4 ,! spee*          
significance is n[ hidd5 9 a curr5t ( a 
frequ5cy int]m$iate betwe5 radio &      
audio4 ,?is curr5t may be amplifi$ _m   
times & !n suppli$ to ! grid ( a        
detector :i* obta9s f x a curr5t (      
audio-frequ5cy :i* has a spee*          
significance4 ,9 ,fig4 #abh ,i h        
indicat$ ! sev]al op]ations4  ,we c n[  
see :y ?is me?od p]mits %>p] tun+4 ,!   
:ole idea ( tun+1 ( c\rse1 is to arrange
 t ! incom+ signal %all cause ! l>ge/   
possible curr5t & at ! same "t to       

provide t any signals at o!r wave-l5g?s 
%all cause only negligible curr5ts4 ,:at
 we want a receiv+ set to d is to       
di/+ui% betwe5 two signals :i* diff]    
slightly 9 wave-l5g? & to respond to    
only "o ( !m4  ,suppose we set up a tun$
 circuit =m$ by a coil & a cond5s] & try
 x out = v>i\s frequ5cies ( signals4 ,y 
"k h[ x w respond f our discussion 9    
connection ) ! tun+ curve ( ,fig4 #ea ( 
,lett] #ac4 ,we might f9d f a numb] (   
su* te/s t ! be/ we c expect any tun$   
circuit to d is to discrim9ate betwe5   
signals :i* diff] ab\t t5 p]c5t 9       
frequ5cy1 t is1 to receive well ! desir$
 signal & to fail practically entirely  
to receive a signal ( a frequ5cy ei!r t5
 p]c5t high] or ! same am\nt l[]4  ,=   
example1 if ! signal is at #cj1jjj      
cycles a tun$ circuit might be expect$  
to discrim9ate aga9/ an int]f]+ signal (
 #cc1jjj4 ,if ! signal is at #cjj1jjj   
cycles a tun$ circuit might discrim9ate 
aga9/ an int]f]+ signal ( #ccj1jjj      
cycles1 b an int]f]5ce at #cjc1jjj      

cycles w\ld be v tr\blesome inde$4 ,x   
c\ldn't be tun$ out at all4  ,n[ suppose
 t ! desir$ signal is at #cjj1jjj cycles
 & t !re is int]f]5ce at #cjc1jjj       
cycles4 ,we provide a local oscillator (
 #bgj1jjj cycles a second1 receive by   
?is sup]-het]odyne me?od :i* ,i h j     
describ$1 & s obta9 an int]m$iate       
frequ5cy4 ,9 ! output ( ! fir/ detector 
we h !n a curr5t ( #cjj1jjj--bgj1jjj or 
#cj1jjj cycles due to ! desir$ signal & 
also a curr5t ( #cjc1jjj--bgj1jjj or    
#cc1jjj cycles due to ! int]f]5ce4 ,bo? 
^! curr5ts we c supply to ano!r tun$    
circuit :i* is tun$ = #cj1jjj cycles a  
second4 ,x c receive ! desir$ signal b x
 c discrim9ate aga9/ ! int]f]5ce because
 n[ ! latt] is t5 p]c5t (f ! tune ( !   
signal4  ,y see ! "q is n "o ( h[ f>    
ap>t two signals are 9 numb] ( cycles p]
 second4 ,! "q always is3 ,h[ l>ge 9    
p]c5t is ! diff]5ce betwe5 ! two        
frequ5cies8 ,! matt] ( sep>at+ two      
effects ( diff]5t frequ5cies is a "q ( !
 int]val betwe5 ! frequ5cies4 ,to f9d ! 

int]val betwe5 two frequ5cies we divide 
"o by ! o!r4 ,y c see t if ! quoti5t is 
l>g] ?an #a4a or small] ?an #j4i !      
frequ5cies diff] by t5 p]c5t or m4 ,!   
high] ! frequ5cy ! l>g] ! numb] ( cycles
 :i* is repres5t$ by a giv5 size (      
int]val4  ,:ile ,i am writ+ ( frequ5cy  
int]vals ,i want to tell y "o ?+ m (    
importance4 ,y rememb] t 9 human spee*  
!re may ent]1 & be necess>y1 any        
frequ5cy betwe5 ab\t #bjj & #bjjj cycles
 a second4 ,t we might call ! range ( ! 
necess>y notes 9 ! voice4 ,:5ev] we want
 a good reproduction ( ! voice we mu/   
reproduce all ! frequ5cies 9 ?is range4 
,suppose we h a radio-curr5t ( #ajj1jjj 
cycles modulat$ by ! frequ5cies 9 !     
voice range4 ,we f9d 9 ! output ( our   
transmitt+ set n only a curr5t (        
#ajj1jjj cycles b curr5ts 9 two o!r     
ranges ( frequ5cies4 ,"o ( ^! is above !
 signal frequ5cy & ext5ds f #ajj1bjj to 
#ajb1jjj cycles4 ,! o!r is ! same am\nt 
bel[ & ext5ds f #ih1jjj to #ii1hjj      
cycles4 ,we say !re is an upp] & a l[]  

b& ( frequ5cies4  ,all ^! curr5ts are 9 
! complex wave :i* comes f !            
radio-transmitt]4 ,= ?is /atem5t y w h  
to take my ^w until y c h&le ! =m (     
ma!matics kn[n z trigonometry4 ,:5 we   
receive at ! di/ant /ation we receive n 
only curr5ts ( ! signal frequ5cy b also 
curr5ts ^: frequ5cies lie 9 ^! side-b&s4
  ,no matt] :at radio-frequ5cy we may   
use we mu/ transmit & receive side-b&s (
 ?is range if we use ! app>atus ,i h    
describ$ 9 ! pa/ lett]s4 ,y c see :at t 
means4 ,suppose we transmit at a        
radio-frequ5cy ( #ej1jjj cycles &       
modulate t ) spee*4 ,we %all really ne$ 
all ! range f #dh1jjj cycles to #eb1jjj 
cycles = "o telephone message4 ,on ! o!r
 h& if we modulat$ a #ejj1jjj cycle wave
 by spee* ! side-b&s are f #dih1jjj to  
#dii1hjj & #ejj1bjj to #ejb1jjj cycles4 
,if we transmit at #ej1jjj cycles1 t is1
 at #fjjj met]s1 we really ne$ all !    
range betwe5 #eggj met]s & #fbej met]s1 
z y c see by ! frequ5cies ( ! side-b&s4 
,at #ajj1jjj cycles we ne$ only ! range 

( wave-l5g?s betwe5 #bidj m4 & #cjfj m4 
,if ! radio-frequ5cy is #ejj1jjj cycles 
we ne$ a /ill small] range ( wave-l5g?s 
to transmit ! necess>y side-b&s4 ,!n !  
range is f #eih m4 to #fjc m4  ,9 ! case
 ( ! transmission ( spee* by radio we   
are int]e/$ 9 hav+ no int]f]5ce f o!r   
signals :i* are )9 #bjjj cycles ( !     
frequ5cy ( our radio-curr5t no matt] :at
 _! wave-l5g?s may be4 ,! "p ( !        
wave-l5g? range :i* mu/ be kept cle> f  
int]f]+ signals becomes small] ! high] !
 frequ5cy :i* is be+ modulat$4  ,y c see
 t v few telephone messages c be s5t 9 !
 long-wave-l5g? "p ( ! radio range & _m 
m1 al?\gh n v _m aft] all1 9 ! %ort     
wave-l5g? "p ( ! radio range4 ,y c also 
see :y x is desirable to keep amateurs 9
 ! %ort wave-l5g? "p ( ! range :]e m (  
!m c transmit simultane\sly )\t int]f]+ 
) ea* o!r or ) comm]cial radio /ations4 
,!re is ano!r reason1 too1 = keep+      
amateurs to ! %orte/ wave-l5g?s4        
,transmission ( radio signals ov] %ort  
di/ances is be/ accompli%$ by %ort      

wave-l5g?s b ov] long di/ances by !     
long] wave-l5g?s4 ,= trans-oceanic "w ! 
v longe/ wave-l5g?s are be/4 ,!         
long-haul /ations1 !re=e1 "w 9 !        
frequ5cy range imm$iately above #aj1jjj 
cycles a second & transmit ) wave l5g?s 
( #cj1jjj m4 & %ort]4  ,illu/ration3    
,pl4 ,,xii4--,broadca/+ ,/ation ( !     
,am]ican ,telephone & ,telegraph        
,company on ! ,ro( ( ! ,walk]-,lisp5>d  
,bldg4 9 ,new ,york ,city ,:]e !        
,long-di/ance ,telephone ,l9es ,t]m9ate4
     ,,lett] #bd  ,,by ,,wire ,,& ,,by  
,,radio   ,,de> ,,boy3  ,! simple/ wire 
telephone-circuit is =m$ by a           
transmitt]1 a receiv]1 a batt]y1 & !    
connect+ wire4 ,if two p]sons are to    
c>ry on a conv]sation ea* mu/ h ?is     
am\nt ( equipm5t4 ,! app>atus might be  
arrang$ z 9 ,fig4 #abi4 ,?is set-up1    
h[ev]1 requires f\r wires betwe5 ! two  
/ations & y "k ! telephone company uses 
only two wires4 ,let u f9d ! pr9ciple ^u
 :i* its sy/em op]ates because x is !   
solution ( _m diff]5t problems includ+ t

 ( wire-to-radio connections4           
,illu/ration3 ,fig #abi  ,imag9e f\r    
wire resi/ances connect$ toge!r to =m a 
squ>e z 9 ,fig4 #acj4 ,suppose !re are  
two pairs ( equal resi/ances1 namely    
,r#a & ,r#b1 & ,z#a & ,z#b4 ,if we      
connect a g5]ator1 ,g1 betwe5 !         
junctions a & b !re w be two sep>ate    
/reams ( electrons1 "o ?r\gh ! ,r-side &
 ! o!r ?r\gh ! ,z-side ( ! circuit4 ,^! 
/reams1 ( c\rse1 w n be ( ! same size = 
! l>g] /ream w fl[ ?r\gh ! side :i* (f]s
 ! small] resi/ance4  ,illu/ration3 ,fig
 #acj  ,half ! e4 m4 f4 betwe5 a & b is 
us$ up 9 s5d+ ! /ream half ! di/ance4   
,half is us$ betwe5 a & ! po9ts c & d1 &
 ! o!r half betwe5 c & d & ! o!r end4 ,x
 doesn't make any diff]5ce :e!r we foll[
 ! /ream f a to c or f a to d1 x takes  
half ! e4 m4 f4 to keep ?is /ream go+4  
,po9ts c & d1 !re=e1 are 9 ! same       
condition ( be+ half-way electrically f 
a to b4 ,! result is t !re c be no      
curr5t ?r\gh any wire :i* we connect    
betwe5 c & d4  ,suppose1 !re=e1 t we    

connect a telephone receiv] betwe5 c &  
d4 ,no curr5t fl[s 9 x & no s\nd is     
emitt$ by x4 ,n[ suppose ! resi/ance (  
,z#b is t ( a telephone l9e :i* /ret*es 
f "o telephone /ation to ano!r4 ,suppose
 also t ,z#a is a telephone l9e exactly 
l ,z#b except t x doesn't g any:]e at   
all because x is all %ut up 9 a little  
box4 ,we'll call ,z#a an artificial     
telephone l9e4 ,we ought to call x1 z   
little *ildr5 w\ld say1 a make-believe  
telephone l9e4 ,x doesn't fool u b x    
does fool ! electrons = !y c't tell !   
diff]5ce betwe5 ! real l9e ,z#b & !     
artificial l9e ,z#a4 ,we c make a v good
 artificial l9e by us+ a cond5s] & a    
resi/ance4 ,! cond5s] introduces some?+ 
( ! capacity effects :i* ,i told y 7    
always pres5t 9 a circuit =m$ by a pair 
( wires4  ,illu/ration3 ,fig #aca  ,at !
 o!r telephone /ation let u duplicate   
?is app>atus1 us+ ! same real l9e 9 bo? 
cases4 ,in/ead ( j any g5]ator ( an     
alt]nat+ e4 m4 f4 let u use a telephone 
transmitt]4 ,we connect ! transmitt]    

?r\gh a trans=m]4 ,! sy/em !n looks l t 
( ,fig4 #aca4 ,:5 "s "o talks at /ation 
#a !re is no curr5t ?r\gh 8 receiv]     
because x is connect$ to c & d1 :ile !  
e4 m4 f4 ( ! transmitt] is appli$ to a &
 b4 ,! transmitt] sets up two electron  
/reams betwe5 a & b1 & ! /ream :i* fl[s 
?r\gh ! ,z-side ( ! squ>e goes out to   
/ation #b4 ,at ?is /ation ! electrons h 
?ree pa?s betwe5 d & b4 ,i h m>k$ ^! by 
arr[s & y see t "o ( !m is ?r\gh !      
receiv]4 ,! curr5t :i* is />t$ by !     
transmitt] at /ation #a w !re=e op]ate !
 receiv] at /ation #b b n at its own    
/ation4 ,( c\rse /ation #b c talk to #a 
9 ! same way4  ,! actual set-up us$ by !
 telephone company is a little diff]5t f
 t :i* ,i h %[n because x uses a s+le   
common batt]y at a c5tral (fice betwe5  
two subscrib]s4 ,! g5]al pr9ciple1      
h[ev]1 is ! same4  ,illu/ration3 ,fig   
#acb  ,x won't make any diff]5ce if we  
use equal inductance coils1 in/ead ( !  
,r-resi/ances1 & connect ! transmitt] to
 !m inductively z %[n 9 ,fig4 #acb4 ,s  

f> z t is conc]n$ we c also use a       
trans=m] betwe5 ! receiv] & ! po9ts c & 
d1 z %[n 9 ! same figure4  ,illu/ration3
 ,fig #acc  ,we are n[ ready to put 9   
radio equipm5t at /ation #b4 ,9 place ( 
! telephone receiv] at /ation #b we     
connect a radio transmitt]4 ,!n :atev] a
 p]son at /ation #a says goes by wire to
 #b & on out by radio4 ,9 place ( !     
telephone transmitt] at /ation #b we    
connect a radio receiv]4 ,:atev] t      
receives by radio is detect$ & goes by  
wire to ! li/5] at /ation #a4 ,9 ,fig4  
#acc ,i h %[n ! equipm5t ( /ation #b4   
,!re y h ! connections = wire to radio &
 vice v]sa4  ,"o ( ! mo/ int]e/+        
developm5ts ( rec5t ye>s is t ( wir$    
wireless or c>ri]-curr5t telephony ov]  
wires4 ,suppose t in/ead ( broadca/+ f !
 ant5na at /ation #b we arrange to h its
 radio transmitt] supply curr5t to a    
wire circuit4 ,we use ?is same pair (   
wires = receiv+ f ! di/ant /ation4 ,we c
 d ?is if we treat ! radio transmitt] & 
receiv] exactly l ! telephone in/rum5ts 

( ,fig4 #acb & connect !m to a squ>e (  
resi/ances4 ,"o ( ^! resi/ances is1 (   
c\rse1 ! l9e betwe5 ! /ations4 ,i h %[n 
! g5]al arrangem5t 9 ,fig4 #acd4  ,y see
 :at ! squ>e ( resi/ances1 or bridge    
really does = u4 ,x lets u use a s+le   
pair ( wires = messages :e!r !y are com+
 or go+4 ,x does t because x lets u     
connect a transmitt] & also a receiv] to
 a s+le pair ( wires 9 su* a way t !    
transmitt] c't affect ! receiv]4 ,:atev]
 ! transmitt] s5ds out goes along !     
wires to ! di/ant receiv] b doesn't     
affect ! receiv] at ! s5d+ /ation4 ,?is 
bridge p]mits ?is :e!r ! transmitt] &   
receiv] are radio in/rum5ts or are !    
ord9>y telephone in/rum5ts4             
,illu/ration3 ,fig #acd  ,by its aid we 
may s5d a modulat$ high-frequ5cy curr5t 
ov] a pair ( wires & receive f ! same   
pair ( wires ! high-frequ5cy curr5t :i* 
is g5]at$ & modulat$ at ! di/ant end ( !
 l9e4 ,x lets u s5d & receive ov] ! same
 pair ( wires ! same sort ( a modulat$  
curr5t z we w\ld supply to an ant5na 9  

radio-telephone transmitt+4 ,x is ! same
 sort ( a curr5t b x ne$ n be any:]e ne>
 z l>ge because we ar5't broadca/+2 we  
are s5d+ directly to ! /ation ( ! o!r   
p>ty to our conv]sation4  ,if we        
duplicate ! app>atus we c use ! same    
pair ( wires = ano!r telephone          
conv]sation )\t int]f]+ ) ! fir/4 ,(    
c\rse1 we h to use a diff]5t frequ5cy ( 
alt]nat+ curr5t = ea* ( ! two           
conv]sations4 ,we c s5d ^! two diff]5t  
modulat$ high-frequ5cy curr5ts ov] !    
same pair ( wires & sep>ate !m by tun+  
at ! di/ant end j z well z we d 9 radio4
 ,i won't sket* out = y ! tun$ circuits 
by :i* ?is sep>ation is made4 ,x's en\gh
 to give y ! idea4  ,9 t way1 a s+le    
pair ( wires c be us$ = transmitt+1     
simultane\sly & )\t any int]f]5ce1      
sev]al diff]5t telephone conv]sations4  
,x takes v mu* less p[] ?an w\ld radio  
transmission & ! conv]sations are       
secret4 ,! ord9>y telephone conv]sation 
c g on at ! same "t )\t any int]f]5ce ) 
^? :i* are be+ c>ri$ by ! modulations 9 

high-frequ5cy curr5ts4 ,a total ( five  
conv]sations ov] ! same pair ( wires is 
! pres5t practice4  ,?is me?od is us$   
betwe5 _m ( ! l>ge cities ( ! ,u4 ,s4   
because x lets "o pair ( wires d ! "w ( 
five4 ,t means a sav+1 = copp] wire co/s
 money4 ,( c\rse1 all ! special app>atus
 also co/s money4 ,y c see1 !re=e1 t ?is
 me?od w\ldn't be economical betwe5     
cities v close toge!r because all t is  
sav$ by n hav+ to buy s mu* wire is sp5t
 9 build+ special app>atus & 9 tak+ c>e 
( x aft]w>ds4 ,= long l9es1 h[ev]1 by n 
hav+ to buy five times z mu* wire1 !    
,bell ,company saves m ?an x co/s to    
build & ma9ta9 ! extra special app>atus4
  ,i impli$ a mom5t ago :y ?is sy/em is 
call$ a c>ri]-curr5t sy/em2 x is because
 ! high-frequ5cy curr5ts c>ry 9 _!      
modulations ! spee* significance4       
,sometimes x is call$ a sy/em (         
multiplex telephony because x p]mits m  
?an "o message at a "t4  ,?is same g5]al
 pr9ciple is also appli$ to ! mak+ ( a  
multiplex sy/em ( telegraphy4 ,9 !      

multiplex telephone sy/em we pictur$    
transmitt+ & receiv+ sets v mu* l       
radio-telephone sets4 ,if in/ead (      
transmitt+ spee* ea* transmitt] 0 op]at$
 z a ,c-,w transmitt] !n x w\ld transmit
 telegraph messages4 ,9 ! same frequ5cy 
range !re c be m telegraph sy/ems op]at$
 simultane\sly )\t int]f]+ ) ea* o!r1 = 
y rememb] h[ _m cycles ea*              
radio-telephone message requires4 ,= t  
reason ! multiplex telegraph sy/em :i*  
op]ates by c>ri]-curr5ts p]mits z _m z  
t5 diff]5t telegraph messages           
simultane\sly4  ,y rememb] t ,i told y  
h[ capacity effects rob ! di/ant end ( a
 pair ( wires ( ! alt]nat+ curr5t :i* is
 be+ s5t to !m4 ,t is always true b !   
effect is n v great unless ! frequ5cy ( 
! alt]nat+ curr5t is high4 ,x's en\gh1  
h[ev]1 s t e few hundr$ miles x is      
necess>y to connect into ! circuit an   
audion amplifi]4 ,?is is true ( c>ri]   
curr5ts especially1 b also true ( !     
voice-frequ5cy curr5ts ( ord9>y         
telephony4 ,! latt]1 h[ev]1 are n       

weak5$1 t is1 att5uat$1 z mu* &         
consequ5tly d n ne$ to be amplifi$ z mu*
 to give good intelligibility at !      
di/ant receiv]4  ,illu/ration3 ,fig #ace
  ,9 a telephone circuit ov] su* a long 
di/ance z f ,new ,york ,city to ,san    
,francisco x is usual to ins]t amplifi]s
 at ab\t a doz5 po9ts along ! r\te4 ,(  
c\rse1 ^! amplifi]s mu/ "w =            
transmission 9 ei!r direction1 amplify+ 
spee* on its way to ,san ,francisco or 9
 ! opposite direction4 ,at ea* ( !      
amplify+ /ations1 or repeat] /ations1 z 
!y are usually call$1 two vacuum tube   
amplifi]s are us$1 "o = ea* direction4  
,to connect ^! ) ! l9e s t ea* may "w 9 
! "r direction !re are us$ two ( !      
bridges or resi/ance squ>es4 ,y c see f 
! sket* ( ,fig4 #ace h[ an alt]nat+     
curr5t f ! ea/ w be amplifi$ & s5t on to
 ! we/1 or vice v]sa4  ,illu/ration3    
,fig #acf  ,!re are a l>ge numb] ( su*  
repeat] /ations 9 ! ,unit$ ,/ates along 
! important telephone r\tes4 ,9 ,fig4   
#acf ,i am %[+ y ! location ( ^? along !

 r\te ( ! fam\s transcont95tal          
telephone-circuit4 ,?is %[s also a      
radio-telephone connection betwe5 ! coa/
 ( ,cali=nia & ,catal9a ,isl&4          
,conv]sations h be5 held betwe5 ?is isl&
 & a %ip 9 ! ,atlantic ,ocean1 z %[n 9 !
 sket*4 ,! conv]sation 0 made possible  
by ! use ( ! vacuum tube & ! bridge     
circuit4 ,"p ( ! way x 0 by wire & "p by
 radio4 ,wire & radio tie nicely toge!r 
because bo? op]ate on ! same g5]al      
pr9ciples & use mu* ( ! same app>atus4  
,blank ,page     ,,index      ,a-batt]y 
= tubes1 #db      ,accumulator1 #bi     
,acid1 action ( hydrog5 91 #g      ,air1
 con/itution (1 #aj      ,ammet]1       
alt]nat+ curr5t1 #bjf2       calibration
 (1 #ec2       con/ruction (1 #bje      
,amp]e1 #di1 #ed      ,amplification1   
#ahb2 "o /age (1 #ahe      ,amplitude ( 
vibration1 #aee      ,ant5na curr5t     
v>iation1 #ada      ,arl+ton te/s1 #bcc 
,artificial telephone l9e1 #beb         
,atom1 conception (1 #f2       nucleus  
(1 #aj2       neutral1 #cd      ,atomic 

numb]1 #ac      ,atoms1 diff]5ce betwe51
 #ab2       k9ds (1 #f1 #aj2            
motion (1 #ce      ,att5uation ( curr5t 
9 wires1 #bei      ,audibility met]1    
#bah      ,audio-frequ5cy amplifi]1     
#ahe2       limitations (1 #ahe         
,audion1 #ce1 #dj1 #db      ,audion1    
amplifi]1 #ahb2       detector1 !ory (1 
#abf2       modulator1 #bcb2            
oscillator1 !ory (1 #hi2       frequ5cy 
control (1 #ii      ,b-batt]y = tubes1  
#dc2       effect ^u *>act]i/ic1 #abh   
,bank$ w\nd coils1 #bbh      ,batt]y1   
con/ruction ( gravity1 #af2       dry1  
#bg2       rev]sible or /orage1 #bi     
,b& ( frequ5cies1 #bdi      ,beat note1 
detection (1 #bba1 #bde      ,bell      
sy/em1 ,arl+ton transmitt]1 #bdi        
,block+ ( tube1 reason =1 #aga          
,blue vitriol1 #af      ,bridge circuit1
 #bee      ,bureau ( ,/&>ds1 #ej        
,c-batt]y = tubes1 #df1 #aff2           
v>iation (1 #ge2       = detection1 #ff 
,calibration ( a receiv]1 #bad          
,capacity1 effect ^u frequ5cy1 #ajj2    

measurem5t (1 #ajd2       unit (1 #ajd2 
v>iable1 #ajg      ,capacity effects1   
#bdc2       elim9ation (1 #bbh          
,c>ri] curr5t1 modulation (1 #adf2      
telephony1 #bee      ,*>act]i/ic1 (     
vacuum tube1 #fh1 #gd2       effect (   
,b-batt]y ^u1 #abh2       h[ to plot a1 
#gj      ,*>act]i/ic curve ( trans=m]1  
#fd      ,*emi/ry1 #h      ,*oke coils1 
#baj1 #bba      ,circuit1 ,a1 ,b1 ,c1   
#ahg2       c\pl$1 #aae2       def9$1   
#dc2       oscillat+1 #aac2       plate1
 #de2       %ort1 #cj2       tune ( a1  
#aag      ,cond5s]1 def9$1 #gg2         
*>g+ curr5t (1 #gh2       dis*>ge curr5t
 (1 #hj2       imp$ance (1 #ace2        
!ory (1 #gh2       tun+1 #bbd           
,common batt]y sy/em1 #bed              
,connection = wire to radio1 #bed       
,cont9u\s waves1 #hf      ,copp]1 atomic
 numb] (1 #ac      ,copp] sulphate1 9   
solution1 #ba      ,cry/als1 atomic     
/ructure1 #adg      ,cry/al detectors1  
#adf2       *>act]i/ic (1 #adh2         
circuit (1 #aej2       !ory (1 #adg     

,curr5t1 transi5t1 #aad2       radio1   
#add      ,cycle1 #id1 #ig      ,damp$  
oscillations1 #aad      ,demodulation1  
#bca      ,detection1 expla9$1 #adf     
,detectors1 audion1 #abf2       cry/al1 
#adf      ,direct curr5ts1 #bje         
,dissociation1 #bb      ,di/ortion1 (   
wave =m1 #afc      ,dry batt]y1 #bg     
,e>?1 atomic con/itution1 #aa           
,effective value1 ( amp]e1 #bjg2       (
 volt1 #bjg      ,effici5cy1 (          
reg5]ative circuit1 #ahb                
,electrical *>ge1 #bb      ,electricity1
 curr5t (1 #ae1 #af      ,electrodes1 ( 
vacuum tube1 #da2       def9ition (1 #da
      ,electrolyte1 def9ition (1 #cd    
,electrons1 prop]ties (1 #d2            
planet>y1 #aj1 #ab2       rate ( fl[1   
#dh2       vapor (1 #ci2       w&]+ (1  
#ad      ,electron /reams1 laws (       
attraction1 #bjj      ,e4 ,m4 ,f41 #ei2 
alt]nat+1 #gf2       ( self-induction1  
#bch      ,en]gy1 exp5d$ 9 tube1 #bce2  
( electrons1 #aac2       radiation (1   
#abe      ,e!r1 #hh      ,fe$-back      

circuit1 #ahb      ,frequ5cy1 #ih1 #aeh2
       effect ^u pit*1 #acc2            
int]val1 #bdg2       natural1 #aag2     
( voice1 #afc      ,fundam5tal note1 (  
/r+1 #aeg      ,gravity batt]y1 !ory (1 
#bc      ,grid1 action (1 #dg2          
cond5s]1 #afi2       curr5t1 #agc2      
leak1 #aga2       leak1 con/ruction1    
#agb1 #baf2       ( audion1 #da         
,h>monics1 #afj      ,helium1 prop]ties 
(1 #i      ,h5ry1 #hc      ,het]odyne1  
#aha      ,hot-wire ammet]1 #ea         
,human voice1 me*anism (1 #aeb          
,hydrog51 action ( 9 acid1 #g2          
atom (1 #g      ,imp$ance1 ( coil1 #acf2
       ( cond5s]1 #acf2       (         
trans=m]1 #aie2       effect ( iron core
 ^u1 #bjg2       mat*+ (1 #aif          
,int]m$iate-frequ5cy amplification1 #bdb
      ,inductance1 def9$1 #hc2          
effect ^u frequ5cy1 #ajj2       imp$ance
 (1 #ace2       mutual1 #aji2       (   
coils1 #aja2       self1 #hc2           
table ( values1 #bbg2       unit (1 #hc2
       v>iable1 #ajh      ,induction1   

pr9ciple (1 #bjh      ,inducto-met]1    
#aji      ,input circuit1 #ahg          
,int]f]5ce1 #bdi      ,int]nal          
resi/ance1 #aia      ,ion1 def9ition (1 
#ai2       positive & negative1 #bj1 #ba
      ,ionization1 #bj      ,l>ynx1 #aec
      ,laws ( attraction1 #bjd          
,load+ coil1 #bbd      ,loop ant5na1    
#aih      ,magnet1 pole (1 #bjc2       (
 s(t iron1 #bje2       ( /eel1 #bje     
,magnetism1 #bjb      ,matt]1           
con/itution (1 #e      ,megohm1 #agb    
,micr(>ad1 #ajd      ,mil-amp]e1 #ga    
,mil-h5ry1 #hc      ,modulation1 #ade1  
#bcj1 #bcg1 #bci      ,molecule1 k9ds (1
 #f2       motion (1 #ce      ,greek3   
mu1 #aij      ,multiplex telegraphy1    
#beh2       telephony1 #beh      ,mutual
 inductance1 #aji2       v>iation (1    
#aaj      ,natural frequ5cy1 #afa       
,nitrog51 #aj      ,nucleus ( atom1 #aj1
 #ab      ,ohm1 def9$1 #fd      ,organ  
pipe1 #afj      ,oscillations1 #hg2     
damp$1 #aad2       to />t1 #aad2        
int5sity (1 #bcf2       natural frequ5cy

 (1 #aag      ,output circuit1 #ahg     
,ov]tones1 #aei      ,oxyg51 p]c5tage 9 
air1 #aj      ,phase1 #ahj      ,plate1 
( an audion1 #da      ,plung] type (    
in/rum5t1 #bje      ,pol>ity ( a coil1  
#bjd      ,p[]1 def9$1 #bcd2            
electrical unit (1 #bce      ,proton1   
prop]ties (1 #d      ,radio curr5t1     
modulation (1 #ade      ,radio-frequ5cy 
amplification1 #bdc2       limitations1 
#bdc      ,radio-frequ5cy amplifi]1     
#ahf1 #aih      ,radio /ation connect$  
to l& l9e1 #bed      ,rat+ ( tubes1 #bce
      ,reception1 ess5tial op]ations 91 
#bce      ,reg5]ative circuit1 #agf2    
frequ5cy (1 #agi      ,repeat] /ations1 
#bfa      ,resi/ance1 measurem5t (1 #fd2
       non-inductive1 #ajc2       squ>e1
 #bea      ,resonance1 #afa             
,resonance curve1 #aag      ,ret>d      
coils1 #baj      ,salt1 atomic          
con/ruction (1 #ag2       cry/al        
/ructure1 #adg2       molecule 9        
solution1 #ai2       p]c5tage 9 sea     
wat]1 #aa      ,saturation1 #ch         

,sea wat]1 atomic con/itution (1 #aa    
,self-inductance1 #hc2       unit (1 #hc
      ,side b&s1 #bdh2       relation to
 wave l5g?s1 #bdi      ,silicon1        
p]c5tage 9 e>?1 #aa      ,sodium        
*loride1 9 solution1 #ai      ,s\nd1    
production (1 #aeb      ,spee*1 to      
transmit by radio1 #bcj      ,spe$ (    
light1 #abb      ,/&>d cell1 #eh        
,/orage batt]y1 #bh1 #cj      ,sulphuric
 acid1 #bb      ,sup]-het]odyne1 #bdb2  
advantages (1 #bdb      ,telephone      
receiv]1 #acj2       !ory (1 #aca       
,telephone transmitt]1 #adb             
,telephony1 by wire1 #bec      ,tickl]  
coil1 #ahb      ,transcont95tal         
telephone l9e1 #bfa      ,transmission1 
ess5tial op]ations 91 #bcj              
,transmitt]1 ,arl+ton1 #bcc2            
cont9u\s wave1 #id1 #aai2       = high  
p[]1 #bcc      ,trans=m]1 #ahe2         
/ep-up1 #aic      ,tubes1 connect$ 9    
p>allel1 #bcd      ,tun+1 curve1 #aag2  
%>p1 #bad2       ) s]ies cond5s]1 #bbd  
,undamp$ waves 7see cont9u\s waves71 #hf

      ,vacuum tube1 #ce1 #dj2           
*>act]i/ics (1 #fg2       con/ruction (1
 #bje2       modulator1 #bci2           
?ree-electrode1 #da2                    
two-electrode1 #db      ,v>iomet]1 #ajh 
,vibrat+ /r+1 /udy (1 #aed      ,vocal  
cords1 #aec      ,voice frequ5cies1 #afc
      ,volt1 def9ition (1 #eg2          
measurem5t (1 #fa      ,voltmet]1       
calibration (1 #fb2       con/ruction (1
 #bje      ,watt1 #bce      ,wave =m1   
#ahb      ,wave l5g?1 relation to       
frequ5cy1 #ih1 #abb2       def9$1 #abb  
,wire1 inductance (1 #ajd      ,wire1   
movem5t ( electrons 91 #ad2             
emission ( electrons f1 #cg      ,wire  
telephony1 #bec      ,wir$ wireless1    
#bee2       advantages (1 #beg          
,x-rays1 #adg      ,z]o c\pl+1 #agg     
,z9c1 electrode = batt]y1 #bc