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...     *-.<-)  '-^f^.  ..,    --f  •_-"    ;-*•---•-  *->-(    •• 


OF  THE 

UNIVERSITY 

OF 


PRINCIPLES   OF  WIRELESS  TELEGRAPHY 


Published    by  the 

McGraw-Hill    Book.  Company 


\Succ  arsons  to  the  Book  Departments  of  the 

McGraw  Publishing  Company  Hill  Publishing-  Company 

Publishers  of  Books  for 

Electrical  World  The  Engineering  and  Mining  Journal 

The  Engineering  Record  Rjwer  and  The  Engineer 

Electric  Railway  Journal  American   Machinist 


PRINCIPLES  OF 


WIBELESS  TELEGRAPHY 


BY 
GEORGE    W.   PIERCE,    A.M.,  PH.D. 

ASSISTANT    PROFESSOR    OF    PHYSICS    IN    HARVARD    UNIVERSITY 


Of    THE 

I   UNIVERSITY  J 

Of 
[UFORNifc* 


McGRAW-HILL   BOOK   COMPANY 

239  WEST   39TH  STREET,    NEW   YORK 

6  BOUVERIE  STREET,  LONDON,  E.G. 

1910 


COPYRIGHT,  1910, 

BY  THE 
McGRAW-HILL  BOOK   COMPANY 


Stanbopc  iPms 

F.    H.   GILSON     COMPANY 
BOSTON.      U.S.A. 


PREFACE 


THIS  volume  comprises  the  non-mathematical  portions  of  a 
course  of  lectures,  entitled  "  Electric  Waves  and  their  Application 
to  Wireless  Telegraphy/'  which  for  several  years  have  been  given 
by  the  author  to  classes  at  Harvard  University.  In  giving  the 
lectures  and  in  preparing  this  volume,  the  design  has  been :  — 
First,  to  present,  in  as  elementary  a  form  as  possible,  the  course  of 
reasoning  and  experimentation  that  has  led  to  the  conception  of 
electric  waves;  second,  to  follow  this  with  a  discussion  of  the 
properties  of  electric  waves  and  electric  oscillations;  third,  to  give 
a  history  of  the  application  of  electric  waves  to  wireless  telegraphy ; 
and  fourth,  to  elaborate  the  general  principles  and  methods  of 
electric-wave  telegraphy  in  sufficient  detail  to  be  of  possible  use 
to  elementary  students  of  electricity  and  to  amateur  and  pro- 
fessional electricians  engaged  in  operating  and  constructing  wire- 
less telegraphic  apparatus. 

The  author  wishes  to  express  his  sincere  thanks  to  Commander 
S.  S.  Robison  of  the  United  States  Navy,  to  Mr.  Elliott  Woods 
of  Washington,  and  to  Chief  Inspector  D.  M.  Mahood  of  the  New 
York  Navy  Yard  for  their  kindness  in  supplying  photographs 
for  some  of  the  illustrations.  Also,  the  author  is  grateful  to  the 
Editors  of  the  Physical  Review  for  the  loan  of  Plates  I  and  II,  and 
to  Mr.  Greenleaf  Whittier  Pickard  for  the  privilege  of  consulting 
his  manuscript  account  of  experiments  on  the  effects  of  daylight 
on  transmission.  Finally,  the  author  takes  great  pleasure  in 
expressing  his  gratitude  to  his  friend  Mr.  George  Francis  Arnold, 
who  has  kindly  read  the  proofs  and  made  many  valuable  sug- 
gestions. 

G.  W.  PIERCE. 

HARVARD  UNIVERSITY,   CAMBRIDGE,   MASS. 
July,  1910. 


TABLE   OF   CONTENTS 


CHAPTER   I 

Page 

INTRODUCTION 1 

CHAPTER  II 
ON  THEORIES  AS  TO  THE  NATURE  OF  ELECTRICITY 6 

CHAPTER   III 
ON  THE  RELATION  BETWEEN  ELECTRICITY  AND  MAGNETISM 12 

CHAPTER   IV 
ON  THE  RESEMBLANCE  OF  SELF-INDUCTION  TO  MECHANICAL  INERTIA  .       20 

CHAPTER  V 
ON  ELECTROSTATIC  CAPACITY > 23 

CHAPTER  VI 

ON  THE  DISCHARGE  OF  A  CONDENSER  THROUGH  AN  INDUCTANCE  AND 

RESISTANCE 28 

CHAPTER  VII 

MAXWELL'S    THEORY.     ELECTRIC    WAVES.     THE    ELECTROMAGNETIC 

THEORY  OF  LIGHT 36 

CHAPTER  VIII 
THE  EXPERIMENTS  OF  HERTZ 42 

CHAPTER  IX 
EXPERIMENTS  ON  THE  IDENTITY  OF  ELECTRIC  WAVES  AND  LIGHT 51 

CHAPTER  X 
ON  THE  PROPAGATION  OF  ELECTRIC  WAVES  ON  WIRES 62 

CHAPTER  XI 

WIRELESS  TELEGRAPHY  BEFORE  HERTZ 75 

vii 


viii  TABLE  OF  CONTENTS 

CHAPTER  XII 

Page 

WIRELESS  TELEGRAPHY  BY  HERTZIAN  WAVES.    MARCONI,  1896-1898 . .       80 

CHAPTER  XIII 
ELECTRIC  WAVE  TELEGRAPHY  BY  RESONANT  CIRCUITS 92 

CHAPTER  XIV 
NATURE  OF  THE  OSCILLATION.    THE  GROUNDING  OF  CIRCUITS 108 

CHAPTER  XV 
PROPAGATION  OVER  THE  EARTH 122 

CHAPTER  XVI 
ON  DETECTORS 140 

CHAPTER   XVII 

ON  DETECTORS  (Continued).     CRYSTAL  RECTIFIERS 157 

CHAPTER   XVIII 

ON  DETECTORS  (Continued) .     FURTHER  EXPERIMENTS  ON  THE  CRYSTAL 

RECTIFIERS 175 

CHAPTER   XIX 

ON    DETECTORS    (Concluded).     THE    ELECTROLYTIC    DETECTOR,    AND 

VACUUM  DETECTORS 201 

CHAPTER  XX 

ELECTRICAL    RESONANCE.    .WAVE    METERS.     RESONANCE    IN    SIMPLE 

CONDENSER  CIRCUITS 215 

CHAPTER   XXI 

ON  RESONANCE  (Continued).     ON  THE  ELECTRICAL  OSCILLATIONS  OF 

CONNECTED  SYSTEMS  OF  CONDENSER  CIRCUITS 228 

CHAPTER  XXII 
TUNING  THE  SENDING  STATION 243 

CHAPTER  XXIII 

SOME  RECENT  METHODS  OF  EXCITING  ELECTRIC  WAVES.     THE  SING- 
ING ARC,  THE  SINGING  SPARK,  AND  THE  QUENCHED  SPARK 253 

CHAPTER   XXIV 

RESONANCE    OF    RECEIVING    CIRCUITS.     THE    POSSIBILITY    OF  PRE- 
VENTING INTERFERENCE 271 


TABLE  OF  CONTENTS  ix 

CHAPTER  XXV 

Page 

DIRECTED  WIRELESS  TELEGRAPHY 296 

CHAPTER   XXVI 
WIRELESS  TELEPHONY 305 

CHAPTER  XXVII 

SOME  DETAILS  OF  CONSTRUCTION  OF  WIRELESS  TELEGRAPHIC  APPA- 
RATUS      312 

CHAPTER  XXVIII 
CONCLUSION v 327 

APPENDIX  I 

ELEMENTARY  FACTS  ABOUT  ELECTRICITY,  AND  DEFINITIONS  OF  UNITS     329 

APPENDIX  II 

CONCERNING  THE  CALCULATION  OF  RESISTANCE,  SELF-INDUCTANCE  AND 

CAPACITY.  .  337 


OF   THE 

UNIVERSITY 

OF 


WIRELESS   TELEGEAPHY 


CHAPTER   I 
INTRODUCTION 

ALMOST  every  one  has  seen  and  heard  the  noisy,  brilliant  spark 
produced  by  the  discharge  of  a  Leyden  jar.  The  experiment, 
shown  in  elementary  courses  in  physics,  is  usually  performed  as 
follows:  The  inner  and  outer  coatings  of  the  Leyden  jar  are 
connected  to  the  terminals  of  a  static  electric  machine.  The 
machine  is  set  in  rotation  and  the  jar  is  charged.  After  the  jar 
has  been  charged,  the  electric  machine  is  disconnected,  and  one 
end  of  a  metallic  rod,  held  by  an  insulated  handle  (see  Fig.  1),  is 


FIG.  1.     Leyden  jar  and  discharger. 


FIG.  2.     Leyden  jar  with  coil  in 
discharge  circuit. 


touched  against  the  outer  coating  of  the  jar,  while  the  other  end 
of  the  rod  is  made  to  approach  a  knob  connected  with  the  inner 
coating.  Before  the  conductor  to  the  inner  coating  is  actually 
touched,  a  discharge  occurs  through  the  metallic  rod,  producing  a 
vivid  spark  at  the  gap  intervening  between  the  knob  and  the  dis- 
charge rod.  As  a  variation  of  the  experiment,  in  the  place  of  the 
straight  or  slightly  curved  metallic  rod  used  in  the  discharge  appa- 
ratus of  Fig.  1,  a  coil  consisting  of  a  few  turns  of  heavy  wire  may 


2  WIRELESS  TELEGRAPHY 

be  employed  to  form  a  part  of  the  circuit  between  the  two  coatings 

of  the  jar,  as  is  shown  in  Fig.  2. 

The  flow  of  electricity  in  a  circuit  of  the  form  of  Fig.  1  or  Fig.  2 

has  been  the  subject  of  many 
interesting  theoretical  and  ex- 
perimental investigations  di- 
rectly applicable  to  the  subject 
under  consideration. 

The  Experiments  of  Joseph 
Henry.  —  Some  experiments 
performed  by  Professor  Joseph 
Henry1  of  Princeton  Univer- 
sity in  the  year  1842  gave 
intimation  that,  under  certain 
conditions,  the  discharge  of  the 
Leyden  jar  takes  place  in 
an  oscillatory  fashion.  Let 
us  give  a  brief  description  of 
Henry's  experiment.  A  small 
sewing  needle  was  placed 
within  the  coil  of  wire  of  the 
discharge  circuit,  as  is  shown 
at  N,  Fig.  2,  so  that  the  elec- 
tricity from  the  Leyden  jar 
was  made  to  flow  in  the  coil 
around  the  needle.  At  the 
time  of  Henry's  experiment  it 
was  already  well  known  that 
a  current  of  electricity  from 
an  ordinary  galvanic  battery, 
when  caused  to  flow  in  a  coil 
of  wire  encircling  a  steel 
needle,  magnetizes  the  needle. 
Henry's  experiment  showed 

FIG.  3.    Rotating-mirror  photograph     that  the  current  of  electricity 
of  oscillatory  discharge.  from    the    Leyden     jar    also 

produced  magnetization  of  the  needle.  It  was  partly  in  search 
of  this  fact,  showing  the  identity  of  static  and  galvanic  elec- 
tricity, that  Henry's  experiment  was  undertaken.  In  the  experi- 
ment, however,  Henry  discovered  the  additional  fact,  that  with 

1  See  Scientific  Writings  of  Jos.  Henry,  Vol.  I,  p.  201,  Washington,  1886. 


INTRODUCTION  3 

the  Leyden  jar  always  charged  in  the  same  direction  by  the 
electric  machine  used  to  charge  the  jar,  the  needle  was  some- 
times found  to  be  magnetized  in  one  direction  and  sometimes 
in  the  opposite  direction,  indicating  that  the  current  that  pro- 
duced the  magnetization  of  the  needle  was  flowing  in  the  coil  in 
the  one  case  from  the  outside  of  the  jar  towards  the  inner  coat- 
ing, while  in  the  other  case  it  was  flowing  from  the  inner  coating 
to  the  outer  coating.  This  effect  could  be  explained  by  supposing 
that  the  current  from  the  Leyden  jar  was  oscillatory,  having  first 
one  direction  and  then  the  other,  and  that  the  magnetization  of 
the  needle  was  reversed  .at  each  reversal  of  the  current,  the  direc- 
tion of  the  magnetization  at  the  end  of  the  experiment  being 
fortuitously  determined  by  the  direction  last  taken  by  the  current. 
Professor  Henry's  experiment,  though  not  conclusive,  gave  strong 
evidence  of  the  oscillatory  character  of  the  discharge;  and  the 
opinion  that  the  discharge  is  oscillatory  was  repeatedly  expressed 
and  defended  by  Professor  Henry  in  a  number  of  papers  and 
scientific  addresses  delivered  between  1842  and  1850. 

Sir  William  Thomson's  Theoretical  Proof  of  the  Oscillatory 
Nature  of  the  Discharge  of  the  Leyden  Jar.  —  In  1853  Sir  William 
Thomson,1  who  was  afterwards  Lord  Kelvin,  proved  by  mathe- 
matical reasoning  that  under  certain  conditions  the  discharge  of  a 
Leyden  jar  occurs  in  an  oscillatory  manner.  Under  certain  other 
conditions  the  discharge  is  non-oscillatory.  In  the  case  of  the 
oscillatory  discharge  the  electricity  does  not  simply  flow  from  one 
coating  to  the  other  until  the  jar  is  in  a  condition  of  electric  neu- 
trality, but  rushes  back  and  forth  between  the  two  coatings  a 
great  number  of  times,  with  a  frequency  depending  on  the  dimen- 
sions of  the  jar  and  the  dimensions  and  form  of  the  coil  through 
which  the  discharge  occurs. 

Feddersen's  Revolving-Mirror  Experiment.  —  In  1859  Doctor 
Feddersen  of  the  University  of  Leipzig,  by  a  very  beautiful  experi- 
ment, proved  the  correctness  of  the  surmise  of  Henry  and  the 
mathematical  predictions  of  Thomson.  Feddersen's  experiment 
consisted  in  photographing  the  spark  produced  by  the  discharge 
of  the  Leyden  jar.  A  photograph  similar  to  that  obtained  by 
Feddersen  is  shown  in  Fig.  3.  A  sketch  of  the  apparatus  used  in 
taking  the  picture  is  shown  in  Fig.  4.  Instead  of  employing  an 
ordinary  camera  to  take  the  picture,  the  light  from  the  spark  S, 
produced  by  the  discharge  of  the  jar,  was  allowed  to  fall  upon 

1  Wm.  Thomson:  Philosophical  Magazine  [4],  5,  p.  393,  1853. 


4  WIRELESS  TELEGRAPHY 

a  rapidly  revolving  concave  mirror  M  of  Fig.  4,  and  was  received 
upon  a  photographic  plate  P  after  reflection  from  the  mirror.  Just 
as  the  light  of  a  sunbeam  entering  a  room  may  be  reflected  upon 
the  wall  or  ceiling  of  the  room  by  a  mirror  held  in  the  hand,  and 
may  be  made  to  move  rapidly  up  the  wall  or  across  the  ceiling  by  a 
rotation  of  the  mirror,  so  in  Feddersen's  experiment  the  motion 
of  the  mirror  caused  the  image  of  the  spark  to  trail  rapidly  across 
the  photographic  plate.  If  the  spark  had  been  steady  and  unidi- 
rectional, the  image  on  the  plate  would  have  been  simply  a  band  of 
light  with  a  length  depending  on  the  speed  of  the  mirror  and  the 
duration  of  the  spark.  The  picture  (compare  Fig.  3)  shows,  on 
the  contrary,  that  the  conditions  at  the  spark  reversed  several 
times  during  the  discharge,  so  that  first  one  terminal  of  the  spark 


FIG.  4.     Rotating  mirror  apparatus. 

was  bright  and  then  the  other,  —  the  bright  terminal  being  evi- 
denced by  the  bright  spots  on  the  photograph  of  Fig.  3.  The  suc- 
cessive alterations  of  the  bright  spots  from  one  side  to  the  other 
of  the  photograph  showed  the  successive  reversals  of  the  current 
across  the  spark  gap  during  the  discharge. 

Feddersen's  photographs  proved  beyond  doubt  the  correctness 
of  Thomson's  prediction  of  the  oscillatory  nature  of  the  discharge, 
and  gave,  as  we  shall  see  later,  a  very  beautiful  method  of  measur- 
ing the  periodic  time  of  oscillation  of  the  discharge,  —  a  time  which 
may  be  only  a  small  fraction  of  a  millionth  of  a  second,  and  which 
is  yet  subject  to  accurate  physical  measurement. 

It  is  by  means  of  electric  oscillations  similar  to  those  produced 
by  the  Ley  den-jar  discharge  that  wireless  telegraph  signals  are 
produced. 


INTRODUCTION  5 

Electric  Waves.  Maxwell's  Theory.  —  In  a  letter  to  C.  H.  Cay, 
Esq.,  dated  5th  of  January,  1865,  James  Clerk  Maxwell,  then  Pro- 
fessor of  Physics  in  the  University  of  Edinburgh,  wrote: 

"  I  have  also  a  paper  afloat  with  an  electromagnetic  theory  of 
light,  which  till  I  am  convinced  to  the  contrary,  I  hold  to  be  great 
guns." 

This  paper  to  which  Maxwell  referred  contained  a  prediction, 
based  on  careful  mathematical  reasoning,  that  electric  oscillations 
in  a  circuit  produce  electric  waves  in  surrounding  space,  that  these 
waves  travel  away  with  the  velocity  of  light,  and  that  light  itself 
is  simply  a  train  of  electric  waves  of  extremely  short  wave  length. 
This  prediction  of  Maxwell,  correlating  the  phenomena  of  light 
and  electricity,  is  one  of  the  most  beautiful  philosophic  specula- 
tions in  the  history  of  science,  and  long  remained  without  direct 
experimental  confirmation;  but  now,  thanks  to  the  brilliant  experi- 
ments of  Heinrich  Hertz,  the  existence  of  electric  waves  with 
properties  intimately  related  to  those  of  light  waves  is  a  well- 
established  fact  of  experience  capable  of  verification  in  even 
very  elementary  physical  laboratories. 

It  is  by  means  of  these  electric  waves  that  the  signals  of  wireless 
telegraphy  and  telephony  are  propagated  through  space. 

In  the  succeeding  chapters,  we  shall  take  up  more  in  detail  the 
course  of  reasoning  that  led  to  Thomson's  and  Maxwell's  pre- 
dictions, the  course  of  experimenting  that  led  to  the  proofs  of  the 
existence  of  their  electric  oscillations  and  electric  waves,  and  the 
development  of  the  very  striking  methods  that  have  been  employed 
in  utilizing  these  electric  oscillations  and  electric  waves  in  the 
transmission  of  signals.  The  discussion  will  introduce  some  details 
apparently  remote  from  commercial  usefulness ;  but1  it  should  be 
borne  in  mind  that  it  has  been  by  means  of  persistent  and  labo- 
rious study  of  these  details  that  the  practical  result  has  been 
attained. 


CHAPTER  II 
ON  THEORIES  AS  TO  THE  NATURE  OF  ELECTRICITY 

IN  the  preceding  chapter  mention  has  been  made  of  the  oscil- 
latory flow  of  electricity  back  and  forth  between  the  two  coatings 
of  a  Ley  den  jar,  when  the  jar  is  allowed  to  discharge  through  a 
conductor.  The  description  there  given  of  the  "  flow  of  elec- 
tricity "  will  probably  call  to  the  mind  of  the  reader  a  picture  of 
a  motion  back  and  forth  of  some  kind  of  material  substance  from 
one  reservoir  to  another.  At  the  same  time,,  it  may  be  difficult  to 
imagine  the  flow  of  any  kind  of  substance  through  the  solid  metal 
of  which  the  conductor  is  composed.  What  then  is  this  electricity 
that  can  flow  through  solid  conductors  ? 

This  is  a  question  that  we  cannot  hope  to  answer  to  our  complete 
satisfaction,  but  we  have  recently  come  to  have  so  much  light 
thrown  upon  the  question  that  it  is  proposed  to  devote  a  few  pages 
to  the  discussion  of  theories  as  to  the  nature  of  electricity. 

In  the  works  of  the  early  writers  on  electricity  two  prominent 
hypotheses  have  been  made  as  to  the  nature  of  electricity.  These 
have  been  called  the  two-fluid  theory  and  the  one-fluid  theory.  The 
chief  facts  that  these  theories  were  at  first  called  upon  to  explain 
were: 

(1)  The  phenomenon  of  electrostatic  attraction  and  repulsion;, 
for  example,  the  attraction  or  repulsion  between  two  charged  pith 
balls,  and 

(2)  The  fact  that  when  electrification  was  produced  in  any  way 
two  opposite  charges  were  always  obtained;  for  example,  when  a 
glass  rod  is  rubbed  with  silk,  a  certain  quantity  of  positive  elec- 
tricity appears  on  the  glass  rod  and  an  equal  quantity  of  negative 
electricity  appears  on  the  silk. 

THE    TWO-FLUID    THEORY 

According  to  the  two-fluid  theory  all  bodies  in  their  unelectrified 
condition  were  supposed  to  contain  equal  quantities  of  two  subtle 
fluids,  one  of  which  was  called  positive  electricity,  and  the  other 
negative  electricity.  On  this  theory  the  process  of  positively  elec- 

6 


THEORIES  AS  TO  THE  NATURE  OF  ELECTRICITY    7 

trifying  a  body  consists  in  adding  to  it  a  quantity  of  the  positive 
fluid  or  taking  from  it  a  quantity  of  the  negative  fluid.  The  state 
of  electrification  of  a  body  is  hence  determined  by  the  excess  in 
amount  of  one  of  the  fluids  over  the  other.  In  order  to  account 
for  the  fact  that  the  appearance  of  electrification  of  one  sign  is 
always  accompanied  by  the  appearance  of  an  equal  amount  of 
electrification  of  the  opposite  sign,  the  two  fluids  were  supposed 
to  be  uncreatable  and  indestructible,  so  that  the  accumulation  of 
positive  electricity  in  one  body  is  always  accompanied  by  the  loss 
of  positive  electricity  in  some  other  body.  This  is  the  principal 
property  that  the  electrical  fluids  were  supposed  to  have  in  com- 
mon with  ordinary  material  fluids;  namely,  the  property  of  conserva- 
tism in  amount  according  to  which  the  total  amount  of  electricity 
in  a  given  system  can  only  be  changed  by  the  transfer  of  electricity 
through  the  boundary  of  the  system. 

The  electrical  fluids,  on  the  other  hand,  must  possess  properties 
that  do  not  belong  to  the  material  fluids;  for  example,  portions  of 
the  positive  fluid  must  be  supposed  to  repel  each  other,  as  do  also 
portions  of  the  negative  fluid,  while  the  two  unlike  fluids  attract 
each  other.  Another  property  of  the  electrical  fluids  still  more  at 
variance  with  the  known  properties  of  material  fluids  is  found  in 
the  fact  that  if  we  add  equal  quantities  of  the  two  electrical  fluids 
to  the  same  body,  the  condition  of  the  body  will  be  unchanged,  so 
that  according  to  this  theory  we  must  suppose  that  "  the  mixture 
of  the  two  fluids  in  equal  proportions  is  something  so  devoid  of 
physical  properties  that  its  existence  has  never  been  detected." 

THE    ONE-FLUID    THEORY 

Benjamin  Franklin  attempted  to  describe  the  phenomena  of 
electricity  in  terms  of  a  single  fluid.  According  to  his  theory, 
one  of  the  fluids,  the  positive,  was  retained  and  called  the  electric 
fluid,  while  the  other,  the  negative  fluid  of  the  two-fluid  theory, 
was  replaced  by  ordinary  matter.  Quantities  of  the  electric  fluid 
were  supposed  to  repel  other  quantities  of  the  fluid  according  to 
the  law  of  the  inverse  square  of  the  distance  and  to  attract  matter 
according  to  the  same  law.  Quantities  of  matter  were  supposed 
to  repel  each  other  and  attract  the  electric  fluid.  According  to 
Franklin's  theory  an  excess  of  the  electric  fluid  rendered  the  body 
positive,  while  a  deficiency  rendered  it  negative. 

1  J.  J.  Thomson,  Electricity  and  Matter,  Charles  Scribner's  Sons,  1904. 


8  WIRELESS  TELEGRAPHY 


THE  ATOMIC  STRUCTURE  OF  ELECTRICITY 

Both  of  the  theories  sketched  above  are  useful  in  supplying  a 
terminology  for  electricity  and  in  affording  a  simple  mode  of  presen- 
tation of  some  of  the  phenomena,  but  both  theories  are  charac- 
terized by  indefiniteness  as  to  the  physical  properties  of  electricity. 

Recently,  however,  a  number  of  phenomena  have  been  studied 
that  have  led  to  a  somewhat  bolder  statement  as  to  the  nature  of 
electricity.  In  accordance  with  data  obtained  chiefly  from  the 
study  of  the  conduction  of  electricity  by  liquids  and  gases,  elec- 
tricity is  now  generally  supposed  to  have  a  structure  that  may  be 
called  atomic. 

The  first  evidence  pointing  in  this  direction  was  obtained  by 
Faraday  in  the  course  of  a  research  on  the  conduction  of  electricity 
by  decomposable  liquids.  When  an  electric  current  is  passed 
through  water,  the  water  is  decomposed  into  hydrogen,  given  off 
at  one  electrode,  and  oxygen,  given  off  at  the  other.  A  great 
many  other  liquids  —  for  example,  the  aqueous  solutions  of  vari- 
ous salts  —  are  similarly  decomposed  by  the  action  of  the  current. 
An  electrically  decomposable  liquid  is  called  by  Faraday  an  Elec- 
trolyte. Faraday  discovered  the  following  laws  of  electrolytic 
decomposition . 

I.  In  a  given  electrolyte,  the  amount  of  substance  decomposed 
by  various  electric  currents  is  proportional  to  the  quantity  of 
electricity  sent  tnrough  the  electrolytes. 

II.  If  the  same  amount  of  electricity  is  sent  through  various 
electrolytes,  the  amount  of  the  several  decomposition  products 
obtained  from  the  various  electrolytes  is  proportional  to  the  com- 
bining weights  of  the  products  obtained.     For  example,  if  hydro- 
gen (2  H)  and  Oxygen  (0)  are  obtained  in  one  electrolytic  cell, 
and  silver  (Ag)  and  chlorine  (Cl)  in  another  cell,  the  amounts  of 
these  various  substances  obtained,  when  a  given  electric  current 
is  sent  through  both  cells,  are   in  the  ratio  of   their  chemical 
combining  weights. 

According  to  the  atomic  theory  of  matter,  these  two  laws  may 
be  interpreted  by  supposing  that  each  of  the  decomposition  prod- 
ucts carries  a  charge  that  is  an  integral  multiple  of  the  charge 
carried  by  the  hydrogen  atom;  so  that,  if  the  hydrogen  atom,  in 
the  process  of  carrying  a  current  electrolytically,  is  supposed  to 
have  associated  with  it  a  definite  small  quantity  of  electricity, 
any  combination  of  atoms,  when  carrying  a  current,  have  asso- 


THEORIES  AS  TO  THE  NATURE  OF  ELECTRICITY          9 

elated  with  them  an  equal  small  quantity  of  electricity  or  an  inte- 
gral multiple  thereof.  That  is,  the  charges  we  meet  with  are  never 
fractional  parts  of  the  charge  carried  by  the  hydrogen  atom; 
whence  we  may  suppose  that  the  latter  charge  is  an  elemental 
quantity  of  electricity.  In  discussing  the  evidence  afforded  by 
Faraday's  experiments  Helmholtz  1  says  that  "  if  we  accept  the 
hypothesis  that  the  elementary  substances  are  composed  of  atoms, 
we  cannot  avoid  the  conclusion  that  electricity,  positive  as  well 
as  negative,  is  divided  into  definite  elementary  portions  which 
behave  like  atoms  of  electricity." 

The  study  of  the  conduction  of  electricity  through  gases  gives 
still  stronger  e  vide  ace  of  the  atomic  character  of  electricity.  Gases 
under  the  action  of  certain  agencies  —  Roentgen  rays,  ultra-violet 
light,  radium,  high  electromotive  forces,  electric  spark,  etc.  — 
become  conductive  and  retain  their  conductivity  long  enough  to 
permit  a  study  of  the  mechanism  by  which  the  electricity  is  con- 
ducted. As  in  the  case  of  the  study  of  conduction  in  liquids,  we 
are  again  "  led  to  the  conception  of  a  natural  unit  or  atom  of 
electricity  of  which  all  charges  are  integral  multiples,  just  as  the 
mass  of  a  quantity  of  hydrogen  is  an  integral  multiple  of  the  mass 
of  a  hydrogen  atom."  2 

By  the  study  of  conduction  in  gases  definite  information  is 
obtained  in  regard  to  the  magnitude  of  this  charge.  In  a  series 
of  experiments  performed  chiefly  at  the  Cavendish  Laboratory  of 
Cambridge  University  the  quantity  of  electricity  in  one  electrical 
atom  is  found  to  be  3.4  X  10~10  electrostatic  c.  g.  s.  units.3  This 
quantity  obtained  from  experiments  on  conduction  in  gases  is  the 
same  as  the  quantity  of  electricity  carried  by  one  hydrogen  atom 
in  the  electrolysis  of  liquids. 

Mass  of  the  Carriers  of  Electricity.  —  Also  at  the  Cavendish 
Laboratory  evidence  as  to  the  mass  of  the  carriers  of  electricity 
has  been  obtained  by  an  experimental  determination  of  the  ratio 
of  e/m,  in  which  e  is  the  elemental  charge  and  m  is  the  mass  of 
matter  carrying  the  charge.  The  result  obtained  is  that  the  mass 
of  the  carrier,  when  the  electricity  is  negative,  is  about  1/1000  of  the 
mass  of  the  hydrogen  atom.  This  mass  is  apparently  the  same 

1  J.  J.  Thomson,  Electricity  and  Matter,  p.  73,  Charles  Scribner's  Sons, 
1904. 

2  J.  J.  Thomson,  Electricity  and  Matter,  p.  83,  Charles  Scribner's  Sons, 
1904. 

3  The  electrical  units  are  defined  in  Appendix  I. 


10  WIRELESS  TELEGRAPHY 

whatever  the  nature  of  the  gas  in  which  the  particle  happens  to 
be  found.  While  the  mass  of  the  carrier  of  positive  electricity  is 
approximately  the  mass  of  the  atom  of  ordinary  matter,  and 
apparently  differs  from  one  gas  to  another  in  the  same  way  as  the 
atoms  of  the  gas  differ.  J.  J.  Thomson  proposes  the  name  "  cor- 
puscle "  for  the  unit  of  negative  electricity,  and  sums  up  the 
corpuscular  theory  of  electricity  as  follows: 

'  These  results  lead  to  a  view  of  electrification  which  has  a 
striking  resemblance  to  that  of  Franklin's  One-Fluid  Theory  of 
Electricity.  Instead  of  taking,  as  Franklin  did,  the  electric  fluid 
to  be  positive  we  take  it  to  be  negative.  The  Electric  Fluid  of 
Franklin  corresponds  to  an  assemblage  of  corpuscles,  negative 
electrification  being  a  collection  of  these  corpuscles.  The  trans- 
ference of  electrification  from  one  place  to  another  is  effected  by 
the  motion  of  corpuscles  from  the  place  where  there  is  a  gain  of 
positive  electrification  to  the  place  where  there  is  a  gain  of  nega- 
tive. A  positively  electrified  body  is  one  that  has  lost  some  of  its 
corpuscles.  We  have  seen  that  the  mass  and  the  charge  of  the 
corpuscles  have  been  determined  directly  by  experiment.  We  in 
fact  know  more  about  the  electric  fluid  than  we  know  about  such 
fluids  as  air  and  water." 

In  applying  Thomson's  Theory  to  the  flow  .of  electricity  in  con- 
ductors we  must  suppose  that  these  small  charged  bodies,  with  a 
mass  equal  to  about  1/1000  of  the  mass  of  the  hydrogen  atom, 
are  able  under  the  action  of  electric  forces  to  move  through  the 
substance  of  even  such  solid  conductors  as  the  metals,  and  that  a 
stream  of  these  small  charged  bodies  constitutes  or  carries  the 
electric  current.  We  must,  however,  bear  in  mind  that  the  stream 
of  negative  particles  is  in  the  opposite  direction  to  the  direction 
conventionally  ascribed  to  the  electric  current. 

If  we  wish  now  to  picture  to  ourselves  the  flow  of  electricity  in 
the  Ley  den-jar  discharge,  we  may  think  of  a  stream  of  these  small 
negatively  charged  corpuscles  passing  from  the  outer  coating  of 
the  jar  through  the  discharge  rod  and  across  the  spark  gap  and 
accumulating  on  the  inner  coating.  This  charges  the  inner  coating 
negatively  and  leaves  the  outer  coating  deficient  in  corpuscles  and 
therefore  charged  positively.  The  stream  of  corpuscles  then  re- 
verses, flows  from  the  inner  coating  to  the  outer,  and  reverses  the 
charge  on  the  jar.  This  process  continues,  each  time  with  a  loss 

1  J.  J.  Thomson,  Electricity  and  Matter,  p.  88,  Charles  Scribner's  Sons, 
1904. 


THEORIES  AS  TO  THE  NATURE  OF  ELECTRICITY         11 

of  electromotive  force,  until  the  electric  tension  finally  becomes 
too  small  to  force  the  corpuscles  across  the  spark  gap. 

This  description  is  given  merely  so  that  the  reader  may  picture 
an  electric  current  to  his  mind.  The  question  as  to  how  and  why 
the  discharge  of  the  Leyden  jar  oscillates  will  be  discussed  later, 
after  some  more  of  the  elementary  facts  about  electric  currents 
have  been  presented. 


CHAPTER  III 

ON   THE   RELATION    BETWEEN   ELECTRICITY    AND 
MAGNETISM 

IN  the  preceding  chapter  I  have  given  some  of  the  newest  specu- 
lations in  regard  to  the  nature  of  electricity.  The  particular  views 
there  expressed  are  not  essential  to  the  development  of  the  con- 
ception of  electric  oscillations  and  electric  waves,  so  that  the  reader 
may  be  skeptical  about  the  atomic  structure  of  electricity  and  still 
be  able  to  follow  the  arguments  for  Maxwell's  Theory.  In  the 
present  chapter  I  wish  to  return  to  surer  ground,  and  give  some 
of  the  older  experiments  on  electricity  and  magnetism  and  on  the 
relation  of  electricity  to  magnetism. 

Prior  to  1820  the  phenomena  of  electricity  and  magnetism  were 
not  known  to  be  related  to  each  other.  The  familiar  facts  about 
magnetism  were:  that  there  is  a  mineral  called  loadstone  that 
has  the  power  of  attracting  pieces  of  iron;  that  a  piece  of  soft  iron 
brought  near  the  loadstone  becomes  also  a  magnet  with  the  power 
to  attract  iron,  but  only  temporarily,  for  the  piece  of  soft  iron  loses 
most  of  its  magnetism  when  it  is  removed  to  a  distance  from  the 
loadstone;  while  a  piece  of  hardened  steel  brought  near  the  load- 
stone or  another  magnet  becomes  a  so-called  permanent  magnet, 
and  retains  a  considerable  part  of  its  magnetism  even  when 
at  a  great  distance  from  the  loadstone.  It  was  also  known 
that  a  steel  needle,  magnetized  by  rubbing  it  on  a  loadstone 
or  another  magnet,  and  pivoted  so  as  to  be  free  to  rotate  in  a 
horizontal  plane,  points  in  approximately  a  north  and  south 
direction. 

About  electricity  it  was  known  that  amber,  glass  and  sealing 
wax  were  capable  of  being  electrified  by  rubbing  them  with  silk, 
flannel,  fur,  etc. ;  that  the  electrifications  so  produced  were  of  two 
kinds,  positive  and  negative;  that  unlike  charges  attract  each 
other  and  like  charges  repel;  that  these  positive  and  negative 
charges  could  be  stored  in  an  apparatus  of  the  form  of  a  Leyden 
jar;  that  certain  bodies,  such  as  metals,  carbon,  water  and  so  forth, 
were  conductors  cf  electricity,  so  that  the  electricity  would  flow 
freely  through  such  bodies.  Also  the  galvanic  cell  was  known,  and 

12 


RELATION  BETWEEN  ELECTRICITY  AND  MAGNETISM      13 

was  employed  to  produce  a  continuous  flow  of  electricity  in  wires. 
This  continuous  flow  of  electricity  in  a  wire  or  other  conductor 
is  an  electric  current,  and  was  known  to  produce  heating  of  the 
conductor  through  which  it  flows. 

In  1820  a  new  impetus  was  given  to  a  study  of  electricity  and 
magnetism  by  the  discovery  by  Hans  Christian  Oersted  of  Copen- 
hagen that  magnetism  and  electricity  are  interrelated.  This  dis- 
covery and  some  of  its  consequences  is  described  in  the  succeeding 
paragraphs. 

On  the  Production  of  a  Magnetic  Field  by  a  Current  of  Elec- 
tricity. —  Oersted's  discovery  was  nothing  less  than  the  important 
fact  that  when  a  pivoted  magnetic  needle  is  placed  near  a  wire 
carrying  a  current  of  electricity,  the  magnetic  needle  tends  to  set 
itself  at  right  angles  to  the  wire  which  carries  the  electric  current. 
If  the  current  is  reversed,  the  direction  of  the  deflection  of  the  mag- 
netic needle  is  reversed.  If  the  wire  carrying  the  current  is  moved 
from  a  position  below  the  needle  to  a  position  above  the  needle, 
the  deflection  of  the  needle  is  again  reversed. 

Oersted's  discovery  has  been  utilized  in  the  construction  of  the 
galvanometer,  which  is  a  very  delicate  instrument  for  detecting 
and  measuring  small  electric  currents.  The  principle  of  the  gal- 
vanometer is  as  follows:  A  magnetic  needle  pivoted  as  in  the 
ordinary  compass,  so  as  to  be  free  to  move 
in  a  horizontal  plane,  will,  if  undisturbed, 
take  up  a  position  in  the  magnetic  me- 
ridian of  the  earth;  that  is,  the  needle 
will  point  approximately  north  and  south, 
(M,  Fig.  5).  Suppose,  now,  that  a  wire 
is  passed  alternately  above  and  below  the 
needle  several  times  so  as  to  form  a  coil  ^.^.^^-^^j 

(C,  Fig.  5),  with  its  windings  in  the  plane  SXL- 

of  the  magnetic  meridian.  Let  a  current  -piGJS.  Coil  and  needle 
be  passed  through  the  coil,  so  as  to  flow  of  galvanometer, 
north  above  the  needle  and  south  below  it;  the  north  current 
above  the  needle  and  the  south  current  below  it  both  tend  to 
deflect  the  north-seeking  end  of  the  magnetic  needle  to  the  west,  so 
that  the  effect  of  the  current  on  the  needle  is  multiplied  by  the 
combined  action  of  the  several  turns  of  the  conductor  around 
the  needle.  For  a  highly  sensitive  galvanometer,  the  magnetic 
needle  instead  of  being  pivoted  is  delicately  suspended  by  a  fine 
fiber  of  spun  quartz. 


14 


WIRELESS  TELEGRAPHY 


In  addition  to  its  application  to  the  construction  of  the  galva- 
nometer, Oersted's  principle  is  utilized  in  the  construction  of 
almost  every  kind  of  electromagnetic  device. 

Interpretation  of  Oersted's  Experiment.  —  The  results  of  Oer- 
sted's experiment  are  now  usually  expressed  by  saying  that  a 
current  of  electricity  in  a  conductor  produces  a  field  of  magnetic  force 

in  the  neighborhood  of  the  con- 
ductor. In  explanation  of  this 
statement  the  reader  is  asked 
to  recall  the  familiar  experi- 
ment in  which  a  sheet  of  paper 
laid  upon  a  bar  magnet  is  cov- 
ered with  iron  filings.  The 
filings  become  magnetized  and 
arrange  themselves  in  curved 
lines  stretching  from  one  pole 
of  the  magnet  to  the  other,  as 
shown  in  Fig.  6.  The  direc- 
tion of  these  lines  traced  by 
the  filings  is  approximately 
the  direction  of  the  magnetic 
force  about  the  magnet.  These 
lines,  delineated  by  the  filings,  are  the  lines  along  which  a  small 
suspended  magnetic  needle  would  orient  itself  if  brought  near  the 
bar  magnet. 

The  region  in  which  cuch  a  magnetic  force  exists  is  called  a 
field  of  magnetic  force.  A  piece  of  unmagnetized  steel  when  placed 
in  such  a  field  becomes  magnetized,  and  retains  some  of  its 
magnetism  even  after  it  is  removed  from  the  field  of  magnetic 
force. 

To  show  the  form  of  the  field  of  magnetic  force  about  a  wire 
carrying  a  current,  as  in  Oersted's  experiment,  iron  filings  may  also 
be  used  with  the  results  given  in  Figs.  7,  8,  and  9.  Figure  7  is 
obtained  with  a  straight  conductor  running  perpendicular  to  the 
plane  of  the  paper  on  which  the  filings  are  disposed.  Tne  picture 
shows  that  when  a  current  of  electricity  is  sent  through  the  straight 
conductor  the  lines  of  magnetic  force  are  circles  about  the  con- 
ductor. The  magnetic  force  is  stronger  near  the  conductor  and 
weaker  at  a  distance  from  the  conductor.  Figure  8  is  obtained 
with  a  coil  of  a  few  turns  of  wire.  Figure  9  shows  the  magnetic 
field  produced  by  a  long  helical  coil  called  a  solenoid.  With  the 


FIG.  6.     Magnetic  field  about  a  bar 
magnet,  as  depicted  by  iron  filings. 


RELATION  BETWEEN  ELECTRICITY  AND   MAGNETISM      15 

solenoid  the  field  of  magnetic  force  is  seen  to  be  remarkably  like 
that  obtained  with  the  bar  magnet. 

It  may  be  observed  that  in  the  case  of  each  of  the  coils  the  lines 


FIG.  7.    Magnetic  field  about      FIG.  8.     Magnetic  field  linking  with  a  coil  of 
a  straight  conductor  carry-  two  turns  carrying  a  current, 

ing  an  electric  current. 

of  magnetic  force  depicted 
by  the  filings  interlink  with 
the  electric  current. 

This  conception  of  a  field 
of  magnetic  force  about  a 
conductor  carrying  an  elec- 
tric current  is  of  funda- 
mental importance  in  the 
study  of  electric  waves,  in 
which  the  action  in  the 
medium  rather  than  the  ac- 
tion in  the  wires  is  the  chief 
factor  to  be  reckoned  with. 

So  long  as  the  electric 
current  in  the  conductor 
remains  steady,  the  mag- 
netic field  remains  steady. 
With  changes  in  the  elec- 
tric current,  the  magnetic 

field  changes.  This  changing  magnetic  field  about  a  conductor 
carrying  an  oscillatory  current  will  later  be  shown  to  be  one  of 
the  components  of  the  electric  waves  produced  at  the  sending 
station  of  a  wireless  telegraph  system. 


FIG.  9.     Magnetic   field  produced  by  a 
solenoid. 


16  WIRELESS  TELEGRAPHY 

On  the  Production  of  an  Electric  Current  by  a  Variation  of  the 
Magnetic  Field.  —  Bearing  in  mind  that  an  electric  current  pro- 
duces a  field  of  magnetic  force  about  it,  let  us  turn  now  to  the 
question  whether  an  electric  current  can  be  produced  by  the  action  of 
a  magnetic  field. 

For  a  period  of  ten  years  succeeding  Oersted's  discovery,  experi- 
ments directed  to  this  question  gave  the  answer  in  the  negative. 
Finally,  in  1831,  Faraday  in  England  and  Joseph  Henry  in  America 
succeeded,  almost  simultaneously,  in  obtaining  electric  currents  by 
the  action  of  a  magnetic  field,  and  in  explaining  the  cause  of  pre- 
vious failures.  Faraday  and  Henry  showed  that  an  electric  cur- 
rent in  a  conductor  in  a  magnetic  field  is  obtained  as  the  result  of 
a  change  in  the  magnetic  field,  whereas  the  previous  experiments 
had  sought  to  produce  the  effect  by  the  magnetic  field  in  a  steady 
state. 

One  way  of  producing  the  required  change  of  magnetic  field  in 
the  neighborhood  of  the  electric  circuit  is  by  the  motion  of  a  per- 
manent magnet,  with  its  accompanying  field,  toward  or  away  from 

the  circuit.  This  was  done 
in  some  of  Faraday's  and 
Henry's  experiments  and  is 
here  described  with  the  aid 
of  Fig.  10.  A  coil  of  wire 
C  is  connected  to  a  galva- 

FIG.  10.    Apparatus  for  showing  the  pro-    nometer  G.   When  the  north 
duction  of  a  transient  electric  current          i        r   xt.  Arc    • 

by  the  motion  of  a  permanent  magnet.     Pole  of  the   magnet  NS   IS 

made  to  approach  and  enter 

the  coil  C,  the  needle  of  the  galvanometer  is  deflected,  showing  that 
an  electric  current  is  produced.  The  current  is,  however,  only  tran- 
sient, and  after  the  magnet  NS  has  arrived  at  its  final  position  and 
ceased  to  move,  the  needle  of  the  galvanometer  comes  back  to  its 
zero  position,  showing  that  the  current  has  subsided.  Now,  however 
long  the  magnet  NS  is  left  stationary  within  the  coil  C,  no  current 
is  produced.  But  if  the  magnet  is  quickly  withdrawn,  the  galva- 
nometer registers  a  current  in  the  direction  opposite  to  the  current 
obtained  by  the  introduction  of  the  magnet.  This  current  is  also 
transient,  and  subsides  when  the  magnet  NS  becomes  stationary. 
If  the  south  pole  of  the  magnet  is  now  introduced  into  the  coil,  the 
galvanometer  shows  a  transient  current  opposite  to  that  produced 
by  the  introduction  of  the  north  pole.  The  withdrawal  of  the 
south  pole  gives  a  transient  current  opposite  to  that  caused  by 


RELATION  BETWEEN  ELECTRICITY  AND  MAGNETISM      17 


its  own  introduction,  and  in  the  same  direction  as  that  given  by 
the  introduction  of  the  north  pole. 

Another  way  of  obtaining  a  similar  result  is  to  employ  two  coils 
of  wire  placed  near  each  other  but  not  electrically  connected,  as 


FIG.  11.    Apparatus  for  showing  electromagnetic  induction. 

shown  in  Fig.  11.  One  of  these  coils,  S,  which  we  will  call  the 
secondary,  is  connected  with  the  galvanometer  (7,  while  the 
other,  called  the  primary,  P,  may  be  connected  with  the  ter- 
minals of  a  galvanic  battery  B.  No  current  is  shown  in  the  gal- 
vanometer when  a  constant  current  is  sent  through  the  primary; 
but  when  the  current  in  the  primary  is  made,  broken  or  reversed, 
transient  currents  are  obtained  in  the  galvanometer.  That  is  to 
say,  the  current  in  the  primary  sets  up  a  magnetic  field  linking 
with  the  secondary  circuit.  While  the  primary  current  is  steady, 
this  field  is  steady  and  no  effect  is  obtained  in  the  secondary. 
But  variations  of  the  current  in  the  primary  cause  variations  of 
the  magnetic  field  and  consequently  currents  in  the  secondary. 

The  variable  currents  in  the  secondary  are  said  to  be  induced 
by  the  variable  currents  in  the  primary,  and  the  phenomenon  is 
referred  to  as  electromagnetic  induction.  It  is  in  part  by  action  of 
this  kind  that  currents  at  the  receiving  station  of  a  wireless  tele- 
graph system  are  produced  by  the  action  of  variable  currents  at 
the  sending  station.  The  extension  of  the  effects  of  electromag- 
netic induction  to  the  case  of  two  circuits  widely  separated  from 
each  other  we  shall  see  to  be  the  result  of  the  use  of  extremely 
rapid  electric  oscillations  at  the  sending  station. 

On  Mutual  Induction.  —  Let  us  examine  a  little  more  specifi- 
cally the  case  of  electromagnetic  induction  described  hi  the  gal- 
vanometer experiment  cited  above. 

This  experiment  shows  that  when  the  current  in  the  primary 
coil  is  increasing,  the  current  induced  in  the  secondary  coil  is  hi 


18  WIRELESS  TELEGRAPHY 

the  opposite  direction  to  the  primary  current;  while,  if  the  current 
in  the  primary  is  decreasing,  the  current  in  the  secondary  is  in 
the  same  direction  as  the  primary  current.  Perhaps  it  would  be 
better  to  speak  of  the  electromotive  force  l  in  the  secondary  rather 
than  the  current,  because  the  electromotive  force  in  the  secondary 
bears  a  simple  relation  to  the  current  in  the  primary.  The  simple 
relation  is,  that  the  electromotive  force  in  the  secondary  is  pro- 
portional to  the  time  rate  of  change  of  the  current  in  the  primary. 
If  E2  is  the  electromotive  force  induced  in  the  secondary,  /i  the 
current  in  the  primary,  /i  the  time  rate  of  increase  of  the  current  /i, 
then  theory  and  experiment  show  that 

#2=  -  Mi,,  (i) 

in  which  M  is  a  constant  depending  on  the  form  and  position  of 
the  two  circuits. 

M  is  called  the  coefficient  of  mutual  induction,  or,  more  briefly, 
the  mutual  inductance  of  the  two  circuits.  M  is  found  to  have 
the  same  value  if  the  variable  current  is  sent  through  the  second- 
ary and  the  electromotive  force  examined  in  the  primary. 

Consistent  with  the  above  equation,  the  Mutual  Inductance  of 
two  circuits  is  defined  as  the  electromotive  force  induced  in  one  of  the 
circuits  when  the  current  in  the  other  is  changing  at  the  rate  of  one 
unit  current  per  second. 

The  mutual  inductance  between  two  circuits  is  increased  by 
increasing  the  number  of  turns  on  either  or  both  of  the  circuits 
or  by  bringing  the  circuits  nearer  together,  or  by  introducing  iron 
or  other  magnetizable  metals  within  the  circuits.  Methods  of 
calculating  the  mutual  inductance  of  circuits  of  various  forms  are 
given  in  Appendix  II. 

By  a  reference  to  equation  (1)  given  above  it  is  seen  that  the 
electromotive  force  induced  in  the  secondary  is  increased  by  in- 
creasing the  rate  of  change  of  current  in  the  primary.  That  is, 
in  order  to  get  a  large  induced  electromotive  force  at  our  receiving 
station  we  should  have  as  large  a  current  as  possible  at  our  send- 
ing station  and  change  it  as  rapidly  as  possible.  This  result  is 
best  attained  by  the  use  of  currents  of  high  frequency  at  the 
sending  station,  such  as  are  obtained  by  the  discharge  of  a 
Ley  den  jar. 

Self-induction.  —  In   the   case  of  the  two  coils  placed  near 
together  in  the  preceding  discussion,  it  was  found  that  the  elec- 
1  This  term  is  defined  in  Appendix  I. 


RELATION  BETWEEN  ELECTRICITY  AND    MAGNETISM     19 

tromotive  force  in  the  secondary  is  produced  by  a  variable 
magnetic  field  from  the  primary  interlinking  with  the  secondary. 
Now,  if  instead  of  two  coils  we  have  one  coil  alone  carrying  a  vari- 
able current,  the  variable  current  produces  a  variable  magnetic 
field  linking  with  the  circuit  itself,  and  in  consequence  a  back 
electromotive  force  is  produced  in  this  coil  tending  to  oppose  the 
variation  of  the  current  in  it.  This  action  of  the  current  on 
itself  is  called  self-induction.  The  back  electromotive  force  due 
to  self-induction  in  the  circuit  is  connected  with  the  current  in  the 
circuit  by  the  formula 

#1=  -  Li/!,  (2) 

in  which  L^  is  called  the  coefficient  of  self-induction,  or,  more 
briefly,  the  self-inductance  of  the  circuit.  7t  is  an  abbrevia- 
tion for  the  time  rate  of  change  of  the  current.  The  subscripts  1 
show  that  all  the  quantities  refer  to  the  same  circuit. 

Consistent  with  equation  (2),  the  self-inductance  of  a  circuit  may 
be  defined  as  the  back  electromotive  force  of  induction  in  the  circuit 
when  the  current  in  the  circuit  is  changing  at  the  rate  of  one  unit 
current  per  second. 

The  numerical  value  of  the  self -inductance  depends  on  the  geo- 
metrical form  of  the  circuit.  In  Appendix  II  formulas  are  given 
for  calculating  the  self-inductance  of  some  simple  forms  of  circuit. 

This  discussion  of  self-inductance  is  here  introduced  in  quanti- 
tative terms,  because  this  quantity  is  of  fundamental  importance 
in  the  study  of  oscillatory  currents.  I  am  aware  that  the  semi- 
mathematical  form  in  which  the  idea  is  presented  may  fail  to  give 
a  clear  conception  of  the  phenomenon,  so  I  propose  to  attempt 
in  the  next  chapter  to  describe  self-induction  by  the  aid  of  cer- 
tain familiar  analogies. 


CHAPTER  IV 

ON  THE  RESEMBLANCE  OF  SELF-INDUCTION  TO 
MECHANICAL  INERTIA 

IN  the  previous  chapter  it  has  been  pointed  out  that  sejf -induc- 
tion is  the  action  of  a  variable  current  on  itself  due  to  the  produc- 
tion of  a  variable  magnetic  field  by  the  current.  When  a  current 
of  electricity  flows  in  a  circuit  of  any  form,  a  field  of  magnetic 
force  is  set  up  and  links  with  the  circuit.  The*  manner  in  which 
the  flow  of  current  in  the  wire  produces  magnetic  effects  in  the 
surrounding  medium  is  not  completely  understood,  but  that  such 
effects  exist  is  made  evident  by  bringing  a  magnetic  compass 
needle  up  near  the  circuit;  the  compass  needle  tends  to  set  itself 
in  certain  intangible  lines  called  lines  of  magnetic  force.  The 
lines  of  magnetic  force  produced  by  a  current  are  closed  curves 
linking  with  the  wire  carrying  the  current,  as  is  shown  by  the 
compass  needle  or  by  the  distribution  of  the  iron  filings  depicted 
in  Figs.  7,  8  and  9. 

Experiments  similar  to  those  cited  in  the  previous  chapter 
show  that  when  a  change  is  made  in  the  electric  current  in  the 
wire,  the  magnetic  field  surrounding  the  wire  is  changed,  and  that 
these  changes  in  the  magnetic  field  impress  back  upon  the  circuit 
an  electromotive  force  opposing  the  change  of  current.  The  self- 
induction  of  an  electric  circuit  may  thus  be  described  as  a  property 
that  tends  to  prevent  a  change  of  the  electric  current  in  the  circuit. 

In  this  respect  self-induction  resembles  the  property  of  inertia 
in  matter.  The  inertia  of  a  body  is  that  property  by  virtue 
of  which  a  body  tends  to  persist  in  its  state  of  rest  or  motion. 
If  a  body  is  at  rest  or  is  moving  with  a  given  velocity,  the  inertia 
of  the  body  opposes  a  change  of  its  state  of  rest  or  motion.  In  a 
similar  manner,  the  self-induction  in  an  electric  circuit  opposes  a 
change  of  the  electric  current  in  the  circuit. 

From  this  it  need  not  be  inferred  that  electricity  itself  is  a  form 
of  matter  possessing  inertia,  because  in  the  case  of  the  electric 
current  we  may  believe  that  the  inertia  resides  primarily  not  in  the 
electricity  but  in  the  magnetic  field  set  up  by  the  current. 

20 


SELF-INDUCTION  TO  MECHANICAL  INERTIA  21 

The  correctness  of  this  belief  is  evidenced  by  the  fact  that  with 
a  fixed  current  flowing  in  a  wire  the  self-induction  may  be  greatly 
increased  by  bending  the  wire  into  the  form  of  a  coil.  Now  mak- 
ing the  wire  into  a  coil  does  not  change  the  amount  of  electricity 
flowing  in  the  wire,  but  it  does  change  the  strength  of  the  mag- 
netic field  about  the  wire.  The  inertia  of  the  current,  therefore, 
has  its  existence  not  primarily  in  the  conductor  but  in  the  medium 
surrounding  the  conductor. 

The  Contrast  of  Self-induction  with  Resistance  and  its 
Resemblance  to  Inertia.  —  The  self-induction  of  a  circuit  acts 
upon  the  current  in  a  manner  entirely  different  from  the  manner 
in  which  resistance  acts.  The  resistance  of  a  circuit  always  op- 
posed the  flow  of  the  current,  and  when  a  current  is  sent  through 
a  conductor,  some  of  the  energy  of  the  current  is  used  up  in  over- 
coming the  resistance  of  the  conductor;  or, more  properly  speaking, 
some  of  the  electric  energy  is  converted  into  heat.  This  is  true 
whether  the  current  is  increasing  or  diminishing  or  is  steady ;  and 
the  heat  developed  is  not  again  completely  available  for  producing 
electric  current,  so  that  a  continuous  supply  of  energy  is  needed 
at  the  source  of  the  electric  current  to  keep  up  the  current 
against  the  resistance  of  the  circuit. 

Self-induction,  on  the  other  hand,  does  not  change  the  electrical 
energy  into  heat.  When  the  current  is  steady,  self-induction  has 
no  effect.  If,  however,  the  current  is  increasing,  some  of  the 
energy  supplied  to  the  system  is  employed  in  establishing  the 
magnetic  field.  If  now  the  current  is  allowed  to  decrease  by  an 
equal  amount,  the  energy  stored  up  in  the  magnetic  field  is  re- 
stored to  the  conductor  and  helps  to  maintain  the  current.  Thus, 
during  a  cyclic  *  change  of  the  current  as  much  energy  may  be 
obtained  from  the  magnetic  field  as  was  given  to  it. 

Hence,  if  we  have  an  oscillatory  current  in  a  circuit,  none  of  the 
energy  of  the  current  is  consumed  by  the  action  of  the  self-induc- 
tion, and  the  supply  of  energy  at  the  source  is  wasted  only  in 
overcoming  the  resistance  of  the  circuit.2 

It  is  apparent  that  in  respect  to  the  consumption  of  energy  self- 
induction  resembles  inertia  in  matter.  Energy  is  required  in  order 

1  A  cyclic  change  is  a  change  from  any  value  A  to  any  other  value  By  and 
from  B  back  to  A  again. 

2  Later  we  shall  see  that  for  some  forms  of  circuit  this  statement  is  not 
strictly  true,  because  some  of  the  energy  may  be  radiated  as  electric  waves. 
Also  in  the  case  of  some  media,  as  iron,  in  the  field  of  magnetic  force,  some 
of  the  energy  is  converted  into  heat  by  hysteresis. 


22  WIRELESS  TELEGRAPHY 

to  set  a  heavy  body  in  motion,  but  this  energy  is  recovered  when 
the  body  is  stopped,  and  the  only  loss  of  availability  of  energy  in 
a  cyclic  process  in  which  a  body  is  started  in  motion  and  stopped 
again  is  that  lost  in  overcoming  friction  in  the  machinery  used 
for  starting  and  stopping  the  body. 

When  a  body  is  set  in  motion,  the  energy  supplied  in  producing 
the  motion  is  stored  up  in  the  body  as  kinetic  energy,  so  that  analo- 
gously many  writers  refer  to  the  energy  of  the  magnetic  field  as 
kinetic  in  character.  Without  necessarily  committing  ourselves  to 
this  specific  proposition  as  to  the  kinetic  character  of  the  magnetic 
field,  it  will  still  be  useful  to  keep  in  mind  that  self-inductance 
opposes  changes  in  the  electric  current  in  the  same  general  manner 
as  inertia  opposes  changes  in  the  motion  of  bodies. 

Keeping  this  analogy  in  mind,  we  can  easily  foresee  many  of  the 
facts  about  the  flow  of  electricity;  for  example,  suppose  that  a 
rapidly  alternating  electromotive  force  is  applied  to  a  circuit  con- 
taining a  large  self-inductance;  usually  only  a  small  current  will 
flow,  just  as  only  a  small  motion  will  generally  be  communicated 
to  a  heavy  body  by  a  rapidly  varying  material  force.  There  are. 
however,  special  cases  in  which  the  periodic  force  will  set  up  a 
large  motion  of  the  material  body.  This  happens  when  the 
period  of  the  force  is  the  same  as  the  natural  period  of  the  body. 
But  in  order  for  the  body  to  have  a  natural  period  something 
besides  inertia  is  required ;  namely,  the  body  must  be  elastic  or 
must  be  elastically  attached  to  something.  So  in  the  case  of 
the  electric  circuit  it  is  also  possible  to  get  a  large  current  with 
a  rapidly  varying  electromotive  force,  provided  the  circuit  con- 
tains besides  its  self-inductance  a  suitable  amount  of  electrostatic 
capacity,  which  will  be  shown  to  supply  the  factor  required  to 
give  periodicity  to  the  electric  circuit. 

Before  developing  further  our  notions  in  regard  to  self-induc- 
tance, it  is  proposed  to  introduce  this  other  phenomenon  that 
enters  prominently  into  the  discussion  of  electric  waves;  namely, 
the  phenomenon  of  electrostatic  capacity. 


CHAPTER  V 
ON  ELECTROSTATIC  CAPACITY 

THE  last  two  chapters  have  been  devoted  to  a  discussion  of 
electric  currents  and  the  magnetic  field  accompanying  such  cur- 
rents. In  order  to  arrive  at  a  conception  of  the  nature  of  electric 
waves  it  is  necessary  also  to  give  some  attention  to  the  action  of 
electric  charges  at  rest.  This  is  the  subject  of  electrostatics.  Here 
again  we  must  look  to  Faraday  for  the  fundamental  discoveries. 
In  the  beginning  paragraph  of  his  most  important  research  on  this 
subject  Faraday  says: l 

"  To  those  philosophers  who  pursue  the  inquiry  zealously  yet 
cautiously,  combining  experiment  with  analogy,  suspicious  of  their 
preconceived  notions,  paying  more  respect  to  fact  than  to  theory, 
not  too  hasty  to  generalize,  and  above  all  things,  willing  at  every 
step  to  cross-examine  their  own  opinions,  both  by  reasoning  and 
by  experiment,  no.  branch  of  knowledge  can  afford  so  fine  and  ready 
a  field  for  discovery  as  this." 

Influence  of  Intervening  Medium  on  Electric  Attraction.  —  The 
result  obtained  by  Faraday  in  the  research  referred  to  is  that  the 
electrostatic  repulsion  or  attraction  between  two  charged  bodies 
is  influenced  by  the  medium  intervening  between  the  charged 
bodies.  If,  for  example,  we  have  two  flat  metallic  plates  placed 
parallel  to  each  other,  and  we  charge  one  of  the  plates  positively 
and  the  other  negatively,  the  electrostatic  attraction  between  the 
two  charges  on  the  plates  will  be  less  when  the  plates  are  separated 
by  glass  than  when  they  are  separated  by  air,  provided  the  plates 
are  charged  with  the  same  quantity  of  electricity  in  the  two  cases. 
The  attraction  between  the  charges  on  the  plates  with  glass  inter- 
vening will  be  about  one-sixth  as  much  as  that  with  the  same  thick- 
ness of  air  intervening;  so  that  in  order  to  get  the  same  force 
between  the  charges  on  the  plates  in  the  two  cases  we  must  put 
upon  the  plates  with  glass  between  them  six  times  as  much  elec- 
tricity as  is  required  with  air  between. 

1  Faraday:  Experimental  Researches  in  Electricity  and  Magnetism,  Vol.  I, 
Eleventh  Series,  Nov.,  1837. 

23 


24  WIRELESS  TELEGRAPHY 

We  thus  come  to  the  result  that  the  insulating  medium  between 
the  oppositely  charged  metallic  plates  serves  not  merely  to  separate 
the  plates  and  prevent  them  from  losing  their  charge,  but  serves 
also  to  determine  the  charge  the  plates  will  receive  for  a  given 
electromotive  force;  for  example,  a  given  battery  connected 
between  the  plates.  And  since  the  insulating  medium  between 
the  plates  has  other  functions  than  merely  to  insulate,  Faraday 
proposes  to  designate  the  insulating  medium  by  the  name  dielec- 
tric, when  reference  is  made  to  the  force  acting  through  it.  He 
says,  "  I  use  the  word  dielectric  to  express  that  substance  through 
or  across  which  the  forces  are  acting." 

On  Condensers.  —  The  apparatus  consisting  of  two  conducting 
bodies  separated  by  a  dielectric  is  called  a  condenser.  An  ordinary 
Ley  den  jar,  consisting  of  two  metallic  coatings  separated  by  glass, 
is  a  familiar  case  of  an  electric  condenser.  Any  two  conducting 
bodies  with  a.  dielectric  between  constitute  a  condenser.  As  an 
extreme  case,  a  single  conducting  body  isolated  in  space  is  con- 
sidered a  condenser,  with  empty  space  as  dielectric,  and  with  the 
other  conductor  removed  to  an  infinite  distance.  As  another 
example,  a  charged  body  in  the  neighborhood  of  the  earth  is  a 
condenser,  with  the  earth  for  the  other  conductor  and  with  air  as 
dielectric. 

Capacity  of  Condenser.  —  Different  condensers  are  said  to  have 
different  capacities,  which  term  does  not  refer  to  the  total  amount 
of  electricity  that  the  condensers  can  contain,  but  to  the  quantity  of 
electricity  they  will  take  under  the  action  of  a  given  electromotive 
force;  namely,  a  unit  electromotive  force.  In  the  practical  system 
of  units  (see  Appendix  I),  the  unit  of  electromotive  force  is  the 
volt,  the  unit  of  quantity  of  electricity  is  the  coulomb,  and  the 
unit  of  capacity  the  farad.  A  farad  is  the  capacity  of  a  condenser 
that  can  be  given  a  charge  of  one  coulomb  under  the  action  of 
electromotive  force  of  one  volt.  The  farad  is  a  very  large  unit 
of  capacity;  for  example,  the  electrostatic  capacity  of  the  whole 
earth  is  only  about  .000708  farad.  That  is  to  say,  it  would  take 
only  about  seven  ten-thousandths  of  a  coulomb  to  raise  the  poten- 
tial of  the  earth  one  volt.  Since  the  farad  as  a  unit  is  very  large, 
the  capacity  of  a  condenser  is  often  designated  in  millionths  of  a 
farad,  or  microfarads. 

The  quantity  of  electricity,  Q,  on  each  plate  of  a  condenser  of 
capacity  C  is  Q  =  CV,  where  V  is  the  difference  of  potential 
between  the  plates. 


ELECTROSTATIC  CAPACITY  25 

Dielectric  Constant.  —  Returning,  now,  to  the  function  of  the 
dielectric  in  determining  the  capacity  of  a  condenser,  the  term 
dielectric  constant  of  a  substance  is  used  to  designate  the  capacity 
of  a  condenser  with  the  substance  as  dielectric  relative  to  the 
capacity  of  the  same  condenser  with  empty  space  as  dielectric. 
The  dielectric  constant  of  air  and  all  the  gases  at  ordinary  pressure 
is  approximately  unity;  this  means  that  the  capacity  of  a  con- 
denser with  a  gas  as  dielectric  is  not  much  changed  when  the  gas 
is  pumped  away.  In  the  example  cited  above  the  dielectric  con- 
stant of  a  particular  glass  is  given  as  six;  that  is,  the  quantity  of 
electricity  that  a  condenser  will  contain  under  a  given  electro- 
motive force  with  this  glass  as  dielectric  is  six  times  the  quantity 
the  condenser  will  contain  under  the  same  electromotive  force 
when  air  is  substituted  for  the  glass.  A  table  of  dielectric  con- 
stants, together  with  some  numerical  formulas  for  calculating  the 
capacity  of  some  simple  forms  of  condenser  and  rules  for  combina- 
tions of  condensers  in  series  and  parallel,  is  given  in  Appendix  II. 

General  Facts  about  Energy  and  Electromotive  Force  of 
Charged  Condenser.  —  In  order  to  send  a  charge  of  electricity  into 
a  condenser,  energy  is  required,  but  the  energy  is  not  converted 
into  heat,  as  it  is  in  the  case  of  a  current  of  electricity  flowing 
through  a  resistance ;  for  the  energy  of  the  charge  may  be  recovered 
as  electric  energy  when  the  condenser  is  allowed  to  discharge.  In 
a  cyclic  process  in  which  a  condenser  is  charged  and  discharged 
again,  there  is  no  loss  of  availability  of  energy  in  the  processes  that 
occur  in  the  condenser.  And  when  a  condenser  charges  and  dis- 
charges several  times  in  an  oscillatory  manner,  it  is  necessary  to 
supply  energy  from  without  only  in  so  far  as  the  electric  energy 
is  radiated  or  is  converted  into  heat  in  flowing  through  some  resist- 
ance in  the  circuit.1 

It  has  undoubtedly  been  observed  by  the  reader  that  in  respect 
to  the  reception  of  energy  from  the  circuit  and  the  return  of  the 
same  amount  of  energy  to  the  circuit  again  the  medium  of  the 
condenser  behaves  somewhat  like  the  medium  of  the  magnetic 
field.  There  is,  however,  one  marked  difference.  In  the  case  of 
the  magnetic  field,  the  opposing  electromotive  force  called  into  play 
•by  self-induction  is  proportional  to  the  rate  at  which  the  current  is 
changing;  while,  hi  the  case  of  the  condenser,  the  electromotive  force 
V  opposing  the  flow  of  electricity  into  the  condenser  is  proportional 

1  This  statement  is  not  always  strictly  true,  because  in  some  forms  of  con- 
denser a  small  part  of  the  energy  is  consumed  by  hysteresis  in  the  dielectric. 


26  WIRELESS  TELEGRAPHY 

to  the  quantity  Q  of  electricity  in  the  condenser.     Numerically  V  ~  Q, 

Q 

and  in  proper  units  V  =  ^ ,  where  C  is  the  capacity  of  the  con- 

C 

denser. 
Mechanical  Systems  Analogous  to  an  Electrical  Condenser.  — 

I.  We  have  a  condition  of  things  analogous  to  the  charging  of  a 
condenser  in  the  act  of  supplying  water  to  a  tall  cylindrical  reser- 
voir. The  force  P  required  to  send  water  into  the  reservoir  against 
the  hydrostatic  pressure  of  the  water  already  in  the  reservoir  is 
proportional  to  the  height  h  of  water  in  the  reservoir,  which  is 
proportional  to  the  amount  of  water  M  in  the  reservoir.  Numeri- 
cally, in  suitable  units  P  =  — -,  where  S  is  the  area  of  cross  section 

>S 

of  the  reservoir.     S  may  be  looked  upon  as  analogous  to  C. 

II.  Another  analogue  to  the  action  of  a  condenser  is  found  in 
the  forces  called  into  play  in  the  act  of  compressing  an  elastic 
spring.  The  restoring  force  F  of  the  spring  is  proportional  to  the 
amount  x  by  which  the  spring  is  compressed.  Numerically, 
F  =  ex,  where  e  is  the  stiffness  of  the  spring. 

In  the  case  of  the  condenser  it  should  be  borne  in  mind  that  the 
greater  the  capacity  of  the  condenser  the  less  the  electromotive 
force  required  in  order  to  charge  it  with  a  given  amount  of  elec- 
tricity. In  this  respect  capacity  of  the  condenser  resembles  the 
reciprocal  of  the  stiffness  of  the  spring,  for  the  greater  the  stiffness 
e  of  the  spring  the  greater  the  force  F  required  to  compress  it  by  a 
given  amount. 

Flow  of  Current  in  a  Circuit  Containing  a  Condenser.  —  The 
reader  will  note  the  following  fundamental  facts  in  regard  to  the 
action  of  a  condenser.  If  a  battery  having  a  constant  electromo- 
tive force  E  has  its  positive  pole  connected  to  one  plate  of  a 
condenser  and  its  negative  pole  connected  to  the  other  plate, 
electricity  will  flow  into  the  condenser  and  charge  it.  As  the  con- 
denser charges  it  gives  rise  to  a  back  electromotive  force  opposing 
the  flow,  so  that  the  current  is  diminished  more  and  more  by  the 
opposing  e.m.f.  of  the  condenser,  as  the  condenser  is  charging. 
The  e.m.f.  at  each  instant  is  proportional  to  the  quantity  q  of 
electricity  in  the  condenser  and  is  inversely  proportional  to  the 
capacity  C  of  the  condenser.  When  this  opposing  e.m.f.  becomes 
equal  to  the  e.m.f.  of  the  battery,  E,  the  flow  of  electricity  ceases. 

Then  E  =  —  where  Q  is  the  final  charge  attained  by  the  con- 
C 


ELECTROSTATIC  CAPACITY  27 

denser.  After  this  condition  is  reached,  no  further  current  flows. 
This  process  of  charging  the  condenser  is  described  as  gradual 
because  time  is  required  for  the  final  condition  to  be  established, 
but  this  time  is  usually  very  short. 

Work  Done  in  Charging  Condenser.  —  During  this  process  of 
charging  the  condenser,  the  average  e.m.f.  of  the  condenser  was 
£  E;  the  work *  done,  which  is  the  charge  introduced  multiplied  by 
the  e.m.f.  of  the  condenser,  is  Q  X  k  E',  or,  substituting  for  Q  its 
value  EC,  the  work  W  done  in  charging  the  condenser  is 

W  =  J  E2C. 
1  See  definitions  of  electrical  work,  in  Appendix  I. 


CHAPTER    VI 


ON  THE  DISCHARGE  OF  A  CONDENSER  THROUGH  AN 
INDUCTANCE  AND  RESISTANCE 

The  Oscillatory  Discharge.  —  We  are  now  ready  to  undertake 
a  more  critical  examination  of  the  proposition  set  down  in  the 
first  chapter  that,  under  certain  conditions,  the  discharge  of  a 
Ley  den  jar  is  oscillatory.  As  a  mechanical  analogy,  let  us  con- 
sider the  motion  of  a  heavy  bob  attached  to  an  elastic  spring.  Let 
the  position  of  rest  of  the  bob  be  the  position  a, 
Fig.  12.  If  now  the  bob  is  pulled  down  to  a  posi- 
tion b  and  released,  the  spring  draws  it  back  again 
to  a.  During  this  process  the  bob  acquires  a  ve- 
locity determined  by  the  stiffness  of  the  spring 
and  the  mass  of  the  bob.  When  the  bob  reaches  a, 
the  spring  ceases  to  pull,  but  the  bob  by  reason  of 
its  inertia  moves  on  up  to  a  position  c,  during 
which  process  the  spring  is  compressed.  When 
the  bob  has  reached  c,  it  has  lost  its  velocity  and 
is  now  driven  back  by  the  compressed  spring.  In 
this  way  the  vibratory 
motion  is  kept  up  for 
some  time,  and  would  be 
kept  up  indefinitely  but 
LJLJ  for  the  fact  that  the  re- 

FIG.  12    Spring  sistance  of  the  air  and 
and  bob  for  .  ,       .   . 

illustrating  the  imperfect  elasticity 
oscillatory  of  fag  spring  convert 
motion.  _  • 

some  of  the  energy  into 

heat  during  each  excursion,  so  that  the  amplitude  of  the  motion 
is  diminished  more  and  more  until  the  body  finally  comes  to 
rest  at  a. 

As  another  illustration,  suppose  a  body  of  water  to  be  contained 
in  a  bent  tube  of  the  form  of  Fig.  13.  Let  the  surface  of  the  water 
in  its  position  of  rest  be  at  a  and  a'  in  the  two  arms  of  the  tube. 
Suppose  now  that  the  water  is  moved  into  the  position  W  and 

28 


FIG.  13.  Water  column 
showing  vibratory 
motion. 


DISCHARGE  OF  CONDENSER 


29 


released.  The  column  of  water  will  vibrate  back  and  forth  in  the 
tube  so  that  its  level  in  the  left-hand  arm  of  the  tube  comes  suc- 
cessively above  and  below  the  position  a.  During  each  excursion 
the  amplitude  of  the  motion  is  diminished  till  the  water  finally 
comes  to  rest  in  its  initial  position. 

Both  of  these  forms  of  mechanical  vibratory  motion  are  easily 
realized  in  practice,  and  both  bear  a  marked  resemblance  to  the 
flow  of  electricity  in  the  discharge  of  a  condenser  through  an 
inductance  and  resistance. 

In  order  now  to  understand  how  a  condenser  discharge  may  be 
oscillatory  in  character,  suppose  a  Ley  den  jar,  or  other  form  of 
electrical  condenser,  of  capacity  C  to  be  initially  charged,  say 
from  an  electric  machine,  with  a  quantity  of  electricity  +Qo  on 
one  plate  and  —  Qo  on  the  other. 
And  suppose  that  the  condenser  has 
in  series  with  it  a  self -inductance  L, 
and  a  spark  gap  S.  (Fig.  14.)  At 
first  let  the  spark  gap  be  too  wide 
for  the  spark  to  pass.  Positive 
electricity  will  be  distributed  over 
the  one  coating  and  one  knob  of 
the  spark  gap,  and  negative  elec- 
tricity will  be  distributed  over  the 
other  coating,  the  coil  L  and  the 
other  knob  of  the  spark  gap. 

Let  VQ  be  the  difference  of  po- 
tential between  the  plates  of  the 
condenser.  Before  the  current  starts  there  will  be  the  same  dif- 
ference of  potential  between  the  knobs  of  the  spark  gap,  because 
all  parts  of  a  conductor  in  which  no  current  is  flowing  are  at  the 
same  potential. 

Let  us  suppose,  now,  that  the  knobs  of  the  spark  gap  are  made 
to  approach  each  other  until  the  gap  is  short  enough  for  the  poten- 
tial to  start  a  spark  (i.e.,  about  39,000  volts  to  the  centimeter,  if 
the  terminals  of  the  gap  are  balls  1  cm.  in  diameter).  When  the 
spark  starts,  the  resistance  of  the  gap  suddenly  drops  to  a  very 
small  value,  in  some  cases  to  a  small  fraction  of  an  ohm,1  and  the 
electric  current  begins  to  flow  across  the  gap  under  the  action  of 
the  high  difference  of  potential  between  the  plates. 

1  We  have  seen  in  Chapter  II  that  a  spark  is  one  of  those  agencies  that 
render  gases  conductive. 


FIG.  14.     Leyden  jar,  inductance 
coil,  and  spark  gap. 


30  WIRELESS  TELEGRAPHY 

The  current  flowing  through  the  circuit  has  a  small  value  when 
the  spark  first  begins  to  pass.  If  it  were  not  for  the  self-induction 
of  the  circuit,  the  current  would  spring  to  a  large  value,  because 
the  electromotive  force  of  the  circuit  is  high  and  its  resistance  low. 
We  have  seen,  however,  that  the  self-induction  acts  in  such  a  man- 
ner as  to  oppose  rapid  changes  in  the  current.  As  a  result  the 
current  requires  time  to  attain  its  maximum.  When  the  current 
reaches  its  maximum,  the  condenser  is  completely  discharged, 
but  there  is  a  large  current  flowing.  This  current  cannot  stop 
at  once,  for  the  self-induction  now  acts  in  the  reverse  direction 
and  opposes  the  decrease  of  the  current,  so  that  the  current  con- 
tinues to  flow  after  the  electromotive  force  of  the  condenser  has 
become  zero.  This  process  charges  the  condenser  oppositely  to 
its  original  charge,  and  when  the  current  in  this  direction  ceases, 
the  back  electromotive  force  of  the  condenser  starts  the  current 
in  the  reverse  direction.  The  condenser  is  again  charged  in  its 
original  direction,  the  current  again  reverses  and  the  process  con- 
tinues for  a  number  of  oscillations  depending  on  the  resistance, 
self -inductance  and  capacity  of  the  circuit. 

The  essential  factors  entering  into  the  production  of  the  oscilla- 
tory discharge  are  the  self-inductance  and  the  capacity  of  the  cir- 
cuit, characterized  in  their  actions  by  the  fact  that  they  are  out 
of  phase  with  each  other,  so  that  when  the  effect  of  the  capacity 
is  a  maximum  that  of  the  induction  is  a  minimum,  and  vice 
versa. 

On  account  of  the  resistance  of  the  circuit  some  of  the  electrical 
energy  is  converted  into  heat  during  each  flow  of  the  current,  so 
that  the  maximum  attained  by  the  current  at  each  oscillation  falls 
lower  and  lower  until  the  spark  ceases.  The  decrease  of  the  ampli- 
tude of  the  oscillation  under  the  action  of  the  resistance  is  referred 
to  as  damping  of  the  oscillation  by  the  resistance.  It  will  be  seen 
later  that  the  radiation  of  energy-  as  elecbric  waves  acts  also  in  a 
manner  to  damp  the  oscillations. 

Criterion.  —  In  his  mathematical  investigation  of  this  problem 
Sir  William  Thomson  showed  that  the  discharge  occurs  in  the 
oscillatory  manner  here  described  only  when  the  resistance  of  the 
circuit  does  not  exceed  a  certain  value  relative  to  the  ratio  of  the 
self-inductance  to  the  capacity  of  the  circuit.  The  exact  expres- 
sion of  this  condition  under  which  the  discharge  is  oscillatory  is, 

R2  <  4  L/C. 


DISCHARGE  OF  CONDENSER  31 

Non-oscillatory  Discharge.  —  If,  on  the  other  hand,  J?2  is 
greater  than  4  L/C,  Thomson  showed  that  the  discharge  is  unidi- 
rectional ;  that  is,  no  reversal  of  the  sign  of  the  charge  takes  place. 
We  should  have  an  analogous  condition  of  affairs  with  the  elastic 
spring  used  as  an  illustration  if  the  bob  B  (Fig.  12)  should  be 
submerged  in  a  liquid,  provided  the  liquid  should  offer  sufficient 
resistance  to  the  passage  of  the  bob  through  it.  Evidently  the 
amount  of  resistance  required  to  prevent  the  oscillation  of  the  bob 
will  increase  with  increase  of  the  inertia  of  the  bob  and  with 
increase  of  the  stiffness  of  the  spring.  The  former  of  these  cor- 
responds to  L,  and  the  latter  to  the  reciprocal  of  C,  so  that  the 
fact  that  L/C  will  occur  in  the  condition  for  the  oscillation  or  non- 
oscillation  of  the  electrical  system  might  have  been  anticipated. 

In  the  case  of  the  water  column,  if  the  connectiag  tube  EF 
between  the  two  vertical  cylinders  in  Fig.  13  is  made  sufficiently 
small  to  offer  enough  friction,  the  motion  of  the  water  will  also 
be  non-oscillatory.  This  is  analogous  to  the  case  of  the  non- 
oscillatory  discharge  of  the  condenser. 

Mathematical  Formulas  for  the  Discharge  of  the  Condenser.— 
Thomson  derived  the  following  equations  for  the  current  i  at  any 
time  t,  where  t  is  measured  in  seconds  from  the  time  when  the  dis- 
charge begins: 

Case  1.     If  R2  <  4  L/C, 

2V  ~Rt 

i 


in  which  F0=  the  initial  difference  of  potential, 
R  =  the  resistance, 
L  =  the  self-inductance, 
C   =  the  capacity,  and 
e   =  2.718281  .  .  .  (base  of  natural  logarithms). 

This  is  the  case  of  the  oscillatory  discharge. 
Case  II.     If  R2  >  4  L/C, 


^O 


32  WIRELESS  TELEGRAPHY 


2  LC 

in  which  TI  =  -  ~  ,  and 

RC-  V#2C2-4  LC 


T, 


RC 


This  is  the  general  case  of  non-oscillatory  discharge. 
Case  III.     If  R2  =  4  L/C, 

V°*  if 
i  =  -—e2L  (5) 

LI 

This  is  the  critical  case,  in  which  the  discharge  is  just  non- 
oscillatory. 

Graphical  Representation  of  Results.  —  By  the  aid  of  the  equa- 
tions (3),  (4)  and  (5)  the  current  in  the  condenser  circuit  at  any 
time  can  be  calculated  in  any  case  in  which  the  constants  of  the 
circuit  and  the  initial  difference  of  potential  of  the  plates  of  the 
condenser  are  known;  of  the  calculated  values  so  obtained  we  can 
construct  a  table,  in  the  first  column  of  which  we  may  place  the 
time  in  convenient  fractions  of  a  second,  and  in  the  second  column 
we  may  write  the  different  values  of  the  current  corresponding  to 
these  different  values  of  the  time. 

There  is,  however,  another  method  of  representing  the  results, 
which  affords  an  easier  comprehension.  This  is  the  graphical 
method,  and  consists  in  constructing  a  curve  on  a  sheet  of  squared 
paper  with  a  scale  of  time  and  a  scale  of  current  at  right  angles 
to  each  other.  As  an  example  of  this  method  of  showing  results, 
let  us  refer  to  Fig.  15,  which  is  a  graphical  representation  of  the 
flow  of  current  in  a  condenser  circuit  in  which  the  resistance  is 
supposed  to  be  zero.  The  horizontal  scale  through  the  center  of 
the  figure  gives  the  time  in  millionths  of  a  second;  the  vertical 
scale  at  the  left  of  the  figure  gives  the  current.  Such  a  diagram 
gives  the  current  at  any  time;  for  example,  when  the  time  is  zero, 
the  current  is  zero.  To  get  the  current  at  one  one-millionth  of  a 
second,  one  goes  out  on  the  horizontal  line  to  one  one-millionth 
second  (which  is  halfway  between  0  and  2),  and  at  this  point 
one  erects  a  vertical  line  which  will  be  seen  to  cut  the  curve  at  a 
point  the  same  height  as  150  amperes  on  the  margin.  This  150 
amperes  is,  then,  the  current  at  TI>  oo  OOF  sec.  In  like  manner,  at 
TI>  o£ooo  sec.,  the  current  is  seen  to  be  about  minus  130  amperes. 

From  this  description  of  the  method  of  interpreting  the  curves 


DISCHARGE  OF  CONDENSER 


33 


it  will  be  evident  how  the  curves  are  drawn;  namely,  a  table  is 
made  of  current  for  different  values  of  time,  by  the  aid  of  formula 
(3),  and  then  for  each  value  of  time  plotted  horizontally  the  cor- 
responding value  of  current  is  erected  vertically,  and  through  the 
points  so  obtained  a  smooth  curve  is  drawn.  This  process  resem- 
bles the  method  employed  by  navigators  to  show  the  route  of  a 
ship.  Each  day,  or  oftener,  an  observation  of  latitude  and  longi- 
tude is  made,  and  a  point  is  put  on  the  map  at  the  intersection  of 
the  given  latitude  and  longitude;  and  through  the  points  thus 
obtained  at  successive  observations  a  smooth  curve  is  drawn,  which 
represents  the  course  of  the  ship,  and  from  which  the  position  of 


!?00r-- 


-160 


-200 1- 


FIG.  15.  Current  from  a  condenser  of  capacity  .01  microfarad  discharging 
through  an  inductance  of  .0001  henry.  Initial  potential  20,000  volts. 
Resistance  zero. 

the  ship  at  points  intermediate  between  the  observations  may  also 
be  approximately  obtained. 

Curves  Showing  Condenser  Discharge.  —  The  manner  in  which 
the  discharge  of  a  condenser  occurs  under  different  conditions  is 
represented  graphically  in  the  curves  of  Figs.  15,  16,  17  and  18. 
In  these  curves  the  time  in  millionths  of  a  second  is  plotted  hori- 
zontally, and  the  current  in  amperes  is  plotted  vertically.  These 
curves  are  calculated  from  the  formulas  given  on  page  31.  In  all 
four  cases  the  capacity,  self-inductance  and  initial  potential 
are  the  same;  namely,  C  =  10  ~  8  farads,  L  =  10  ~  4  henrys, 
V0  =  20,000  volts.  The  only  difference  between  the  conditions 
of  the  discharge  in  the  four  cases  is  the  difference  in  resistance  of 
the  circuit  through  which  the  discharge  occurs. 

In  Fig.  15  the  resistance  is  supposed  to  be  zero,  and  we  have 


34 


WIRELESS  TELEGRAPHY 


as  a  result  what  is  called  an  undamped  oscillation.  The  current 
oscillates  back  and  forth  between  a  positive  maximum  of  200 
amperes  and  a  negative  maximum  of  200  amperes. 

In  Fig.  16  the  resistance  of  the  circuit  is  10  ohms,  and  in  Fig.  17 


200 


150 


100 

S60 

<5 
q 


£-60 

a 

5 
-100 


-160 


7V~A 


4  C/  8         \10        12/        14          \16        18  /     20 

Time\     in      /MillionthB\     of     /a      SecondX  / 


4          \16        18/20  22         24/26         28 

--\J/ \JZ--~ 


k/V  t^— 

FIG.  16.     Same  as  Fig.  15,  except  that  the  resistance  is  10  ohms. 

this  resistance  is  20  ohms.  These  two  curves  show  how  the  current 
is  damped  by  the  resistance  of  the  circuit.  The  curves  of  Figs. 
15,  16  and  17  all  come  under  the  conditions  of  Case  I. 

If,  however,  the  resistance  be  200  ohms,  we  have  the  condition 


g-60 

o 

-100 
-150 
-200 


2-         \4  0/8          \10        12  /      14 

Time\     in      /Millionths\    of    /a      Second 


2          4          6 

Millionths  Second 

FIG.  18.  Same  as  Fig.  15, 
except  that  the  resist- 
ance is  200  ohms. 


FIG.  17.     Same  as  Fig.  15,  except  that  the 
resistance  is  20  ohms. 

for  the  current  to  be  just  non-oscillatory,  R2  =  4  L/C,  and  the 
equation  of  the  curve  is  then  the  equation  given  under  Case  III. 
This  kind  of  discharge  is  shown  in  the  curve  of  Fig.  18.  This 


DISCHARGE  OF  CONDENSER  35 

case  has  also  the  same  capacity,  self-inductance  and  initial  voltage 
as  the  preceding  cases,  but  the  current  is  seen  to  rise  only  to  about 
75  amperes  and  then  gradually  to  approach  zero. 

If  the  resistance  be  made  greater  than  200  ohms,  we  have 
Case  II,  in  which  the  discharge  is  also  non-oscillatory.  A  curve 
representing  this  case  is  not  given;  the  form  of  such  a  curve  is 
somewhat  like  that  of  Fig.  18,  with  the  exception  that  the  curve 
does  not  rise  to  so  great  a  value  and  does  not  approach  zero  so 
rapidly  as  does  the  curve  in  Fig.  18. 

The  Period  of  Oscillation.  —  From  equation  (3),  p.  31,  it  can 
be  shown  that  the  period  of  a  complete  oscillation  of  the  current, 
in  case  the  discharge  is  oscillatory,  is 

T  =  2*    /     2LC=,  (6) 

V4  LC  -  R2C2 

in  which  T  is  the  time  of  a  complete  oscillation  in  seconds;  L,  C 
and  R  are  measured  in  the  same  set  of  units;  e.g.,  henry s,  farads 
and  ohms  respectively;  TT  is  3.1416  .  .  .  ,  the  ratio  of  the  circum- 
ference to  the  diameter  of  a  circle. 

Equation  (6)  is  the  exact  expression  for  the  period,  but  in  most 
practical  cases  that  occur  in  the  use  of  electric  waves  it  is  found 
that  the  effect  of  the  resistance  is  inappreciable  in  its  effect  on 
the  period;  that  is,  in  equation  (6),  R2C2  is  small  in  comparison 
with  4  LC,  so  that  the  expression  for  the  time  of  a  complete  oscil- 
lation simplifies  to 

T  =  27rV/LC.  (7) 

This  formula  is  usually  sufficiently  accurate.  For  example,  in  the 
case  plotted  in  Fig.  16,  the  period  of  oscillation  calculated  by  equa- 
tion (7)  differs  from  the  exact  value,  obtained  from  equation  (6), 
by  one-fourth  of  one  per  cent. 

The  various  formulas  given  in  this  chapter  were  first  obtained 
mathematically  by  Sir  William  Thomson  in  1855.  In  1859  Fed- 
dersen  demonstrated  the  oscillatory  character  of  the  discharge  by  a 
revolving  mirror  photograph  of  the  spark,  similar  to  the  photo- 
graph shown  in  Fig.  3  of  Chapter  I.  Since  then  all  o"  Thomson's 
equations  have  been  submitted  to  careful  tests  and  have  been  found 
to  be  accurate. 


CHAPTER    VII 

MAXWELL'S  THEORY.    ELECTRIC  WAVES.     THE  ELECTRO- 
MAGNETIC THEORY  OF   LIGHT 

IN  the  preceding  chapter  we  have  seen  that  when  a  condenser, 
in  series  with  a  self-inductance  and  resistance,  is  charged  and 
allowed  to  discharge,  the  current  obtained,  if  the  resistance  is  not 
too  large,  will  be  oscillatory  in  character.  In  this  arrangement 
of  apparatus  we  have  a  mechanism  that  serves  as  the  source  of 
electric  waves. 

In  1865  Maxwell  predicted,  .by  mathematical  reasoning  based 
on  some  experiments  of  Faraday,  that  variable  currents  in  a  con- 
ductor produce  electric  waves  in  space,  that  these  electric  waves 
travel  with  the  velocity  of  light,  and  that  light  itself  consists  of 
electric  waves  of  extremely  short  wave  lengths.  While  direct 
experimental  verification  of  this  theory  —  by  the  actual  discovery 
of  electric  waves  —  did  not  come  during  Maxwell's  lifetime,  Max- 
well yet  showed  that  his  predictions  were  strongly  supported  by 
many  of  the  known  facts  about  electricity  and  light. 

Without  the  aid  of  mathematics  it  is  difficult  to  follow  the  steps 
of  Maxwell's  reasoning,  so  that  the  discussion  here  given  will 
undoubtedly  appear  to  the  reader  to  be  inconclusive.  In  the  next 
chapter  we  hope  to  remedy  this  defect  of  the  theoretical  discus- 
sion by  a  description  of  the  actual  experimental  demonstration  of 
the  chief  propositions  of  Maxwell's  theory. 

In  the  derivation  of  his  theory  Maxwell  makes  use  of  the  two 
facts  about  the  relation  of  electricity  to  magnetism  that  we  have 
given  in  Chapter  III ;  namely, 

I.  An  electric  current  in  a  conductor  produces  a  magnetic  field 
in  the  neighborhood  of  the  conductor,  and 

II.  A  variable  magnetic  field  in  the  neighborhood  of  a  con- 
ductor produces  an  electromotive  force  in  the  conductor. 

To  these  two  well-known  experimental  facts  Maxwell  adds  a 
third  proposition  in  the  form  of  an  assumption,  which  has  been 
called  the  displacement  assumption. 

The  Displacement  Assumption.  —  This  assumption  is  an  attempt 
on  the  part  of  Maxwell  to  give  expression  to  the  idea  of  Faraday, 

36 


THE  ELECTROMAGNETIC   THEORY  OF  LIGHT 


37 


that  when  a  condenser  is  charged,  the  condition  of  things  is  not 
completely  described  by  saying  that  a  positive  charge  is  given  to 
one  plate  and  a  negative  charge  to  the  other  plate  of  the  condenser. 
Faraday  showed  that  something  takes  place  in  the  medium  between 
the  plates,  and  Maxwell  makes  the  assumption  that  the  action  in 
the  medium  partakes  somewhat  of  the  nature  of  an  electric  current, 
although  the  medium  is  an  insulating  substance. 

It  is  difficult  to  determine  just  how  Maxwell  imagined  this 
action  to  take  place,  and  different  writers  have  employed  different 
mechanisms  in  the  description  of  the  current  that  Maxwell  sup- 
posed to  exist  in  the  insulators.  One  way  of  representing  his  idea 
is  to  suppose  that  the  insulating  medium,  whether  a  solid,  liquid, 
or  gaseous  dielectric,  or  even  empty  space,  is  made  up  of  small 
parts,  and  to  suppose  that  the  electricity  in  these  small  parts  of 
the  insulator  may  flow  freely 
in  the  small  parts  but  can- 
not flow  from  one  part  to 
the  next.  If  we  call  these 
small  parts  molecules,  we 
may  describe  the  current  in 
the  insulating  medium  as 
the  act  of  polarizing  the 
molecules.  That  is,  for  ex- 
ample, when  the  left-hand 
plate  of  the  condenser  in  Fig. 
19  is  charged  positively,  the 
positive  electricity  added  to 
this  plate  attracts  the  nega- 
tive electricity  and  repels  the  positive  electricity  of  the  neigh- 
boring molecules,  so  that  the  part  of  each  molecule  near  the  plate 
becomes  negative  and  the  distant  part  becomes  positive.  Mole- 
cules in  this  condition  are  said  to  be  polarized.  The  layer  of 
molecules  so  polarized  acts  on  the  next  layer  and  produces  a  similar 
polarization,  so  that  in  turn  the  molecules  throughout  the  medium 
between  the  plates  become  polarized. 

It  is  seen  that  this  general  transfer  of  positive  electricity  to  the 
right  and  negative  electricity  to  the  left  in  the  molecules  would  have 
an  effect  similar  to  an  electric  current  flowing  from  the  positive  plate 
to  the  negative  through  the  insulator.  Maxwell  called  this  general 
transfer  of  electricity  in  the  dielectric  a  displacement  current.  During 
the  charging  of  the  condenser,  the  displacement  current  is  in  the 


FIG.  19.    Illustrating  displacement  current. 


38  WIRELESS  TELEGRAPHY 

same  direction  as  the  current  in  the  conducting  parts  of  the  circuit, 
so  that  the  displacement  current  may  be  said  to  complete  the  con- 
duction current.  During  the  discharge  of  the  condenser  the  dielec- 
tric loses  its  polarity,  and  according  to  Maxwell's  view,  gives  rise  to  a 
displacement  current  in  the  dielectric.  In  this  case,  also,  the  dis- 
placement current  completes  the  conduction  current,  which  is  now 
flowing  away  from  the  positive  plate  of  the  condenser. 

It  has  been  stated  above  that  the  displacement  current  partakes 
of  the  nature  of  an  electric  current.  The  displacement  current  differs 
from  the  ordinary  current  in  that  there  is  within  the  molecules  nothing 
corresponding  to  ordinary  resistance,  so  that  none  of  the  energy  of 
the  displacement  current  is  converted  into  heat.  The  displacement 
current  also  differs  from  the  conduction  current  in  that  the  displace- 
ment current,  under  a  given  applied  electromotive  force,  sets  up  a 
restoring  force  in  the  dielectric  which,  like  the  reaction  of  a  com- 
pressed spring,  soon  becomes  large  enough  to  equalize  the  electro- 
motive force  and  stop  the  current;  whereas  the  conduction  current 
in  a  circuit  that  is  wholly  conductive  continues  to  flow  as  long  as  the 
electromotive  force  is  applied  to  the  circuit. 

These  are  the  differences  between  the  displacement  current  and 
the  ordinary  current.  On  the  other  hand,  according  to  Maxwell's 
theory,  the  displacement  current  is  exactly  like  an  ordinary  electric 
current  in  respect  to  its  relation  to  the  magnetic  field.  We  may 
thus  add  to  the  two  propositions  stated  on  p.  36,  the  proposition 

III.  In  the  case  of  a  circuit  not  entirely  closed  by  conducting 
parts,  the  current  in  the  conducting  parts  is  completed  by  a  dis- 
placement current  through  the  dielectric.  This  displacement'  cur- 
rent produces  a  magnetic  field  in  its  neighborhood;  and  a  variable 
magnetic  field  in  a  dielectric  produces  displacement  currents  in 
the  dielectric. 

Electric  Waves.  —  In  Maxwell's  treatise  the  propositions  I,  II 
and  III  are  discussed  quantitatively,  with  the  result  that  he  obtains 
a  number  of  quantitative  relations  about  light  and  electricity.  How- 
ever, without  such  a  mathematical  discussion  we  may  be  able  to  see 
how  the  facts  assumed  to  be  correct  in  proposition  III  lead  to  the 
idea  of  electric  waves  in  the  dielectric. 

For  this  purpose  let  us  suppose  that  we  have  two  conducting 
bodies  of  the  form  shown  in  Fig.  20.  A  and  B  are  two  metallic 
rods  with  a  small  spark  gap  between.  Suppose  now  that  A  is  charged 
with  electricity  of  one  sign,  and  B  with  electricity  of  the  other  sign, 
and  suppose  the  charges  are  gradually  increased  until  a  spark 


THE  ELECTROMAGNETIC  THEORY  OF  LIGHT 


39 


passes  between  them.  If  the  resistance  is  not  too  large,  the  current 
that  flows  will  be  oscillatory,  because  the  rods  have  electrostatic 
capacity  and  self-inductance.  The  two  metallic  rods  here  pictured 
constitute  an  electric  "  oscillator." 

According  to  Maxwell's  theory,  the  oscillatory  currents  in  the 
oscillator  will  be  completed  by  displacement  currents  in  surrounding 
space.  A  part  of  this  displacement  current  takes  place  along  the 
black  loops  in  the  direction  of  the  arrows  from  one  end  of  the  oscil- 
lator around  to  the  other.  The  displacement  loops  are  really  sec- 
tions of  a  sheet  such  as  would  be  obtained  if  we  rotated  the  figure 


FIG.  20.     Displacement  current  and  magnetic  force. 

about  the  oscillator  as  an  axis.  These  displacement  currents  in  the 
sheet  will  reverse  their  direction  when  the  current  in  the  oscillator 
reverses,  and  are  accompanied  by  a  magnetic  field  of  which  a  single 
line  is  shown  encircling  the  displacement  sheet.  The  magnetic  field 
produced  by  the  displacement  current  in  the  shaded  region,  being 
oscillatory  in  character,  will  induce  displacement  currents  in  a  portion 
of  the  medium  farther  out  from  the  oscillator,  and  the  latter  current 
will  lag  somewhat  behind  the  former.  Thus,  a  sheet  corresponding 
to  the  shaded  region  will  sustain  a  displacement  current  oscillating 
with  the  period  of  the  oscillator.  The  unshaded  region  farther  out 
will  sustain  similar  oscillations  a  little  later,  so  that  we  have  the 
condition  of  things  that  exists  in  a  wave  motion  traveling  with  a 
finite  velocity;  namely,  a  series  of  disturbances  first  in  one  direction, 
then  in  the  opposite  direction,  taking  place  all  over  a  closed  surface, 
and  traveling  outward  from  the  source. 


40 


WIRELESS  TELEGRAPHY 


Properties  of  the  Electric  Waves.  —  A  masterly  mathematical 
treatment  by  Maxwell  of  this  idea  of  an  electric  displacement  in 

dielectric  media  led  not  only  to  great  prog- 
ress in  the  knowledge  of  electromagnetism, 
but  also  to  a  complete  revision  of  theories 
as  to  the  nature  of  light,  so  that  now  all 
the  phenomena  of  optics  are  describable  in 
terms  of  Maxwell's  electric  waves.  From 
his  theory  Maxwell  deduced  the  following 
facts  in  regard  to  electric  waves: 

1.  The  electric  wave   in  the  dielectric 
consists  of  a  displacement  current  in  one 
direction  with  a  magnetic  force  at  right 
angles  to  it,  both  of  these  quantities  being 
in  the  wave  front;  that  is  to  say,  at  right 

Electric  force  E    angles  to  the  direction  of  propagation  of 

Thus  electric  waves, 
like  light  waves,  are  transverse  waves. 

2.  The  velocity  of  propagation  of  the 


FIG.  21. 

and  magnetic  force  M    ,,  /        ™      91  x 

perpendicular  to  direc-    the  wave  (se    Fig.  21). 
tion  of  propagation  T. 


\JU 

electric  waves  (in  a  non-magnetic  insulating  medium)  is  —  -  ,  where 


a  is  the  ratio  of  the  c.  g.  s.  electromagnetic  unit  of  quantity  to  the 
c.  g.  s.  electrostatic  unit  of  quantity,1  and  k  the  dielectric  constant  of 
the  medium.  In  empty  space,  by  definition,  k  is  unity,  and  the  ratio 
a  was  known  from  older  experiments  to  be  the  velocity  of  light 
(3  X  1010  cm.  per  second)  ;  whence  the  velocity  of  the  electric  waves  in 
free  space  is  identical  with  the  velocity  of  light,  which  is  3  X  1010  cm., 
or  about  186,000  miles  (seven  times  around  the  earth)  in  one  second. 

3.  In  an  insulating  medium  other  than  free  space  (for  example,  in 
glass  or  paraffin)  it  is  seen  from  the  preceding  section  that  the  velocity 
of  the  electric  waves  is 


in  which  VQ  is  the  velocity  of  waves  in  free  space,  and  v  the  velocity 
of  the  waves  in  a  dielectric  of  dielectric  constant  k;  whence, 

v0/v  =  Vk  (9) 

That  is  to  say,  the  index  of  refraction  2  of  a  medium  for  electric  waves 
is  equal  to  the  square  root  of  the  dielectric  constant  of  the  medium. 

1  For  definitions  of  these  units  see  Appendix  I. 

2  The  index  of  refraction  is  the  ratio  VQ/V. 


THE   ELECTROMAGNETIC  THEORY  OF  LIGHT  41 

4.  All  good  conductors  are  opaque  to  electric  waves,  all  good  insu- 
lators are  transparent  to  electric  waves,  and  semiconductors  like 
wood  and  stone  are  semitransparent.  Metallic  surfaces  are  prac- 
tically perfect  reflectors  of  electric  waves. 

The  Electromagnetic  Theory  of  Light.  —  Among  these  several 
properties  of  electric  waves  the  properties  stated  in  1  and  2  are 
identically  true  of  electric  waves  and  light;  while  the  properties  enu- 
merated in  3  and  4  have  also  met  with  very  useful  application  to 
light  as  well  as  to  longer  electric  waves.  Thus  Maxwell  came  to 
the  conclusion  that  light  waves  are  electric  waves  of  short  wave 
length.  This  theory  is  now  generally  accepted. 

It  is  interesting  to  note, on  this  theory,  how  light  can  be  produced. 
We  have  seen  how  electric  waves  may  be  produced  by  oscillating 
electric  currents  in  a  circuit  of  the  form  shown  in  Fig.  20.  Now  if 
we  suppose  the  oscillator  of  Fig.  20  to  be  made  smaller  and  smaller, 
the  capacity  and  inductance  will  both  be  decreased,  and  the  time  of 
oscillation  is  thereby  decreased.  If  then  we  think  of  the  oscillator 
as  possessing  atomic  dimensions,  the  period  of  oscillation  approaches 
that  of  light.  It  is,  however,  not  necessary  to  think  of  an  actual 
electric  discharge  taking  place  between  the  atoms  of  our  atomic 
oscillator,  because  the  rapid  vibratory  motion  of  a  single  charged 
particle,  or  electron,  back  and  forth  would  have  the  same  effect  as 
an  electric  discharge  between  particles,  and  would  produce  electric 
waves  of  which  the  period,  for  a  particular  size  and  velocity  of  the 
vibrating  particle,  would  be  the  period  of  light  of  some  particular 
color. 

Let  us  turn  next  to  the  experimental  demonstration  of  the  exist- 
ence of  the  electrical  waves  predicted  by  Maxwell.  This  did  not 
come  during  Maxwell's  lifetime;  in  fact,  twenty-two  years  elapsed 
between  Maxwell's  remarkably  clear  presentation  of  the  theory  and 
Hertz's  brilliant  confirmation  of  it. 


CHAPTER  VIII 
THE    EXPERIMENTS    OF    HERTZ 

first  direct  experimental  confirmation  of  Maxwell's  theory  of 
electric  waves  was  made  by  Professor  Heinrich  Hertz  1  of  Karlsruhe 
in  1888.  At  Karlsruhe,  and  later  at  Bonn,  Hertz  performed  a  great 
number  of  experiments,  in  which  he  produced  and  detected  electric 
waves;  measured  the  wave  length;  showed  that  the  electric  waves 
were  transverse,  polarized  waves;  that  they  were  capable  of  reflec- 
tion from  metallic  surfaces  and  were  freely  transmitted  through 
insulators;  that  they  could  be  refracted  by  prisms  of  pitch  and  other 
dielectrics;  and  that  as  the  wave  length  of  the  electric  waves  was 
shortened,  the  electric  waves  showed  properties  more  and  more 
analogous  to  the  properties  of  light. 

Lodge's  Resonance  Experiment.  —  Prior  to  the  work  of  Hertz, 
Sir  Oliver  Lodge  2  in  England  had  made  some  experiments  on  the 
inductive  action  between  Ley  den-jar  circuits  which  were  a  close 
approach  to  the  discovery  of  electric  waves.  A  description  of  these 
experiments  will  aid  us  to  understand  Hertz's  apparatus.  Lodge 
employed  two  circuits  of  the  form  shown  in  Fig.  22.  The  Leyden 
jar  A  had  its  two  coatings  connected  with  an  electric  machine,  so  that 
when  the  machine  was  operated,  the  jar  was  charged,  and  when  the 
tension  of  the  charge  reached  a  certain  value,  a  discharge  occurred 
through  the  loop  BCD  and  across  the  spark  gap  S.  This  discharge 
was  oscillatory  and  acted  inductively  upon  a  second  circuit  A'B'C'D' 
placed  parallel  to  the  first.  The  second  circuit  was  provided  also 
with  a  spark  gap  at  S',  which  was  formed  by  a  strip  of  metal  folded 
over  the  jar  so  as  to  touch  the  inner  coating  and  come  near  the  outer 
coating  as  S'.  This  circuit,  which  we  shall  call  the  receiving  circuit, 
had  its  period  of  oscillation  variable  in  that  the  inductance  of  the 
circuit  could  be  changed  by  the  movable  slider  at  C'D'.  When 
sparks  were  passing  in  the  discharge  circuit,  Lodge  found  that  there 
was  a  certain  position  of  the  slider  C'D'  that  gave  a  maximum  effect 
at  the  receiving  circuit,  as  was  shown  by  the  lively  passage  of  sparks 

1  Electric  Waves,  translated  by  D.  E.  Jones,  Macmillan  &  Co.,  1893. 

2  Lodge:  Report  British  Association,  Vol.  50,  p.  567,  1888. 

42 


THE  EXPERIMENTS  OF  HERTZ 


43 


across  the  spark  gap  at  S'.     The  two  circuits  were  then  in  resonance; 
that  is  to  say,  they  had  the  same  period  of  oscillation  as  determined 


FIG.  22.     Sir  Oliver  Lodge's  resonant  Leyden  jars. 

by  the  formula  T  =  2  TiVLC.  The  oscillatory  current  in  the  dis- 
charge circuit  induced  an  electromotive  force  in  the  receiving  circuit, 
and  when  the  circuits  were  in  resonance,  this  induced  electromotive 
force  was  capable  of  forcing  sparks  across  the  gap  at  Sf,  even  when 
the  two  circuits  were  several  meters  apart. 

According  to  Maxwell's  theory,  the  inductive  action  between  the 
two  circuits  consisted  of  electric  waves  sent  out  from  the  discharge 
circuit  and  striking  the  receiving  circuit;  but  Lodge  was  not  able  to 
demonstrate  the  existence  of  these  waves.  To  do  this  it  was  neces- 
sary to  make  the  wave  length  shorter  and  the  radiation  freer  than 
that  produced  by  Lodge's  discharge  circuit. 

Hertz's  Experiments  with  Electric  Waves  in  Air.  —  In  order  to 
produce  shorter  waves  than  those  employed  by  Lodge,  Hertz  made 
use  of  a  discharge  system  with  smaller  capacity  and  self-inductance. 
One  form  of  Hertz's  "  oscillator  "  is  shown  in  Fig.  23.  It  consists 
of  two  flat  metallic  plates,  40  cm.  square,  each  attached  to  a  rod  30 
cm.  long.  The  two  rods  were  placed  in  the  same  line,  and  were 
provided  at  their  nearer  ends  with  balls  separated  by  a  spark  gap 
about  7  mm.  long.  The  oscillator  was  charged  from  the  secondary 
of  a  Ruhmkorff  coil  J  attached  to  the  rods  near  the  spark  gap.  The 


44 


WIRELESS  TELEGRAPHY 


primary  of  the  coil  was  fed  by  a  battery,  and  contained  a  vibrator 
for  interrupting  the  primary  current  so  as  to  produce  a  high  poten- 
tial in  the  secondary.  At  each  interruption  by  the  vibrator  in  the 
primary,  the  two  halves  of  the  oscillator  became  charged,  and  dis- 


1 


FIG.  23.     Hertz  oscillator. 

charged  in  an  oscillatory  manner  across  the  spark  gap  of  the  oscil- 
lator. At  each  spark,  according  to  Maxwell's  theory,  there  was  sent 
out  a  train  of  waves  from  the  oscillator. 

In  order  to  detect  these  waves,  Hertz  employed  a  receiving  circuit, 


Of 


FIG.  24.     Hertz's  circular 
resonator. 


FIG.  25.     Hertz's  apparatus  for  showing  the 
existence  of  electric  waves  in  air. 


now  generally  called  a  "  resonator/'  of  the  form  shown  in  Fig.  24, 
which  is  seen  to  consist  of  a  circular  loop  of  wire  broken  by  a  diminu- 
tive air  gap  at  X.  The  radius  of  the  loop  was  35  cm.,  which  was 
found  by  experiment  to  be  the  proper  size  to  be  in  resonance  with  the 
oscillator. 


THE  EXPERIMENTS  OF  HERTZ  45 

To  demonstrate  the  existence  of  the  electric  waves  Hertz  made  use 
of  the  phenomenon  of  interference.  The  arrangement  of  apparatus 
is  shown  in  Fig.  25.  M  is  a  metallic  reflector,  consisting  of  a  sheet 
of  zinc,  2. meters  wide  by  4  meters  high,  from  which  the  waves  sent 
out  by  the  oscillator  are  reflected.  The  reflected  waves  superimpose 
upon  the  direct  waves,  producing  in  the  region  between  the  oscillator 
and  the  metallic  reflector  certain  positions  where  the  direct  and  the 
reflected  waves  neutralize  each  other  and  certain  other  positions  in 
which  their  effects  add.  In  demonstrating  these  effects  Hertz  per- 
formed a  number  of  beautiful  experiments. 

In  one  experiment  the  plane  of  the  resonator  was  kept  parallel  to 
the  reflector,  with  the  spark  gap  at  the  side,  as  shown  in  Fig.  25. 
Then  wherever  the  resonator  may  be  placed  along  the  line  SNi,  the 
electric  force  F  and  F'  is  the  same  at  the  two  sides  of  the  resonator. 
But  the  force  F',  being  applied  to  a  completely  metallic  part  of  the 
loop,  acts  to  a  greater  advantage  l  than  the  force  F,  so  that  sparks 
are  produced  unless  both  F  and  F'  are  very  small.  With  this  orien- 
tation of  the  resonator,  Hertz  started  with  the  resonator  at  N\  close 
to  the  reflector  and  moved  it  gradually  away  toward  the  oscillator. 

In  the  position  Ni  there  were  no  sparks  in  the  resonator,  showing 
that  there  is  a  node  of  electric  force  at  the  reflector.  This  result  is 
consistent  with  the  fact  that  a  large  difference  of  potential  cannot 
be  set  up  in  the  surface  of  a  good  conductor.  As  the  resonator  is 
moved  away  from  the  reflector,  sparking  begins  in  the  resonator, 
becomes  more  and  more  lively,  until  a  maximum  is  reached  at  LI. 
This  position  LI,  is  called  a  loop  of  electric  force.  On  proceed- 
ing further  in  the  same  direction,  a  second  minimum  of  sparking 
is  found  at  Nz,  and  so  forth. 

Discussion  of  this  Experiment.  —  The  occurrence  of  maxima 
and  minima  in  the  region  between  the  reflector  and  the  oscillator 
is  evidence  of  the  undulatory  nature  of  the  disturbance,  and  the 
distance  NiN*  or  LiL2,  is  the  half  wave  length.  To  make  this 
proposition  clear,  reference  is  made  to  Fig.  26,  which  shows  several 
drawings  of  the  direct  and  the  reflected  wave  and  the  resultant 
obtained  by  their  superposition.  The  reflecting  mirror  is  repre- 
sented by  the  heavy  vertical  line  at  the  right.  The  undulating 
line,  made  up  of  dashes,  represents  the  direct  wave,  which  is 
moving  toward  the  reflector;  and  the  dotted  wavy  line  is  the 
reflected  wave,  moving  from  the  reflector.  The  heavy  line  in  the 

1  In  the  same  way  that  plucking  a  violin  string  at  the  middle  will  produce 
a  greater  motion  than  plucking  it  near  the  end. 


46 


WIRELESS  TELEGRAPHY 


diagram  is  the  resultant  effect  obtained  by  adding  the  two  waves. 
The  distance  from  one  crest  to  the  next  similar  crest  (C3  to  Ci)  is 
called  the  wave  length,  and  the  time  for  the  wave  to  move  one 
wave  length  is  called  the  period  T.  The  different  diagrams,  (a), 
(b),  (c),  (d),  (e),  (/),  (g),  (h),  (i),  show  the  conditions  that  exist  in  the 
region  between  the  oscillator  and  the  mirror  at  different  times.  At 
a  time  that  we  have  called  t  =  0,  as  represented  in  diagram  (a), 
the  direct  and  the  reflected  waves  are  exactly  opposed  to  each 


\ 


\ 


v  /%' 


(a) 
t=0 


d) 


FIG.  26.     Showing  superposition  of  direct  and  reflected  waves. 

other  throughout  the  space  between  the  oscillator  and  the  reflector, 
so  that  the  resultant  electric  force  is  everywhere  zero.  In  (6), 
t  =  778,  the  direct  wave  has  moved  nearer  to  the  mirror  by  a  dis- 
tance equal  to  i  NiLi(=  i  wave  length),  while  the  reflected  wave, 
which  moves  with  the  same  velocity,  has  moved  from  the  mirror  by 
an  equal  amount.  It  is  seen  that  now  the  direct  and  the  reflected 
waves  do  not  oppose  each  other  everywhere  in  the  region.  In 
some  parts  of  the  region,  e.g.,  at _Ni,  N%,  N3,  they  do  oppose  and 


THE  EXPERIMENTS  OF  HERTZ 


47 


neutralize  each  other,  while  at  other  points  their  intensities  add. 
At  Li,  L2  and  L3  the  added  intensities  give  a  resultant  about 
1.4  times  the  maximum  of  either  wave  alone. 

In  (c),  t  =  2  T/8,  the  direct  wave  has  approached  the  mirror  by 
another  eighth  of  a  wave  length,  the  reflected  wave  has  receded 


(f) 


h) 


FIG.  26  (Continued). 

from  the  mirror  by  an  equal  amount,  and  the  two  waves  exactly 
superpose.  The  resultant  intensity  of  electric  force  is  still  zero  at 
Ni,  Nzt  N3  and  N*,  while  at  LI,  L2  and  L3  the  intensity  is  double 
that  of  either  wave  separately. 

In  a -similar  manner  th?  remaining  drawings  (d),  (e),  (/),  (g),  (h), 
(i)  represent  the  progress  of  the  direct  wave  toward  the  mirror  and 
the  recession  of  the  reflected  wave  from  the  mirror  by  successive 
eighths  of  a  wave  length.  The  resultant  intensity  is  always  zero 


48  WIRELESS  TELEGRAPHY 

at  Ni,  NZJ  N3  and  7V4,  while,  on  the  other  hand,  if  we  pass  down 
the  figure  from  the  diagram  (a)  to  (h),  we  see  that  the  intensity  at 
LI,  L2  and  L3  begins  at  zero  (a),  rises  to  double  the  intensity  of  the 
single  wave  (c),  falls  to  zero  (e),  then  to  minus  the  double  intensity 
(g)j  and  finally,  at  the  expiration  of  a  time  equal  to  T,  rises  again  to 
zero  (i) .  To  show  this  more  clearly,  all  the  resultants  are  collected 
in  the  last  diagram  (r)  of  the  figure. 

The  positions  Ni,  N%,  Ns  and  JV4  are  the  positions  in  which  the 
resonator  of  Hertz  gave  no  sparks  at  the  detecting  spark  gap, 
because  the  electric  force  at  these  positions  is  t  constantly  zero. 
The  positions  LI,  L2  and  L3  are  places  where  the  sparking  at  the 
resonator  was  a  maximum,  because  at  these  places  the  electric 
force  fluctuates  up  and  down  during  each  period  of  the  wave.  The 
distance  from  Ni  to  N2  or  from  LI  to  L2  is  half  the  distance  from 
C3  to  Ci,  diagram  (a),  and  is  therefore  equal  to  half  the  wave  length. 
With  the  dimensions  of  apparatus  used  by  Hertz  in  the  experiment 
represented  in  Fig.  25,  this  half  wave  length  was  4.8  meters. 

The  set  of  drawings  given  in  Fig.  26  represents  the  conditions 
that  exist  in  a  "  stationary  wave  system,"  in  which  the  direct  and 
the  reflected  wave  are  both  moving,  while  the  interference  between 
these  two  waves  gives  a  set  of  maxima  and  minima  fixed  in  space. 
The  minima  are  positions  where  there  is  never  any  resultant  force, 
while  at  the  maxima  the  force  fluctuates  between  positive  and 
negative  maxima,  with  a  period  equal  to  T,  the  period  of  the  waves. 

The  conditions  assumed  in  the  drawings  given  in  Fig.  26  are 
somewhat  simpler  than  the  conditions  actually  occurring  in  Hertz's 
experiment,  because  the  direct  and  the  reflected  waves  in  the  case 
represented  in  the  drawings  are  supposed  to  have  the  same  ampli- 
tude, whereas  in  the  actual  experiments  the.  reflected  wave  is 
weaker  than  the  direct  wave,  so  that  JVi,  Nz,  N3  and  N4  are  not 
positions  of  zero  intensity,  but  yet  have  intensity  small  enough  to 
enable  them  to  be  located  by  the  experiment. 

Nature  of  the  Wave.  —  The  experiment  by  Hertz,  just  described, 
shows  that  the  disturbance  sent  out  from  the  oscillator  and  detected 
by  the  resonator  travels  as  a  train  of  waves.  To  give  the  reader 
an  idea  of  the  nature  of  this  wave  motion  reference  is  made  to  the 
diagram  of  Fig.  27,  which  shows  in  part  the  electric  field  about  the 
oscillator  at  a  particular  moment.  This  figure  is  a  simplification 
of  a  diagram  theoretically  obtained  by  Hertz  from  Maxwell's 
equations. 

The  oscillator  is  shown  in  the  center  of  the  diagram,  and  on 


THE  EXPERIMENTS  OF  HERTZ 


49 


either  side  of  the  oscillator  are  shown  the  lines  along  which  Max- 
well's displacement  currents  occur.  These  lines  are  called  lines 
of  electric  induction.  We  have  seen  in  Chapter  VII  how  we  can 
imagine  the  displacement  current  in  the  dielectric  to  complete  the 
conduction  current  in  the  oscillator.  In  that  case  the  lines  of  elec- 
tric induction  terminate  on  a  positive  and  a  negative  charge  at 
their  two  ends.  At  the  instant  represented  in  the  diagram,  the 
two  halves  of  the  oscillator  have  opposite  charges,  and  some  of 
the  lines  of  electric  induction  near  the  oscillator  terminate  upon  the 
charges  on  the  oscillator.  But  a  little  farther  out  from  the  oscil- 
lator the  lines  in  the  diagram  are  represented  as  closed  upon  them- 
selves. This  closing  of  a  loop  on  itself  occurs  when  the  positive 
and  the  negative  charges  on  the  oscillator  come  together  as  the 
current  in  the  oscillator 
reverses.  The  closed  loops 
represented  in  the  diagram 
have  been  produced  by 
successive  oscillations  of 
the  current  on  the  oscilla- 
tor, and  have  been  liber- 
ated from  the  oscillator 
and  are  moving  freely 
away.  The  condition  of 
things  in  the  space  around 
the  oscillator  in  action 
may  be  pictured  to  the 
mind  by  supposing  that 

these  closed  loops  of  electric  induction  move  away  from  the 
oscillator,  and  as  they  move  they  elongate  and  grow  less  intense. 
Their  width,  however,  remains  constant,  so  that  if  a  receiver  be 
placed  in  any  fixed  position,  say  in  the  equatorial  plane,  PP, 
the  inductive  action  of  the  loops,  as  they  successively  pass, 
changes  continuously  from  one  direction  to  the  other  with  a 
period  equal  to  that  of  the  oscillator.  This  train  of  continuously 
reversing  electrostatic  induction  is  one  aspect  of  the  electric-wave 
train. 

Another  aspect  of  the  electric  wave  train  may  be  discovered  by 
examining  the  magnetic  field  about  the  oscillator.  The  lines  of 
magnetic  force  about  the  oscillator  are  circles  in  a  plane  perpen- 
dicular to  the  oscillator,  and  these  lines  in  a  non-magnetic  medium 
are  everywhere  perpendicular  to  the  lines  of  electric  induction,  so 


FIG.  27.     Simplified  diagram  of  electric 
force  about  an  oscillator. 


50  WIRELESS  TELEGRAPHY 

that  the  receiving  circuit,  placed,  for  example,  in  the  equatorial 
plane,  experiences  also  a  series  of  continuously  varying  magnetic 
forces  which  tend  to  induce  an  electromotive  force  in  the  receiving 
circuit  in  the  same  direction  as  that  induced  by  the  electric  induc- 
tion, so  that  both  the  electric  and  the  magnetic  effects  act  together 
and  are  called  the  components  of  the  electric  wave.  One  compo- 
nent is  electric  induction,  which  is  in  the  plane  of  the  oscillator. 
The  other  component  is  magnetic  force,  which  is  perpendicular  to 
the  electric  induction.  Both  of  these  components  are  perpendicu- 
lar to  the  direction  of  propagation  of  the  wave;  that  is  to  say, 
the  wave  is  transverse. 

Attempt  to  Determine  the  Velocity  of  the  Wave  in  Air.  —  Hertz 
attempted  to  determine  the  velocity  of  the  wave.  We  have  seen 
that  he  found  the  wave  length,  X,  to  be  9.6  meters.  This  is  the  dis- 
tance traveled  by  the  wave  during  the  period  of  one  oscillation  of  the 
current  in  the  oscillator,  so  that,  if  we  knew  the  period,  T,  we  could 
calculate  the  velocity  by  the  equation  v  =  \JT.  Hertz  attempted 
to  obtain  the  period,  T,  of  the  oscillator  by  calculation  from  such 
formulas  as  could  be  had  for  oscillators  of  this  shape,  and  he  ob- 
tained the  period  of  complete  oscillation  to  be  2.8  hundred-millionths 
of  a  second.  This  gave  for  the  velocity  of  the  waves  the  value 
340,000  kilometers  per  second.  In  this  calculation,  as  Professor 
H.  Poincare  pointed  out,t  Hertz  made  an  error,  and  overestimated 
the  period  in  the  ratio  of  V2  : 1,  so  that,  with  this  correction,  he 
would  have  obtained  the  velocity  of  the  waves  to  be  480,000 
kilometers,  while  the  velocity  of  light  is  300,000  kilometers  per 
second.  This  apparent  discrepancy  between  the  experiment  and 
Maxwell's  theoretical  conclusion,  that  the  velocity  of  the  waves  is 
equal  to  the  velocity  of  light,  was  due,  as  Hertz  suggested,  to  the 
inapplicability  of  the  formula  used  in  the  calculation  of  the  period 
of  oscillation.  Experiments  which  we  shall  soon  come  to  discuss 
show  that  the  velocity  of  the  electric  waves  is  the  same  as  the  velocity 
of  light,  and  thus  confirm  Maxwell's  predictions. 


CHAPTER  IX 


EXPERIMENTS    ON   THE   IDENTITY   OF   ELECTRIC   WAVES 
AND    LIGHT 

Hertz's  Apparatus  for  Shorter  Electric  Waves.  —  After  Hertz  had 
succeeded  in  proving  that  the  action  of  an  electric  oscillation  spreads 
out  as  a  wave  into  space,  he  planned  experiments  with  the  object 
of  concentrating  this  action  and  making  it  perceptible  to  greater 
distances,  by  putting  the  oscillator  in  the  focal  line  of  a  large  con- 
cave cylindrical  mirror.  In  order  to  avoid  the  disproportion  between 
the  length  of  the  waves  and  the  dimensions  he  was  able  to  give  to  the 


O 

o 


FIG.  28.  Hertz's  rec- 
tilinear oscillator. 


FIG.  29.     Hertz's  cylindrical  mirrors.     Oscillator 
is  at  left;  resonator,  at  right. 


mirror,  Hertz  made  the  oscillator  smaller,  so  that  the  length  of  the 
waves  was  less  than  one-tenth  of  those  first  discovered. 

The  form  of  oscillator  used  in  these  experiments  is  shown  in 
Fig.  28.  The  two  halves  of  the  oscillator  were  cylindrical  bodies 
3  cm.  in  diameter,  terminating  in  spheres  4  cm.  in  diameter.  The 
total  length  of  the  oscillator  was  26  cm.,  and  the  spark  gap  was 
usually  about  3  mm. 

For  a  receiving  circuit,  the  circle  of  wire  used  in  the  previous 
experiments  was  replaced  by  a  linear  resonator,  consisting  of  two 
straight  pieces  of  wire,  each  50  cm.  long  and  5  mm.  in  diameter, 
adjusted  in  a  straight  line  so  that  their  near  ends  were  5  cm.  apart. 

51 


52 


WIRELESS  TELEGRAPHY 


From  these  ends  two  wires,  15  cm.  long  and  1  mm.  in  diameter,  were 
carried  away  parallel  to  each  other  to  a  micrometer  spark  gap  simi- 
lar to  that  used  for  indicating  the  waves  in  the  previous  experiments. 

The  method  of  mounting  the  oscillator  and  resonator  in  the  focal 
line  of  the  cylindrical  mirrors  is  shown  in  Fig.  29.  The  reflecting 
surface  of  the  cylindrical  mirrors  was  of  thin  sheet  metal.  The 
dimensions  of  the  reflectors  are  shown  in  the  diagram.  With  these 
reflectors  about  the  oscillator  and  the  resonator  Hertz  was  able  to  get 
indications  of  waves  up  to  a  distance  of  20  meters.  The  length  of 
the  wave,  measured  by  the  method  of  the  last  experiment,  was  66  cm., 
and  the  period  of  oscillation,  assuming  that  the  waves  travel  with 
the  velocity  of  light,  was  2.2  thousandths  of  a  millionth  of  a  second. 
With  this  wave  length  Hertz  succeeded  in  carrying  out  many  of 
the  elementary  experiments  that  are  commonly  performed  with  light. 

Rays  and  Shadows.  —  With  the  electric  waves,  as  with  light  and 
radiant  heat,  shadows  may  be  cast  by  objects  opaque  to  the  waves. 


FIG.  30.     Plan  of  oscillator,  receiver  and  metallic  screens. 


Hertz  found  that  a  metallic  screen  interposed  between  the  oscillator 
and  the  receiver,  in  the  position  A,  Fig.  30,  stopped  the  sparking  of 


THE  IDENTITY  OF  ELECTRIC  WAVES  AND  LIGHT      53 

the  resonator  completely,  while  the  two  screens  in  the  position  B  and 
B'  did  not  materially  diminish  the  sparks  at  the  resonator.  If,  how- 
ever, the  opening  between  B  and  B'  was  made  narrower,  the  sparks 
became  weaker,  and  disappeared  when  the  opening  was  reduced  below 
a  half  meter.  In  experiments  of  this  kind,  although  the  dimensions 
of  the  screens  are  measured  in  meters,  these  screens  are  yet  not  large 
in  comparison  with  the  wave  length  of  the  waves,  and  the  phenomena 
of  diffraction  are  very  marked,  so  that  there  is  no  sharp  geometrical 
limit  either  to  the  rays  or  to  the  shadows. 

Polarization.  —  Hertz  showed  that  the  electric  waves  produced 
by  his  linear  oscillator  are  polarized  waves.  One  way  employed  by 
him  for  showing  this  was  to  start  with  the  focal  lines  of  the  two  reflec- 
tors parallel,  as  in  Fig.  29,  so  that  there  is  lively  sparking  at  the 


FIG.  31.     Showing  polarization  by  the  absence  of  effects  when  the 
oscillator  and  the  resonator  are  at  right  angles  to  each  other. 

resonator,  and  turn  the  receiving  mirror  about  the  line  joining  oscil- 
lator and  resonator.  During  this  operation  the  resonator  sparks 
become  more  and  more  feeble,  and  when  the  two  focal  lines  are 
at  right  angles,  as  in  Fig.  31,  no  sparks  whatever  are  obtained  at 
the  resonator,  even  when  the  two  mirrors  are  moved  up  close  to 
each  other. 

In  another  method  of  showing  that  the  electric  waves  are  polarized, 
Hertz  made  use  of  a  grating  of  wires.  The  wires  of  the  grating 
were  1  mm.  in  diameter  and  3  cm.  apart,  and  were  mounted  in  an 
octagonal  wood  frame  2  meters  high  and  2  meters  long.  When 
the  grating  was  interposed  between  the  oscillator  and  the  resonator 
so  that  the  direction  of  the  wires  of  the  grating  was  perpendicular  to 
the  oscillator  and  the  resonator,  as  shown  in  Position  1,  Fig.  32,  the 
screen  practically  did  not  interfere  at  all  with  the  sparks  at  the 
resonator.  But  if  the  screen  was  set  up  in  such  a  way  that  its  wires 


54 


WIRELESS  TELEGRAPHY 


were  parallel  to  the  oscillator  and  the  resonator  (Position  2,  Fig.  32) 
it  stopped  the  rays  completely.  With  regard,  then,  to  the  transmis- 
sion of  energy  the  screen  behaves  toward  the  electric  waves  as  a 
tourmaline  plate  behaves  toward  a  plane  polarized  ray  of  light. 

Another  way  of  showing  polarization  of  the  electric  waves  was 
also  devised  by  Hertz.     The  receiver  was  again  placed  so  that  its 


Position  .1  Transparent 


FIG.  32. 


Position  2  Opaque 
Polarization  proved  by  the  interposition  of  a  grating  of  wires. 


focal  Jine  was  perpendicular  to  that  of  the  oscillator,  as  in  Fig.  31. 
Under  these  circumstances,  as  already  mentioned,  no  sparks  appeared. 
Nor  were  any  sparks  produced  when  the  screen  was  interposed  in  the 
path  of  the  waves,  so  long  as  the  wires  of  the  screen  were  either 
horizontal  or  vertical.  But  if  the  frame  was  set  up  in  such  a  position 
that  the  wires  were  inclined  at  45°  to  the  horizontal  on  either  side 
(see  Fig.  33),  then  the  interposition  of  the  screen  immediately  pro- 
duced sparks  at  the  resonator  spark  gap.  Clearly  the  screen  resolves 
the  electric  force  of  the  advancing  wave  into  two  components,  and 
transmits  only  that  component  which  is  perpendicular  to  the  direc- 


THE  IDENTITY  OF  ELECTRIC  WAVES  AND  LIGHT       55 


tion  of  its  wires.  This  component  is  inclined  at  45°  to  the  axis  of 
the  receiver,  and  so  has  a  component  along  the  direction  of  the 
resonator. 

From  these  experiments  it  is  evident  that  the  interposition  of  the 
screen  stops  the  waves  when  the  wires  of  the  screen  are  parallel  to 


FIG.  33.     Rotation  of  plane  of  polarization  by  a  wire  grating  at  45°. 

the  electric  component  of  the  waves.  It  is  in  this  position  that  the 
electric  force  would  produce  currents  in  the  wires.  The  changing 
magnetic  force  at  right  angles  to  the  wires  would  also  produce  cur- 
rents in  the  wires,  so  that  both  the  components,  that  is  to  say,  the 
whole  electric  wave,  would  be  absorbed  or  reflected.  Hertz  showed 
that  the  action  was  one  of  reflection  rather  than  of  absorption;  in  this 
the  wire  screen  differs  from  the  action  of  the  tourmaline  crystal  on 
light,  for  the  extinguished  component  in  that  case  is  absorbed  rather 
than  reflected. 

Refraction.  —  Hertz  also  performed  some  experiments  on  the 
refraction  of  electric  waves,  employing  for  the  purpose  a  large  prism 


30C 


FIG.  34.     Showing  refraction  of  electric  waves  by  prism. 

of  pitch  cast  in -a  wooden  box.      The  base  of  the  prism  was  an  isos- 
celes triangle  1.2  meters  on  the  side,  and  with  a  refracting  angle  of 


56 


WIRELESS  TELEGRAPHY 


nearly  30°.  The  height  of  the  prism  was  1.5  meters,  and  its  weight 
was  1200  pounds.  With  the  arrangement  of  apparatus  as  shown  in 
Fig.  34  the  rays  were  refracted  by  the  prism  through  an  angle  of 
22°.  From  this  value  Hertz  calculated  the  index  of  refraction  of 
the  pitch  to  be  1.69,  while  the  refractive  index  of  pitch-like  materials 
for  light  is  given  as  being  between  1.5  and  1.6. 

In  concluding  this  series  of  experiments  Hertz  says:  "  We  have 
applied  the  term  rays  of  electric  force  to  the  phenomena  which  we 
have  investigated.  We  may  perhaps  further  designate  them  as 
rays  of  light  of  very  great  wave  length.  The  experiments  described 
appear  to  me,  at  any  rate,  eminently  adapted  to  remove  any  doubt 
as  to  the  identity  of  light,  radiant  heat,  and  electromagnetic  wave 
motion.  I  believe  that  from  now  on  we  shall  have  greater  confidence 
in  making  use  of  the  advantages  which  this  identity  enables  us  to 
derive  both  in  the  study  of  optics  and  of  electricity." 

Experiments  of  Righi.  —  Immediately  following  the  discovery  of 
electric  waves  by  Hertz,  a  great  number  of  experiments  were  made 


M 


FIG.  35.  Professor  Righi's 
oscillator  for  short  electric 
waves. 


FIG.  36.     Righi's 
resonator. 


FIG.  37.     Mounting   of 
Righi's  resonator. 


by  various  investigators  in  repetition  of  Hertz's  expermients  and  in 
the  effort  to  extend  his  results,  particularly  in  the  direction  of  the 
study  of  the  properties  of  short  electric  waves,  so  as  to  obtain  a 
further  comparison  of  their  properties  with  the  properties  of  light. 
In  order  to  obtain  electric  waves  shorter  than  those  of  Hertz,  Pro- 
fessor Righi 1  of  the  University  of  Bologna  devised  an  oscillator 
consisting  of  two  spheres  (B,  C,  Fig.  35)  separated  by  a  small  spark 
gap  in  oil.  A  and  D  are  the  terminals  of  an  induction  coil  or  electric 
1  Augusto  Righi:  L'  ottica  delle  Oscillazioni  Elettriche,  Bologna,  1897. 


THE   IDENTITY  OF  ELECTRIC  WAVES  AND  LIGHT      57 

machine  used  to  charge  the  oscillator.  These  terminals  are  provided 
with  the  spheres  A  and  D,  which  are  separated  from  the  spheres  B 
and  C  of  the  oscillator  by  spark  gaps  in  air,  so  that  the  oscillator  BC 
is  without  metallic  connection  with  the  other  parts  of  the  circuit. 
The  spheres  B  and  C  were  fastened  with  shellac  into  the  truncated 
cones  of  glass  EF  and  GH,  which  were  supported  in  an  ebonite  frame. 
The  lower  funnel-shaped  glass  vessel  served  to  contain  the  oil.  The 
spark  length  in  oil  between  B  and  C  could  be  regulated  by  the  screw 
V.  The  advantage  of  having  the  spark  between  the  spheres  take 
place  in  oil  instead  of  in  air,  as  had  already  been  pointed  out  by  MM. 
Sarasin  and  De  la  Rive,  arises  from  the  fact  that  it  takes  a  greater 
difference  of  potential  to  start  a  given  length  of  spark  and  therefore 
gives  a  more  energetic  discharge.  When  the  spark  is  once  started, 
the  oil  is  carbonized  and  becomes  conducting,  so  that  the  succeed- 
ing oscillations  pass  with  comparatively  little  damping.  Also  the  oil 
obviates  the  necessity  of  repeatedly  polishing  the  terminals,  as  Hertz 
found  he  had  to  do  when  he  attempted  to  get  short  waves  with  the 
spark  in  air.  Righi  found  that  vaseline  oil  is  especially  well  adapted 
for  use  with  his  oscillator. 

For  a  receiving  apparatus  Righi  made  use  of  a  resonator  consisting 
of  a  strip  of  silver  AB  deposited  on  glass  and  interrupted  by  a 
diamond  scratch  C  across  the  middle  of  the  strip.  This  provided  an 
extremely  short  spark  gap  between  the  two  parts  of  the  resonator,  as 
shown  in  Fig.  36.  Also  the  spark  across  this  small  gap  will  occur 
more  easily  than  a  spark  of  equal  length  in  free  air.1  Righi's  reso- 
nator is  thus  seen  to  be  an  extremely  sensitive  modification  of  the 
rectilinear  resonator  used  by  Hertz. 

In  most  of  Righi's  experiments  the  oscillator  and  the  resonator  were 
mounted  in  cylindrical  reflectors.  The  mounting  of  the  resonator  is 
shown  in  section  in  Fig.  37.  The  resonator  is  at  A,  and  is  fastened 
upon  a  strip  of  ebonite  BC.  The  observer  looks  through  the  con- 
verging lens  at  H,  which  serves  to  magnify  the  minute  sparks  between 
the  two  halves  of  the  resonator.  The  apparatus  could  be  used  quan- 
titatively by  observing  the  angle  through  which  it  was  necessary  to 
turn  the  resonator  and  its  reflector  in  the  support  LM  in  order  to 
extinguish  the  sparks.  The  angle  of  turning  was  indicated  by  -the 
pointer  N  moving  over  a  graduated  circle  OP. 

1  The  author  has  shown  that  the  potential  required  to  start  a  spark  along 
a  surface  of  glass  is  about  .44  of  the  potential  to  start  a  spark  of  equal  length 
in  free  air.  (Pierce:  Physical  Review,  Vol.  2,  p.  99,  1894.) 


58 


WIRELESS  TELEGRAPHY 


The  following  table  gives  the  dimensions  of  Righi's  apparatus  and 
the  corresponding  wave  lengths  obtained : 


Denomination 
of  the  appa- 
ratus. 

Oscillators. 

Resonators. 

Wave  length 
in  cm. 

Diameter  of 
the  spheres. 

Length   in  cm. 

Width    in  cm. 

I. 

.8 

0.9 

.1 

2.6 

II. 

3.75 

3.6 

.2 

10.6 

III. 

8.0 

10. 

.2 

20. 

3.6 

.6 

11.8 

10. 

.6 

21.4 



In  a  test  of  the  sensitiveness  of  various  combinations  of  this  appa- 
ratus, Righi  found  that  with  the  resonator  III  and  the  oscillator  II 
both  armed  with  their  respective  cylindrical  reflectors,  sparks 
appeared  across  the  minute  diamond  scratch  of  the  resonator  when 
it  was  at  a  distance  of  25  meters  from  the  oscillator.  This  is  a 
distance  of  125  times  the  wave  length  for  this  apparatus.  With  the 
oscillator  III,  the  sparks  were  evident  at  a  greater  distance.  With 
resonator  II  and  oscillator  II,  the  greatest  distance  to  which  indica- 
tions of  the  waves  could  be  obtained  was  20  meters,  which  is  190 
times  the  wave  length.  While  with  the  minute  apparatus,  resonator 
I  and  oscillator  I,  the  maximum  distance  was  about  80  centimeters, 
which  is  31  wave  lengths.  With  this  smaller  apparatus,  in  spite  of 
the  comparative  feebleness  of  the  waves,  many  experiments  that  are 
commonly  performed  with  light  waves  could  be  successfully  carried 
out  with  the  electric  waves.  For  example,  a  small  coin  (10  centes- 
imi)  can  be  used  to  reflect  the  waves.  The  coin  does  not  need  to  be 
polished  as  with  experiments  on  the  reflection  of  light,  because  irregu- 
larities of  the  surface  of  the  coin  are  too  small  to  have  any  effect  on 
-the  reflection  of  the  electric  waves.  Refraction  and  total  internal 
reflection  of  these  short  waves  could  be  shown  with  prisms  of  sulphur 
or  paraffin  that  were  very  little  larger  than  the  glass  prisms  used  in 
optics. 

Righi  also  succeeded  in  demonstrating  the  double  refraction  and 
elliptic  polarization  of  the  waves  by  slabs  of  the  wood  of  the  fir  tree. 

The  Use  of  a  Thermal  Junction  for  Measuring  Electric  Waves.  — 
In  the  experiments  of  Hertz  and  Righi  the  presence  of  the  electric 
waves  was  manifested  by  the  production  of  sparks  across  a  minute 
spark  gap  between  two  parts  of  the  receiving  conductor.  In  1892 


THE  IDENTITY  OF  ELECTRIC  WAVES  AND  LIGHT      59 


Ignaz  Klemencic  l  showed  that  a  thermal  junction  could  be  employed 
to  detect  and  measure  the  waves.  Klemencic's  device,  Fig.  38,  con- 
sists of  two  thin  sheets  of  brass  MM,  10  cm.  broad  and  30  cm.  long, 
placed  3  cm.  apart,  and  having  soldered  to  them  respectively  a  very 
fine  platinum  and  a  very  fine  platinum-nickel  wire,  which  were 
crossed  at  k  and  were  thence  conveyed  off  at  right  angles  and  soldered 
at  their  other  ends  to  the  leads  I,  I  of  a  sensitive  galvanometer.  This 
resonating  system  was  fixed  at  the  focal  line  of  a  suitable  cylindrical 
metallic  reflector.  When  electric  waves,  with 
the  electric  force  parallel  to  MM ,  fall  on  this 
receiver,  electric  oscillations  between  M  and  M 
produce  heating  of  the  knot  k,  which  is  the 


FIG.  38.     Resonator  employing 
thermal  junction. 


FIG.  39.     Oscillator  for  very 
short  electric  waves. 


point  of  contact  of  two  dissimilar  metals,  and  in  consequence  the 
heat  developed  gives  rise  to  a  thermoelectromotive  force  at  the  knot 
and  consequently  to  a  current  in  the  galvanometer.  By  the  use  of 
this  instrument  and  a  Righi  oscillator,  Klemencic  has  studied  the 
reflection  of  electric  waves  from  metals  and  insulators. 

Various  investigators  have  made  use  of  the  Klemencic  thermal 
junction  in  quantitative  experiments  on  electric  waves.  By  reducing 
the  size  of  the  metal  vanes  MM,  Professor  A.  D.  Cole  2  has  applied 
the  apparatus  to  measurements  with  waves  with  a  wave  length  of 
4  cm.  Professor  Lebedew,3  employing  a  slightly  different  form  of 

1  Ignaz  Klemenctf:  Wied.  Ann.,  45,  p.  62,  1892. 

2  A.  D.  Cole:  Wied.  Ann.,  57,  p.  290,  1896,  and  Phys.  Review,  7,  Nov.,  1898. 

3  Peter  Lebedew:  Wied.  Ann.,  56,  p.  1,  1895. 


60 


WIRELESS  TELEGRAPHY 


thermal  junction,  worked  with  waves  of  wave  length  of  only  6  mm., 
and  succeeded  in  showing  the  double  refraction  of  electric  waves  by 
crystals.  A  form  of  oscillator  similar  to  that  used  by  Cole  and  by 
Lebedew  for  producing  their  short  electric  waves  is  shown  at  o,  o, 
Fig.  39. 

Professor  Lampa  and  Professor  Bose  have  also  succeeded  in  mak- 
ing measurements  with  electric  waves  of  only  6  mm.  wave  length. 

Wave  Length  of  Electric  Waves  and  Light.  —  The  following  table 
contains  in  round  numbers  the  value  of  wave  length  and  number 
of  vibrations  per  second  of  some  electric  waves  and  waves  of  radiant 
heat  and  light: 


Electric  waves  produced  by 

Wave  length 
in  cm. 

Number  of  vibrations 
per  second. 

Commercial  Alternating  Current 

200,000,000 

150 

Leyden  Jar  Discharge,  Feddersen  .... 
Hertz's  First  Oscillator  

300,000 
1,000 

100,000 
30,000,000 

Hertz's  Rectilinear  Oscillator  

60 

500,000,000 

Righi's  Oscillator     

2.6 

11,000,000,000 

Lebedew,  Lampa,  and  Bose's  El.  Waves 
Longest  Radiant  Heat   

.6 
.01 

50,000,000,000 
3,000,000,000,000 

Orange-colored  Light   

.00006 

500,000,000,000,000 

Shortest     Ultra-violet,     Schumann, 
Lyman  

.00001 

3,000,000,000,000,000 

Physicists  have  long  been  accustomed  to  recognize  that  the  dif- 
ference between  radiant  heat,  visible  light,  and  the  actinic  ultra-violet 
radiation  is  merely  difference  in  wave  length,  and  that  our  greater 
familiarity  with  the  visible  portion  of  the  spectrum  arises  merely 
from  the  fact  that  we  have  a  particular  set  of  nerves  sensitive  to 
these  rays. 

The  visible  part  of  the  spectrum  lies  between  wave  lengths  .000040 
and  .000076  centimeter.  By  the  aid  of  the  thermopile  and  the 
photographic  plate  the  spectrum  has  been  extended  to  include  all 
the  radiation  with  wave  length  between  .00001  (extreme  ultra-violet) 
and  .01  centimeter  (extreme  infra-red).  This  upper  limit  is  about 
1000  times  the  lower  limit.  It  is  interesting  to  note  that  the  Hertzian 
waves  measured  by  Lebedew,  Lampa,  and  Bose  have  a  wave  length 
only  about  60  times  the  wave  length  of  the  limit  attained  in  the 
infra-red.  That  is  to  say,  the  shortest  Hertzian  waves  that  have 
been  measured  are  nearer  in  wave  length  to  the  longest  measured 
heat  waves  than  these  are  to  the  shortest  measured  ultra-violet. 
Also  in  properties  the  Hertzian  waves  are  nearer  to  the  long  heat 


THE  IDENTITY  OF  ELECTRIC  WAVES  AND  LIGHT       61 

radiations  than  these  are  to  the  ultra-violet  or  even  to  the  visible. 
For  example,  some  of  the  long  heat  waves,  like  the  Hertzian  waves, 
pass  readily  through  vulcanite  and  other  insulators  opaque  to  visible 
light. 

Space  is  lacking  to  consider  further  the  experimental  evidence  in 
favor  of  Maxwell's  proposition  that  electric  waves  are  of  the  same 
nature  as  light  waves,  and  that  the  light  waves  are  in  fact  simply 
electric  waves  of  those  particular  wave  lengths  that  possess  the  prop- 
erty of  being  capable  of  affecting  the  retina  of  the  eye. 


CHAPTER  X 
ON  THE  PROPAGATION  OF  ELECTRIC  WAVES  ON  WIRES 

Wheatstone's  Experiments.  —  Early  in  the  history  of  the  electric 
telegraph  the  question  arose  as  to  the  velocity  of  propagation  of  elec- 
tric disturbances  along  wires.  The  first  attempt  to  measure  this  velo- 
city was  made  by  Wheatstone  1  in  1834.  Wheatstone  attempted  to 
measure  the  velocity  of  electricity  in  a  circuit  consisting  of  a  copper 
wire  about  half  a  mile  long,  and  extended  back  and  forward  so  as  to 
form  twenty  parallel  lines,  15  cm.  apart.  Three  spark  gaps  were 
inserted  in  this  line,  one  at  each  end  and  one  at  the  center.  These 
were  arranged  horizontally,  side  by  side,  in  front  of  a  mirror  mounted 
on  a  horizontal  axis  and  capable  of  being  revolved  at  the  rate  of  800 
revolutions  per  second. 

Upon  discharging  a  condenser  through  the  two  end  spark  gaps 
into  the  circuit,  the  image  of  all  three  of  the  sparks  could  be  seen 
in  the  revolving  mirror,  and  the  image  of  the  central  spark  was  found 
to  be  displaced  with  reference  to  the  other  two,  showing  that  the 
central  spark  occurred  later  than  the  two  end  sparks.  The  amount 
of  the  displacement  of  the*  central  spark,  together  with  the  speed  of 
the  mirror,  furnished  the  data  for  computing  the  speed  of  propagation 
of  the  electric  current.  Wheatstone  had  difficulty  in  determining 
the  amount  of  the  displacement,  which  he  could  obtain  only  by 
eye  observations.  Computations  from  Wheatstone's  observations 
seemed  to  show  that  an  electric  discharge  traversed  the  copper  wire 
at  a  speed  of  288,000  miles  (463,000  kilometers)  per  second,  which 
is  greater  than  the  velocity  of  light;  and  this  was  long  accepted  as 
the  true  "  velocity  of  electricity." 

While  the  numerical  result  obtained  by  Wheatstone  is  now  known 
to  be  incorrect,  the  experiment  is  yet  interesting  in  that  it  showed 
that  time  was  required  for  the  electrical  disturbance  to  traverse  the 
wire.  The  revolving  mirror  employed  in  this  experiment  has  now 
become  a  classical  apparatus  in  physical  investigation. 

Other  Early  Experiments.  —  In  1850  Fizeau  and  Gounelle  like- 
wise made  a  series  of  experiments  on  the  velocity  of  the  electric 

1  Wheatstone:  Phil.  Trans.,  Part  II,  p.  583,  1834;  Pogg.  Ann.,  34,  p.  464. 

62 


THE  PROPAGATION  OF  ELECTRIC  WAVES  ON  WIRES  63 

current,  and  for  this  purpose  availed  themselves  of  the  telegraph 
lines  between  Paris  and  Amiens  (314  kilometers)  and  between  Paris 
and  Rouen  (288  km.).  Their  measurements  gave  a  velocity  of 
101,700  km.  per  second  for  iron  wires,  and  172,000  km.  per  second 
for  copper  wires. 

In  other  similar  measurements  of  the  apparent  velocity  of  the 
electric  current  various  results  have  been  obtained  in  practice  which 
are  much  lower  than  those  of  Wheatstone,  and  Fizeau  and  Gounelle, 
being  in  some  cases  2240  kilometers  per  second,  and  in  others  4800, 
28,000,  96,000  and  so  on.  What,  then,  is  the  explanation  of  this 
great  variability  in  the  experimental  results  ? 

Theoretical  Discussion.  —  In  1855,  in  discussing  the  feasibility  of 
an  Atlantic  cable,  Sir  William  Thomson  gave  a  mathematical  treat- 
ment of  a  case  of  the  propagation  of  electric  disturbances  in  con- 
ductors. In  1857  Kirchhoff,  and  in  1876,  Heaviside,  developed 
extended  theoretical  treatments  of  the  problem.  The  results  ob- 
tained by  these  mathematical  physicists  show  that  the  velocity  of 
propagation  of  electrical  disturbances  in  conductors  depends  on  the 
nature  of  the  disturbance  and  the  A 
relative  values  of  the  capacity, 
self -inductance  and  resistance  of 
the  conductor. 

If  we  have  two  long  parallel    FlG"  40>  Jw.°   Parallel  wires   with 

&  applied  electromotive  force, 

wires  (Fig.  40)  as  in  the  case  of 

land  telegraph  and  telephone  lines,  or  one  wire  in  an  insulating 
sheath  submerged  in  a  conducting  body,  as  in  the  submarine  cable, 
three  important  cases  arise  in  practice. 

Case  I.  Telegraphy.  —  If  the  self-induction  of  the  line  is  negli- 
gible in  comparison  with  its  resistance  and  we  have  an  electromotive 
force  impressed  on  one  end  of  the  line,  the  current  in  the  conductor 
grows  in  a  manner  described  as  "diffusion."  Fig.  41  gives  a  set  of 
curves  1  showing  the  difference  of  potential  between  the  two  conduc- 
tors at  various  positions  along  the  line,  at  different  times  after  the 
application  of  the  electromotive  force.  In  this  case  there  is  no  proper 
velocity  of  the  electricity;  for  at  the  instant  the  battery  is  applied 
some  electricity  appears  all  along  the  line,  and  the  charge  at  a  short 
distance  from  the  origin  grows  faster  than  the  charge  at  a  greater 
distance.  '  This  is  approximately  the  case  that  occurs  in  submarine 

1  Redrawn  from  Professor  A.  G.  Webster's  Electricity  and  Magnetism; 
Macmillan,  1897. 


64 


WIRELESS  TELEGRAPHY 


cabling,  and  Sir  William  Thomson  showed  that  in  the  case  of  the 
proposed  Atlantic  cable,  the  time  required  for  each  signal  would  be 
sixteen  times  as  long  as  the  time  for'a  cable  of  the  same  cross  section 


Distance 


FIG.  41. 


Diffusion  of  electric  current  in  parallel  wires  with  negligible 
inductance. 


with  one-quarter  of  the  length,  such  as  then  existed  in  the  French 
submarine  telegraph  to  Sardinia  and  Africa. 

The  condition  assumed  by  Sir  William  Thomson  is  only  approxi- 
mately realized  in  practice,  for  in  no  line  is  the  action  of  self-induction 


FIG.  42. 


Distance 

Modified  diffusion. 


completely  negligible.  Especially  is  the  action  of  the  self-induction 
not  negligible  at  the  instant  of  applying  the  battery  at  A,  Fig.  40, 
because  this  application  of  the  battery  is  sudden,  and  for  a  sudden 
charging  of  the  conductor  the  effect  of  the  self-induction  is  greater 


THE  PROPAGATION  OF  ELECTRIC  WAVES  ON  WIRES   65 

than  for  a  slow  application  of  the  charge.  For  this  reason  the  prop- 
agation of  the  disturbance  is  more  accurately  represented  by  the  set 
of  curves  given  in  Fig.  42.  In  this  diagram  it  is  seen  that  the 
disturbance  has  a  nearly  square  wave  front,  which,  according  to  the 
theory,  travels  with  the  velocity  of  light,  while  succeeding  parts  of 
the  impulse  lag  more  and  more  behind  the  wave  front.  The  square 
wave  front  itself  becomes  also  more  and  more  attenuated  as  the 
disturbance  progresses  along  the  wires. 

This  same  condition  of  things  exists  to  some  extent  in  the  case  of 
land  telegraph  lines,  and  accounts  for  the  indefiniteness  of  the  results 
that  have  been  obtained  in  the  attempt  to  measure  the  velocity  of 
propagation.  If  for  a  particular  length  of  line  the  apparatus  used 
by  the  experimenter  for  detecting  the  wave  is  sufficiently  sensitive 
to  respond  on  the  arrival  of  the  wave  front,  the  value  obtained  for 
the  velocity  is  the  velocity  of  light ;  while  with  a  greater  length  of 
line  the  wave  front  is  too  feeble  to  affect  the  instrument,  which  then 
responds  to  a  more  intense  part  of  the  wave  arriving  later,  and  hence 
gives  a  smaller  value  for  the  velocity. 

Case  II.  Telephoning.  —  Suppose,  now,  that  instead  of  simply  ap- 
plying a  battery  to  the  line,  as  in  telegraphing,  we  apply  a  telephonic 
electromotive  force  to  the  parallel  wires  of  Fig.  40  or  to  the 
submarine  cable.  This  telephonic  electromotive  force  is  an  alter- 
nating electromotive  force.  Although  the  self-inductance  and  resist- 
ance of  the  circuit  may  be  the  same  as  before,  the  effect  of  the 
self-induction  is  larger  in  the  telephonic  case,  because  of  the  rapidity 
of  the  alternations  of  the  electromotive  force  at  the  source.  Under 
this  condition  Heaviside  finds  that  the  different  waves  generated  by 
the  sounds  of  different  pitch  travel  with  different  velocities,  and  that 
this  results  in  a  distortion  of  the  wave  and  puts  a  limit  to  the  dis- 
tance to  which  the  telephone  can  be  used.  This  distortion  is  caused 
by  the  resistance  and  capacity  of  the  line,  and  is  partially  eliminated 
by  self-induction.  Heaviside  says  that  this  "  self-induction  is  the 
telephonist's  best  friend,"  for  it  tends  to  preserve  the  sharpness  of 
the  wave  and  to  eliminate  the  part  of  the  disturbance  lagging  behind 
the  wave  front.  Heaviside  pointed  out  that  the  addition  of  properly 
distributed  self-induction  was  beneficial  to  prevent  distortion  in 
telephony;  and  in  actual  practice,  by  adding  inductance  coils  at 
intervals  along  telephone  lines,  Professor  Pupin  has  considerably 
increased  the  distance  to  which  distinct  speech  may  be  transmitted. 

In  the  case  of  the  submarine  cable,  on  account  of  the  relatively 
small  value  of  the  self-inductance,  submarine  telephony  is  not  at 


OF    THE 

UNIVERSITY 


66  WIRELESS  TELEGRAPHY 

present  practicable  to  a  greater  distance  than  about  twenty  miles 
(32  kilometers). 

Case  III.  Electric  Waves  of  High  Frequency.  —  As  a  third  case, 
which  is  the  one  in  which  we  are  here  chiefly  interested,  let  us  suppose 
that  the  electromotive  force  applied  to  the  end  of  the  two  parallel 
wires  oscillates  with  very  great  frequency.  The  effect  of  the  resist- 
ance then  becomes  negligible  in  comparison  with  the  effect  of  the  self- 
indue  tion.  The  theoretical  treatment  of  this  case  shows  that  such 
a  disturbance  travels  with  the  velocity  of  light,  and  except  by  a 
decrease  of  amplitude,  the  wave  is  not  distorted  during  its  progress 
along  the  wires.  In  what  follows  we  shall  see  how  experiments  have 
confirmed  this  deduction  from  the  theory. 

Hertz's  Experiments  with  Waves  on  Wires.  —  In  1888,  a  short 
time  before  his  experiments  with  electric  waves  in  air,  which  have 
been  described  in  chapters  VIII  and  IX,  Hertz  performed  a  series  of 


FIG.  43.     Hertz  apparatus  for  waves  on  wires. 

experiments  with  electric  waves  on  wires.  The  form  of  apparatus 
employed  is  shown  in  Fig.  43.  At  a  short  distance  behind  one  of  the 
plates  A  of  the  oscillator,  a  second  plate  P  was  placed.  From  P  a 
copper  wire  was  bent  through  the  arc  mn  and  thence  led  off  horizon- 
tally. When  the  plate  A  is  charged  positively,  negative  electricity 
is  attracted  to  the  nearer  side  of  the  plate  P,  and  an  equivalent  posi- 
tive charge  is  sent  away  along  the  wire.  When  the  charge  on  A 
becomes  negative,  a  similar  negative  charge  moves  away  along  the 
wire,  so  that  during  the  oscillations  between  A  and  A',  in  which  the 
charge  on  A  changes  continuously  back  and  forth  from  positive  to 
negative  values,  a  train  of  positive  and  negative  impulses,  constitut- 
ing a  tram  of  waves,  travels  out  along  the  wire. 

The  train  of  waves  on  the  wire  will  be  reflected  from  the  end  of 

the  wire,  as  may  be  seen  from  the  following  reasoning.     The  current 

< 


THE  PROPAGATION  OF  ELECTRIC  WAVES  ON  WIRES  67 

cannot  flow  past  the  end  of  the  wire,  nor  does  the  electricity  con- 
stituting the  current  merely  flow  out  to  the  end  of  the  wire  and  stop 
in  a  state  of  equilibrium.  Two  forces  are  acting  on  the  current: 

(1)  the  accumulation  of  electricity  near  the  end  of  the  wire  raises 
the  potential  of  the  wire  and  provides  a  force  opposing  the  current; 

(2)  the  slowing  down  of  the  current  causes  change  in  the  magnetic 
field  surrounding  the  wire,  and  this  tends  to  prevent  the  cessation  of 
the  current.     These  two  forces  do  not  act  together, —  when  one  is  a 
maximum,  the  other  is  a  minimum.    As  a  result  first  one  and  then  the 
other  of  these  forces  will  predominate,  so  that  the  charge  will  first 
be  sent  into  the  parts  near  the  end  of  the  wire  by  the  magnetic  field 
(self-induction)  and  will  then  be  sent  out  again  by  the  electrostatic 
rise  of  potential  (reciprocal  of  capacity) .    The  effect  of  this  is  that  the 
periodically  arriving  impulses  will  be  sent  back  again  with  the  same 
period,  and  we  shall  have,  therefore,  a  direct  and  a  reflected  train  of 
waves.     The  direct  and  the  reflected  waves  will  interfere  with  each 
other,  so  as  to  form  a  stationary  system  of  waves  like  that  obtained 
in  the  experiment  with  waves  in  air  reflected  from  a  sheet  of  metal 
(Chapter  VIII).     In  this  case,  however,  the  end  of  the  wire  will  be 
a  loop  of  potential;  whereas  the  metal  reflector  of  the  waves  in  air 
is  a  node  of  potential.     There  is  also  another  difference;  for  in  the 
case  of  the  wire,  the  returning  wave  will  again  be  reflected  at  P,  and 
a  simple  stationary  wave  system  can  only  be  realized  provided  the 
horizontal  wire  has  a  proper  length,  which  may  be  determined  by 
experiment. 

Hertz  studied  the  waves  produced  in  the  wire,  with  the  aid  of  his 
circular  resonator,  shown  in  the  figure.  With  the  resonator  in  the 
vertical  position  C,  Hertz  was  able  to  locate  the  nodes  and  loops  of 
current  in  the  wire  by  the  absence  or  presence  of  sparks  at  the  reso- 
nator. When,  however,  the  resonator  was  placed  in  the  horizontal 
position  B,  the  effect  obtained  was  due  partly  to  the  waves  in  the 
wires  and  partly  to  a  linking  with  the  resonator  of  magnetic  lines 
directly  from  the  oscillator.  The  compound  effect  obtained  in  the 
latter  case  was  utilized  by  Hertz  in  a  study  of  the  interference  between 
the  waves  in  the  wire  and  the  waves  in  the  air.  He  came  to  the  con- 
clusion that  the  wave  length,  and  consequently  the  velocity  of  prop- 
agation, was  different  in  the  two  cases.  This  was  in  contradiction 
of  Maxwell's  theory. 

Later,  by  the  use  of  a  smaller  oscillator  at  A  A' ,  he  found  that 
the  difference  between  the  velocities  of  the  waves  on  wires  and  in 
air  very  nearly  disappeared. 


68  WIRELESS  TELEGRAPHY 

Experiments  of  Sarasin  and  De  la  Rive.  —  While  Hertz  was 
puzzling  over  this  problem,  and  attempting  to  explain  the  dis- 
crepancy between  his  experiment  with  the  long  waves,  which  did 
not  agree  with  Maxwell's  theory,  and  his  experiment  with  the 
shorter  waves,  which  did  agree  with  the  theory,  MM.  Sarasin  and 
De  la  Rive  at  Geneva  repeated  the  experiment  with  the  longer 
waves  in  a  room  larger  than  that  available  to  Hertz,  and  obtained 
from  this  case  also  approximately  the  same  velocity  for  the  waves 
on  the  wire  and  the  waves  in  air.  Hertz's  difficulty  probably  arose 
from  the  disturbing  influence  of  electric  waves  reflected  from  ob- 
jects in  the  room.  Maxwell's  proposition  of  the  equality  of  the  two 
velocities  is  strictly  true  only  provided  the  waves  on  wires  are 
produced  on  two  parallel  wires  close  together,  —  a  positive  impulse 
being  started  along  one  of  the  wires  and  at  the  same  time  an  equal 
negative  impulse  being  started  along  the  other  wire.  Introducing 
this  precaution,  numerous  subsequent  experimenters  have  con- 
firmed Maxwell's  conclusion  that  the  velocity  of  the  electric  waves 
in  a  pair  of  nonmagnetic,  conducting  wires  is  the  same  as  the 
velocity  of  these  waves  in  the  dielectric  surrounding  the  wires. 

Direct  Determination  of  the  Velocity  of  the  Waves  on  Wires.  - 
Blondlot,1  Trowbridge  and  Duane,2  and  Saunders3  have  made 
direct  experimental  determinations  of  the  velocity  of  electric 
waves  on  wires.  In  all  of  these  experiments  the  method  consisted 
in  determining  the  wave"  length  X  of  the  waves  on  the  wires,  and 
in  determining  independently  the  time  of  the  oscillation  T  that 
produced  the  waves.  The  quotient  obtained  by  dividing  the 

wave  length  by  the  time  of  oscillation  gives  the  velocity  rs"*  0)j 

for  the  wave  length  is  the  distance  traveled  in  the  time  of  one 
oscillation,  and  dividing  the  distance  traveled  by  the  time  re- 
quired to  travel  it  gives  the  velocity.  In  all  of  the  experiments 
the  wave  length  X  was  determined  by  exploring  the  stationary- 
wave  system  on  the  wires  by  a  method  like  that  devised  by 
Lecher  (p.  70).  Trowbridge  and  Duane  and  Saunders  determined 
the  period  of  oscillation  T  by  spark  photographs  taken  with  the  aid 
of  the  revolving  mirror,  while  Blondlot  determined  the  period  by 

1  Blondlot:  Comptes  Rendus,  Vol.  117,  p.  543,  1893. 

2  Trowbridge  and  Duane:  American  Journal    of  Science,  Vol.  49,  p.  297, 
1895. 

3  Saunders:  Physical  Review,  Vol.  4,  p.  81,  1896. 


THE  PROPAGATION  OF  ELECTRIC  WAVES  ON  WIRES  69 


a  resonance  method,*  like  that  at  the  present  day  used  in  getting 
the  wave  length  in  a  wireless  telegraph  antenna. 

The  following  results  were  obtained  for  the  velocity  of  electric 


waves  on  wires: 


Observer. 

Velocity  in  kilo- 
meters per  second. 

Blondlot  

(  293,000 

\  298,000 

Trowbridge  and  Duane.  .  .  . 

j  298,800 
I  300,300 

{295,400 

299,400 

Saunders    

299,800 

299,800 

299,500 

299,900 

The  average  of  the  best  determination  of  the  velocity  of  light 
is  about  299,900  kilometers  per  second,  with  which  the  above 
determinations  of  the  velocity  of  the  electric  waves  on  copper 
wires  is  in  good  agreement. 

Velocity  of  Electric  Waves  in  Air.  —  Although  the  velocity  of 
the  electric  waves  in  air  has  not  been  determined  by  a  direct 
method,  the  experiment  of  Sarasin  and  De  la  Rive  showed  that 
the  velocity  of  the  waves  in  air  is  the  same  as  their  velocity  in 
copper  wires  surrounded  by  air,  and  therefore  the  same  as  that  of 
light. 

Waves  on  Iron  Wires.  —  On  account  of  the  magnetic  properties 
of  iron,  the  velocity  of  the  waves  on  small  iron  wires  has  been 
found  to  be  slightly  less  than  the  velocity  of  waves  of  the  same 
period  on  a  nonmagnetic  metal  like  copper.  With  wires  J  milli- 
meter hi  diameter  and  with  115,000,000  oscillations  per  second, 
St.  John  found  that  the  velocity  on  the  iron  wire  was  4  to  5%  less 
than  the  velocity  on  the  copper  wires.  This  result  showed  that 
the  magnetization  of  the  iron  is  able  to  follow  extremely  rapid 
reversals  of  the  magnetizing  current. 

On  Surface  Travel.  —  In  addition  to  this  slight  change  in  veloc- 
ity due  to  the  magnetic  property  of  the  iron,  the  damping  effect 
of  the  resistance  of  the  iron  is  very  large.  In  attempting  to  esti- 
mate the  effect  of  resistance  on  the  damping  of  oscillations  of  high 
frequency,  it  should  be  remembered  that  these  rapid  currents 
travel  in  a  very  thin  film  on  the  outside  of  the  conductor.  By 


TO  WIRELESS  TELEGRAPHY 

electrolytically  coating  an  iron  resonator  with  copper  and  a  copper 
resonator  with  iron,  Bjerknes  found  that  when  this  coating  was 
greater  than  a  hundredth  of  a  millimeter,  the  coated  iron  resonator 
acts  like  one  of  copper  and  the  coated  copper  resonator  like  one 
of  iron.  This  showed,  in  the  case  of  electric  oscillations  of  very 
high  frequency,  that  the  currents  are  confined  to  a  shell  whose 
thickness  is  of  the  order  of  a  hundredth  of  a  millimeter.  The 
thickness  of  this  shell  depends,  however,  on  the  frequency  of  the 
oscillations,  and  on  the  radius  and  material  of  the  conductor.  (See 
Appendix  II.) 

Waves  on  Wires  Studied  with  a  Vacuum  Tube  Detector.  —  A 
form  of  apparatus  devised  by  Professor  Lecher  for  showing  the 
existence  of  stationary  waves  on  wires  is  shown  in  Fig.  44,  which  is 


T' 


FIG.  44.     Lecher  apparatus. 

a  view  of  the  apparatus  from  above.  A' FA  is  an  ordinary  Hertz 
oscillator.  Parallel  to  the  plates  A  A'  of  the  oscillator  are  placed 
two  equal  plates  BB'  connected  to  a  pair  of  parallel  horizontal 
wires.  A  bridge  of  wire,  shown  in  a  separate  drawing  at  the  right, 
and  having  an  insulated  handle,  may  be  placed  across  the  horizontal 
wires.  A  Geissler  tube  gg',  which  is  pumped  to  a  sensitive  vacuum, 
is  placed  across  the  wires  near  their  outer  end,  so  that  the  glass  of 
the  tube  rests  on  the  wire.  When  the  oscillator  is  in  action,  the 
Geissler  tube  will  glow.  Let  us  now  put  the  bridge  across  the 
wires  near  the  Geissler  tube,  the  glow  will  cease,  because  it  is 
short-circuited  by  the  bridge.  If  now  we  move  the  bridge  toward 
the  oscillator,  a  position  will  be  found,  X  X',  for  which  the  Geissler 
tube  at  the  ends  of  the  wires  will  again  light  up  into  a  lively  glow. 
A  slight  motion  of  the  bridge  in  either  direction  from  this  position 
causes  the  glow  to  diminish.  In  explanation  of  this  phenomenon 
we  must  think  of  the  wires  as  divided  into  two  circuits  by  the 
bridge.  One  of  these  circuits,  which  we  will  call  the  "  oscillator 
circuit,"  is  FABXX'B'A'F,  comprising  the  two  condensers  AB 


THE  PROPAGATION  OF  ELECTRIC  WAVES  ON  WIRES  71 

and  A'B'  and  the  spark  gap  F.  This  circuit  has  its  own  definite 
period  of  oscillation.  The  other  circuit,  which  we  will  call  the 
"  resonator  circuit,"  consists  of  the  conductors  gXX'g'.  When 
the  bridge  XX'  is  in  the  position  that  causes  the  Geissler  tube  to 
glow,  the  oscillator  circuit  and  the  resonator  circuit  are  in  reso- 
nance, and  during  one  complete  oscillation  the  electric  wave  goes 
from  the  bridge  out  to  g',  back  across  the  bridge,  out  to  g,  and  back 
again  to  the  bridge.  Whence  it  is  seen  that  the  length  of  the 
conductor  from  g'  across  the  bridge  to  g  is  the  half  wave  length  of 
the  oscillator. 

If  now  the  bridge  is  moved  from  XX'  toward  the  oscillator, 
a  second  position  SSf  of  the  bridge  is  found  for  which  the  tube 
is  caused  to  glow.  During  this  displacement  of  the  bridge,  the 
self-inductance,  and  therefore  the  period,  of  the  oscillator  circuit 
is  diminished,  while  the  length  of  the  wire  to  the  right  of  the  bridge 
is  increased.  Therefore,  the  wire  to  the  right  of  the  bridge  cannot 
be  in  resonance,  as  a  whole,  with  the  oscillator  circuit.  We  can 
show  this  experimentally,  for  if  we  leave  the  first  bridge  at  SS' 
and  place  a  second  bridge  across  the  wires,  a  position  TT'  can  be 
found  for  which  the  presence  of  the  second  bridge  does  not  affect 
the  glow  of  the  tube.  A  slight  motion  of  the  second  bridge  to  the 
right  or  to  the  left  diminishes  the  glow. 

The  two  positions  SS'  and  TT'  are  called  nodes  of  electric 
potential.  In  a  similar  way  with  longer  parallel  wires  several 
nodes  may  be  located.  The  free  end  of  the  wires  is  always 
a  loop  of  potential,  and  other  loops  of  potential  exist  halfway 
between  the  nodes.  The  presence  of  these  nodes  and  loops  at 
equal  intervals  along  the  parallel  wires  shows  the  existence  of  a 
stationary  wave  system  similar  to  that  discovered  by  Hertz  in  his 
experiments  with  electric  waves  in  air. 

Blondlot's  Apparatus.  —  A  modification  of  Lecher's  apparatus 
made  by  Professor  Blondlot  is  shown  in  Fig.  45.  The  two  halves 
of  the  oscillator  are  here  bent  into  semicircles,  while  the  parallel 
wires  lead  out  from  a  secondary  circuit  placed  immediately  be- 
neath the  oscillator.  The  oscillator  and  the  circular  portion  of  the 
secondary  are  submerged  in  a  glass  vessel  containing  oil.  Leads 
from  the  induction  coil  are  brought  into  the  oil  and  connected  to 
the  two  sides  of  the  spark  gap,  —  one  connection  being  made 
directly  at  a  and  the  other  connection  being  through  a  small 
spark  gap  at  b.  In  this  form  of  apparatus  the  waves  on  the  wires 
are  produced  by  electromagnetic  induction  from  the  oscillator. 


72 


WIRELESS  TELEGRAPHY 


Arons'  Tube.  —  A  very  beautiful  method  of  demonstrating  the 
presence  of  waves  on  parallel  wires  was  devised  by  Professor  Arons. 
The  two  parallel  wires  for  the  greater  part  of  their  length  were 
inclosed  in  a  glass  tube  from  which  the  air  could  be  pumped. 
When  the  proper  degree  of  exhaustion  is  attained,  and  the  wires 


W 


w 


FIG.  45.     Blondlot  apparatus. 

in  the  tube  are  made  to  take  the  place  of  the  parallel  wires  WW  in 
air  in  Blondlot 's  apparatus  (Fig.  45),  a  bright  glow,  as  represented 
in  Fig.  46,  appears  at  intervals  along  the  wires  indicating  the  pres- 
ence of  a  large  fluctua-  r, 
tion  of  potential   (loop)       <       ^m> —     >a        j^        >_m^       >^>  - 
at  the  positions  of  glow. 
This  beautiful  apparatus  ' 

of  Professor  Arons  exhibits  to  the  observer  at  a  glance  the  whole 
character  of  the  potential  distribution  in  the  system. 

Exploration  by  the  Bolometer.  —  In  the  place  of  the  vacuum 
tube  in  experiments  with  waves  on  wires,  Paalzow  and  Rubens 1 
l|l have  shown  how  to  adapt  the  bolom- 
eter to  this  purpose,  and  to  obtain 
with  it  striking  quantitative  results. 
The  bolometer  (Fig.  47)  consists  of 
an  accurately  balanced  Wheatstone 
bridge,  so  arranged  that  the  oscilla- 
tory current  to  be  measured  is  made 
to  pass  through  a  fine  wire  EF  con- 
stituting   one    arm    of    the    bridge. 
FIG.  47.     Bolometer.  &M1     .  .  , 

This  oscillating  current  heats  the  fine 

wire,  thereby  changing  its  resistance,  which  throws  the  bridge  out 
of  balance  and  produces  a  deflection  of  the  galvanometer  G. 

1  Paalzow  and  Rubens,  Wied.  Ann.,  Vol.  37,  p.  529. 


THE  PROPAGATION  OF  ELECTRIC  WAVES  ON  WIRES     73 

In  Paalzow  and  Rubens's  arrangement  of  apparatus  (Fig.  48), 
in  order  to  avoid  disturbing  the  waves  on  the  wires  PQRS,  the 
leads  to  the  bolometer  were  not  connected  directly  to  the  wires  under 
examination,  but  were  connected  inductively  by  a  single  turn 
around  capillary  glass  tubes  TT,  sliding  on  these  wires.  The 
glass  tubes  TT  act  as  diminutive  Ley  den  jars  with  the  horizontal 
wires  inside  the  tube  for  one  coating,  and  the  turn  of  wire  on  the 
outside  of  each  tube  for  the  other  coating.  Variations  of  electric 
potential  at  a  point  inside  the  little  tubes  induce  (by  electrostatic 
action)  alternating  potential  in  the  turns  of  wire  outside  and 
produce  alternating  currents  through  one  arm  of  the  bolometer 
bridge. 

Figure  48  shows  a  form  of  apparatus  suitable  for  experiments  with 
this  method.  This  is  the  form  of  apparatus  used  by  Professor 


FIG.  48.     Exploration  of  waves  on  wires  by  bolometer. 

St.  John.  As  has  been  before  mentioned,  in  order  to  get  a  simple 
stationary  wave  system  in  the  parallel  wires,  these  wires  must 
have  a  proper  length  in  comparison  with  the  wave  length  of  the 
waves.  In  St.  John's  experiment  the  proper  length  of  the  wires 
was  determined  by  trial.  The  exploring  terminals  of  the  bolom- 
eter were  put  at  the  ends  P  and  S  of  the  wires  of  Fig.  48.  The 
oscillator  was  set  in  activity,  and  a  reading  of  the  bolometer  was 
taken  for  this  length  of  wire.  A  few  centimeters  of  wire  were  cut 
off,  and  the  reading  again  taken.  This  process  was  repeated  until 
a  maximum  point  was  passed.  A  sharp  and  unmistakable  maxi- 
mum was  found  when  PQ  had  a  certain  length  (859  centimeters). 
The  effect  fell  off  rapidly  when  the  wires  were  shortened  or  length- 
ened from  this  point.  The  result  is  shown  graphically  in  Fig.  49, 


Deflection  of  Bolometer  -^ 

S  g  £  g  §  ^ 

WIRELESS  TELE 

A 

\ 

I 

\ 

1 

\ 

/ 

\ 

* 

' 

\ 

\ 

I 

N 

where  distances  from  Q  are 
plotted  horizontally,  and 
deflections  of  the  galvanom- 
meter  are  plotted  verti- 
cally. 

To  determine  the  char- 
acter of  the  vibration  along 
the  wire,  the  lengths  of  QP 
and  RS  were  fixed  at  859 
centimeters,  the  exploring 
terminals  were  then  moved 
along  the  wires,  and  the 
bolometer  readings  taken 
for  each  position  of  the 
exploring  terminals.  A  dia- 
grammatic representation 
of  the  result  is  shown  in  Fig.  50.  The  curve  is  seen  to  be  simple 
in  form,  with  maxima  and  minima  at  approximately  equal  inter- 


800  900 

Crn.  Distance  from  Q  R 

FIG.  49.     Showing  adjustment  of  parallel 
wires  to  resonance  (Professor  St.  John). 


s 


1234  5678 

Meters  from  Q  R 

FIG.  50.     Curve  of  distribution  of  potential  on  parallel  wires  (St.  John). 

vals  along  the  wires,  and  with  a  maximum  at  the  ends  of  the 
wires. 

The  discussion  of  these  experiments  has  been  given  at  some 
length,  because  a  wave  system  resembling  that  here  described  is 
produced  in  the  antennae  used  in  wireless  telegraphy,  and  the  study 
of  the  resonance  conditions  in  wireless  telegraphy  circuits  will  be 
seen  to  be  closely  related  with  the  study  of  stationary  waves  in  wires. 


CHAPTER   XI 
WIRELESS    TELEGRAPHY    BEFORE    HERTZ 

By  Conduction  through  Water.  —  The  first  successful  attempt  at 
electric  telegraphy l  between  stations  not  connected  by  wires  seems 
to  have  been  made  by  S.  F.  B.  Morse  in  1842.  Morse  describes  his 
experiments  in  a  letter  to  the  Secretary  of  the  Treasury  of  the  United 
States,  which  was  laid  before  the  House  of  Representatives  on  Decem- 
ber 23,  1844.  He  says: 

"  In  the  Autumn  of  1842,  at  the  request  of  the  American  Institute, 
I  undertook  to  give  the  public  in  New  York  a  demonstration  of  the 
practicability  of  my  telegraph,  by  connecting  Governor's  Island  with 
Castle  Garden,  a  distance  of  a  mile;  and  for  this  purpose  I  laid  my 
wires  properly  insulated  beneath  the  water.  I  had  scarcely  begun 
to  operate,  and  had  received  but  two  or  three  characters,  when  my 
intentions  were  frustrated  by  the  accidental  destruction  of  a  part  of 
my  conductor  by  a  vessel,  which  drew  them  up  on  her  anchor,  and 
cut  them  off.  In  the  moments  of  mortification  I  immediately  devised 
a  plan  for  avoiding  such  an  accident  in  the  future,  by  so  arranging 
my  wires  along  the  banks  of  the  river  as  to  cause  the  water  itself  to 
conduct  the  electricity  across.  The  experiments,  however,  were 
deferred  till  I  arrived  in  Washington;  and  on  December  16,  1842, 
I  tested  my  arrangement  across  the  canal,  and  with  success.  The 
simple  fact  was  then  ascertained  that  electricity  could  be  made  to 
cross  the  river  without  other  conductors  than  the  water  itself;  but 
it  was  not  until  the  last  Autumn  that  I  had  the  leisure  to  make  a 
series  of  experiments  to  ascertain  the  law  of  its  passage.  The  follow- 
ing diagram  will  serve  to  explain  the  experiment : 

"  A,  B,  C,  D  (Fig.  51)  are  the  banks  of  the  river;  N,  P,  is  the 
battery;  G  is  the  galvanometer;  ww,  are  the  wires  along  the  banks 
connected  with  copper  plates,  /,  g,  h,  i,  which  are  placed  in  the  water. 
When  this  arrangement  is  complete,  the  electricity,  generated  by  the 
battery,  passes  from  the  positive  pole  P,  to  the  plate  h,  across  the 

1  A  large  part  of  the  historical  information  contained  in  this  chapter  was 
obtained  from  Mr.  J.  J.  Fahie's  excellent  History  of  Wireless  Telegraphy, 
Dodd,  Mead  &  Co.,  1902. 

75 


76  WIRELESS  TELEGRAPHY 

river  through  the  water  to  the  plate  i,  and  thence  around  the  coil  of 
the  galvanometer  to  plate  /,  across  the  river  again  to  plate  g,  and 
thence  to  the  other  pole  of  the  battery,  N. 

"  The  distance  across  the  canal  is  80  feet " 

In  these  experiments  Morse  found  that  it  was  necessary  to  make 
the  wires  along  each  shore  three  times  as  great  as  the  distance  from 
shore  to  shore  across  the  stream. 

Later,  under  Morse's  direction,  his  assistants,  Messrs.  Vail  and 
Rogers,  established  communication  in  the  same  way  across  the  Sus- 

quehanna    River,    a   dis- 
tance of  nearly  a  mile. 

Similar    attempts  to 
send  signals  through 
water    by    utilizing    the 
water    were    made    by 
^M^^^^^^^  James  Bowman   Lindsay 
~^  between    1854  and  1860. 

By  gradually  increasing 
the  power  of  his  plant 
and  the  length  of  his  conductors,  Lindsay  succeeded,  with  an  appa- 
ratus like  that  of  Morse,  in  signaling  across  the  Tay  where  the  river 
is  more  than  a  mile  wide. 

In  1880  Professor  John  Trowbridge  of  Harvard  University  sug- 
gested the  use  of  circuits  resembling  those  of  Morse,  modified  by  the 
employment  of  an  interrupted  current  in  the  sending  circuit  and  a 
telephone  receiver  in  the  receiving  circuit.  This  modification  takes 
advantage  of  the  high  sensitiveness,  portability  and  rapidity  of  action 
of  the  telephone  as  a  current  indicator.  About  1882  Professor 
Graham  Bell  made  some  successful  experiments  with  the  method 
suggested  by  Professor  Trowbridge.  The  following  is  an  extract 
from  Prbfessor  Bell's  description: 

"  Urged  by  Professor  Trowbridge,  I  made  some  experiments  which 
are  of  very  great  value  and  suggestiveness.  The  first  was  made  on 
the  Potomac  River. 

"  I  had  two  boats.  In  one  boat  we  had  a  Leclanche  battery  of 
six  elements  and  an  interrupter  for  interrupting  the  current  very 
rapidly.  Over  the  bow  of  the  boat  we  made  water  connection  by 
a  metallic  plate,  and  behind  the  boat  we  trailed  an  insulated  wire, 
with  a  float  at  the  end  carrying  a  metallic  plate,  so  as  to  bring  these 
two  terminals  about  100  feet  apart.  I  then  took  another  boat  and 
sailed  off.  In  this  boat  we  had  the  same  arrangement,  but  with  a 


WIRELESS  TELEGRAPHY  BEFORE  HERTZ  77 

telephone  in  the  circuit.  In  the  first  boat,  which  was  moored,  I 
kept  a  man  making  signals;  and  when  my  boat  was  near  his  I  would 
hear  those  signals  very  well  —  a  musical  tone,  something  of  this  kind; 
turn,  turn,  turn.  I  then  rowed  my  boat  down  the  river,  and  at  a 
distance  of  a  mile  and  a  quarter,  which  was  the  farthest  distance  I 
tried,  I  could  still  distinguish  those  signals." 

In  these  experiments  of  Morse,  Lindsay,  Trowbridge  and  Bell  the 
signals  were  carried  from  one  station  to  the  other  by  conduction 

through   the  water.     The  current  in  ^- -^^ 

flowing  from  one  submerged  plate  to  /'  '^ 

the  other  at  the  sending  station  spreads         / 

out  through  the  water  in  curves  like        / 

those  of  Fig.  52.     If,  now,  the  termi-        ! 

nals  of  the  receiving  circuit  dip  down        \ 

into  the  conducting  area,  the  current    ^ 

divides, — part  going  through  the  water       x 

and  part  through  the  receiving  circuit,    /  / 

in  the  inverse  ratio  of  their  resistances.         / 

This  method  of  signaling,  though  at-        1 

tempted    with    improved    apparatus         * 

by   Messrs.   Rathenau,  Rubens,   and          FlG-  52-    Lines  of  flow- 

Strecker,  and  by  the  latter  carried  to  a  distance  of  14  kilometers  (8.7 

miles),  has  not  contributed  to  the  art  of  wireless  telegraphy,  as  it  is 

now  practiced. 

Dolbear 's  Apparatus. — A  somewhat  more  suggestive  apparatus  was 
invented  by  the  late  Professor  Dolbear  of  Tufts  College,  Massachu- 
setts, and  was  awarded  a  United  States  patent  in  March,  1882.  Figure 
53,  taken  from  the  patent  specifications,  shows  a  diagram  of  the 
apparatus.  The  transmitting  station,  shown  at  the  left,  consisted 
of  a  condenser  Hf  connected  to  one  terminal  of  the  secondary  t>f  an 
induction  coil  G,  of  which  the  other  terminal  of  the  secondary  was 
grounded  at  C.  The  primary  of  the  induction  coil  contained  a  bat- 
tery /'  and  microphone  transmitter  T.  The  receiving  apparatus, 
shown  at  the  right,  consisted  of  a  telephone  receiver  R  with  one 
terminal  connected  to  ground  at  D,  and  the  other  terminal  connected 
to  a  condenser  H,  which  was  in  turn  connected  through  a  battery  l  B 
with  a  second  condenser  H 2. 

Professor  Dolbear,  in  his  patent  specifications,  describes  the  action 
of  the  apparatus  as  follows : 

"  Now  if  words  be  spoken  in  proximity  to  transmitter  T,  the  vibra- 

1  The  function  of  this  battery  is  not  evident. 


78 


WIRELESS  TELEGRAPHY 


tion  of  its  diaphragm  will  disturb  the  electric  condition  of  the  coil  C, 
and  thereby  vary  the  potential  of  the  ground  at  A,  and  the  variations 
of  the  potential  at  A  will  cause  corresponding  variations  of  the  poten- 
tial of  the  ground  at  B,  and  the  receiver  R  will  reproduce  the  words 
spoken  in  proximity  to  the  transmitter,  as  if  the  wires  CD  were  in 


FIG.  53.     Dolbear's  apparatus  for  wireless  telegraphy. 

contact,  or  connected  by  a  third  wire.  Electrical  communications 
may  be  thus  established  between  points  certainly  more  than  half  a 
mile  apart;  but  how  much  farther  I  cannot  now  say." 

In  some  other  of  Professor  Dolbear's  writings  he  speaks  of  using  an 
automatic  break  and  a  Morse  key  in  the  primary  of  his  coil  instead 
of  the  microphone  transmitter,  and  he  also  speaks  of  using  a  gilt  kite 
carrying  a  fine  wire  from  the  secondary  of  the  Ruhmkorff  coil. 

Professor  Dolbear  thus  made  an  approach  to  the  method  that  was 
subsequently,  in  the  hands  of  Marconi,  to  be  crowned  with  success. 
The  difficulty  with  the  Dolbear  apparatus  was  that  the  elevated  con- 
ductor had  to  discharge  through  the  secondary  of  the  induction  coil, 
and  thus  (as  we  see  now)  had  a  very  slow  frequency,  so  that  the 
inductive  action  of  the  waves  emitted  was  very  feeble.  Dolbear, 
therefore,  did  not  have  in  his  sending  station  the  one  essential  that 
makes  the  Marconi  sender  a  success;  namely,  electrical  oscillations 
of  high  frequency.  Also  the  detector  used  in  Professor  Dolbear's 
apparatus  (his  electrostatic  telephone  receiver)  was  not  of  sufficient 
sensitiveness. 

Sir  William  Preece's  Method.  —  Another  serious  attack  on  the 
problem  of  wireless  telegraphy,  before  the  work  of  Hertz  and 


WIRELESS  TELEGRAPHY  BEFORE  HERTZ  79 

Marconi  had  made  the  way  clear,  was  made  by  Sir  William 
Preece,  engineer-in-chief  of  the  postal  telegraph  system  of  Eng- 
land. Preece  attempted  to  utilize  the  electromagnetic  induction 
between  two  long  horizontal  wires,  one  at  the  sending  station  and 
the  other  at  the  receiving  station.  These  horizontal  wires  were 
supported  parallel  to  each  other  on  telegraph  poles,  and  were 
grounded  at  their  two  ends.  The  sending  wire  contained  a  battery 
and  an  interrupter,  or  else  an  alternating  current  generator,  so  that 
the  line  was  traversed  by  an  interrupted  or  an  alternating  current ; 
while  the  receiving  circuit  contained  an  ordinary  telephone  re- 
ceiver. The  surging  current  in  the  sending  wire  produced  a  vari- 
able magnetic  field  surrounding  it.  This  variable  magnetic  field 
produced  by  the  sending  circuit  cut  or  linked  with  the  receiving 
circuit,  and  induced  a  periodic  electromotive  force  in  it,  which  was 
evidenced  by  sounds  in  the  receiver. 

After  several  years  of  experimenting,  Sir  William  Preece  was 
able  to  utilize  this  apparatus  for  signaling  to  some  of  the  islands 
a  short  distance  off  the  coast  of  England,  and  in  1898  a  regular 
installation  was  established  at  Lavernock  Point  on  the  mainland 
and  at  Flatholm  in  the  Bristol  Channel,  3.3  miles  (5.2  kilometers) 
apart. 

Preece's  experiments  can  be  said  to  have  availed  only  to  show 
the  futility  of  the  attempt  to  get  inductive  action  at  long  distance 
without  the  use  of  oscillations  of  high  frequency. 


CHAPTER    XII 

WIRELESS   TELEGRAPHY    BY   HERTZIAN   WAVES. 
MARCONI,  1896-1898 

WE  come  now  to  the  application  to  wireless  telegraphy  of  the 
principles  discovered  by  Maxwell  and  Hertz.  For  this  application 
we  are  chiefly  indebted  to  the  genius,  skill,  and  forceful  initiative 
of  Signor  Guglielmo  Marconi.  We  are  to  see,  however,  that 
the  achievement  did  not  come  as  a  scientific  revolution,  but  as 
a  steady  development  to  which  many  other  investigators  also 
contributed. 

The  Coherer.  —  In  the  extension  of  the  effects  of  the  Hertzian 
waves  to  great  distances  the  first  need  was  a  detector  of  high 
sensitiveness.  Such  a  detector  was  already  at  hand  in  a  crude 
form;  for  as  long  ago  as  1866  S.  A.  Varley  had  discovered  that 
metallic  filings  in  a  loose  condition  have  a  high  resistance  and  that 
this  resistance  is  decreased  to  a  small  value  under  the  action  of  an 
electric  discharge  sent  through  the  filings.  This  fact  was  utilized 
by  Varley  in  the  construction  of  a  "  lightning  bridge,"  or  lightning 
arrester,  used  to  protect  electrical  apparatus  from  lightning. 
Varley  had  also  noticed  that  the  resistance  of  the  filings,  when 
lowered  by  the  discharge,  could  be  brought  back  to  its  high  value 
by  tapping  or  shaking  the  vessel  containing  them. 

In  1884  Calzecchi-Onesti  also  made  and  published  some  inde- 
pendent experiments  on  this  interesting  phenomenon,  verifying 
and  extending  the  results  of  Varley. 

This  work  of  Varley  and  Onesti  remained  unnoticed  until  1890, 
when  Professor  E.  Branly,  of  the  Catholic  University  of  Paris, 
rediscovered  the  phenomenon.  Professor  Branly  studied  the 
conductivity  of  metallic  filings  placed  in  a  glass  tube  between  two 
metallic  plugs,  by  which  the  filings  could  be  put  into  an  electric 
circuit.  He  found  that  the  filings  were  rendered  conductive  by 
electric  discharges  in  the  neighborhood  of  the  tube,  even  when  the 
discharges  did  not  actually  pass  through  it.  He  also  observed  that- 
tapping  the  tube  restored  the  filings  to  their  high  resistance.  He 
gave  the  name  "  radio-conductor  "  to  the  apparatus,  and  made 

80 


WIRELESS  BY  HERTZIAN  WA VES  —  MARCONI,  1896-1898      81 

many  interesting  experiments  in  the  effort  to  obtain  an  explana- 
tion of  its  action.  Branly's  radioconductor  is  now  familiarly 
known  as  the  "  coherer,"  -  a  name  invented  by  Sir  Oliver  Lodge. 
Coherer  Applied  to  Study  of  Electric  Waves.  — In  1893  and 
1894  Sir  Oliver  Lodge  applied  the  coherer  to  the  study  of  electric 
waves  by  putting  it  in  the  place  of  the  micrometer  spark  gap  in 
a  Hertz  resonator,  as  is  shown  in  Fig.  54.  Under  the  action  of  the 
electric  waves  sent  out  from  a  properly  placed  Hertz  oscillator, 
the  resistance  of  the  metallic  filings  in  the  coherer  fell  to  a  low 
value,  so  that  the  galvanometer  G  connected  in  series  with  a 
battery  B  in  a  local  circuit  through  the  coherer  gave  a  deflection. 
After  the  waves  ceased  the  resistance  of  the  coherer  remained  low, 
so  that  the  galvanometer  remained  deflected.  In  order  to  prepare 


FIG.  54.     Sir  Oliver  Lodge's  apparatus  for  detecting  electric  waves. 

for  another  reading  it  was  necessary  to  restore  the  filings  to  high 
resistance  by  tapping  the  tube.  Lodge  effected  this  restoration 
either  by  a  tapping  mechanism  driven  by  clockwork,  or  by  an 
electric  trembler  (like  an  electric  bell)  mounted  on  the  same  base 
as  the  coherer. 

With  this  apparatus  Professor  Lodge  succeeded  in  detecting 
Hertz  waves  at  a  distance  of  about  55  yards  from  the  source. 

Experiments  of  Popoff.  —  A  still  nearer  approach  to  an  operable 
form  of  receiving  apparatus  for  wireless  telegraphy  was  made  in 
1895  by  Professor  Popoff  of  Kronstadt,  and  a  description  of  the 
apparatus  was  communicated  by  him  to  the  Physico-Chemical 
Society  of  St.  Petersburg  in  April  of  that  year.  Popoff's  apparatus, 
which  was  designed  for  use  in  the  study  of  atmospheric  electricity, 
is  shown  in  the  diagram  of  Fig.  55.  The  left-hand  terminal  of 
the  coherer  was  connected  to  a  metallic  rod  extending  above  the 
house-top ;  the  right-hand  terminal  of  the  coherer  was  connected  to 
earth;  so  that  electric  currents  produced  by  the  atmospheric  elec- 


82 


WIRELESS    TELEGRAPHY 


tricity  were  conducted  to  earth  through  the  coherer.  Through 
the  coherer  there  was  also  a  local  circuit  containing  a  battery 
B  and  a  relay  R.  Under  the  action  of  the  atmospheric  electrical 
disturbances,  the  filings  in  the  coherer  became  conductive,  so 
that  a  current  from  the  battery  flowed  through  the  coherer  and 
around  the  electromagnet  of  the  relay.  This  current  magnetized 


FIG.  55.     Professor  PopofTs  apparatus  for  studying 
atrnospheric  electricity. 

the  core  of  the  relay  and  attracted  the  armature  so  as  to  close 
the  contact  at  D,  which  put  the  battery  in  circuit  with  an  electric 
bell.  The  bell  was  so  placed  that  its  hammer  while  in  vibration 
struck  the  bell,  and  also  struck  the  coherer,  causing  it  to  decohere. 
Thus  the  atmospheric  discharges  caused  the  bell  to  sound,  a^id 
after  the  cessation  of  the  discharge  the  filings  were  decohered 
so  that  the  bell  ceased  to  sound  and  awaited  another  discharge. 

In  addition  to  the  bell,  Popoff  used  also  a  telegraphic  registering 
apparatus  in  shunt  with  the  bell,  so  as  to  get  a  written  record  of  the 
duration  of  each  atmospheric  electric  disturbance. 

In  a  note,  dated  December,  1895,  he  says:  "  I  entertain  the  hope 
that  when  my  apparatus  is  perfected  it  will  be  applicable  to  the 
transmission  of  signals  to  a  distance  by  means  of  rapid  electric 
vibrations  —  when  in  fact  a  sufficiently  powerful  generator  of  these 
vibrations  is  discovered."  1 

1  This  quotation  is  from  Fahie's  History  of  Wireless  Telegraphy,  1892. 


WIRELESS  BY  HERTZIAN  WAVES  —  MARCONI,  1896-1898      83 

Marconi's  1896  Apparatus.  —  We  come  now  to  the  early  work  of 
Marconi.  After  having  made  some  preliminary  experiments  on  his 
father's  estate  near  Bologna  in  Italy,  Signor  Marconi  went  to  Eng- 
land, and  on  June  2, 1896,  filed  in  the  Patent  Office  of  Great  Britain 
a  part  of  his  first  application  for  a  patent  for  "  improvements  in  trans- 
mitting electrical  impulses  and  signals,  and  in  apparatus  therefor." 
The  part  of  the  application  filed  at  this  date  is  without  diagrams, 
and  contains  only  provisional  specifications.  A  complete  speci- 
fication covering  the  same  subject  matter,  amply  illustrated  with 
drawings  and  full  of  details  as  to  the  invention,  was  filed  March  2, 
1897.  This  patent  application  of  Mr.  Marconi  contains  the  first 
published  account  of  a  completed  apparatus  for  successful  wireless 
telegraphy  by  electric  waves,  and  is,  therefore,  a  document  of  con- 
siderable interest.  It  would  seem  to  be  not  unprofitable  to  give 
careful  attention  to  Marconi's  description  of  his  invention. 

In  the  description  that  follows,  the  quotations  are  taken  from  the 
Marconi  patent  specifications;  and  after  some  of  the  paragraphs  of 
quoted  or  paraphrased  description  I  have  added  a  brief  paragraph 
in  the  form  of  a  summary. 

Hertz  or  Righi  Oscillator  and  Receiver.  —  At  the  transmitting 
station  he  employs  "  a  Ruhmkorff  coil  having  in  its  primary  circuit 
a  Morse  key  for  starting  or  interrupting  the  current."  The  secondary 
of  the  coil  he  connects  to  "  pole  appliances  "  for  producing  the  desired 
oscillations.  Under  "  pole  appliances  "  he  mentions  "  insulated  balls 
separated  by  small  air  spaces  or  high  vacuum  spaces,  or  compressed 
air  or  gas,  or  insulating  liquids  kept  in  place  by  a  suitable  insulating 
material,  or  tubes  separated  by  similar  spaces  and  carrying  sliding 
discs." 

This  form  of  the  transmitting  apparatus,  as  may  also  be  seen  by 
reference  to  the  original  drawings,  is  an  ordinary  Hertz  or  Righi 
Oscillator,  actuated  by  a  Ruhmkorff  coil  with  a  Morse  key  in  its 
primary  circuit.  There  is,  however,  also  the  suggestion  of  the 
use  of  a  high  vacuum  or  compressed  air  or  gas  about  the  spark 
gap. 

"  At  the  receiving  instrument  there  is  a  local  battery  circuit  con- 
taining an  ordinary  receiving  telegraphic  or  signaling  instrument 
and  an  appliance  for  closing  the  circuit."  The  appliance  for  closing 
the  circuit  "  consists  of  a  tube  containing  conductive  powder,  or 
grains,  or  conductors  in  imperfect  contact,  each  end  of  the  column 
of  powder  or  the  terminals  of  the  imperfect  contact  or  conductor 
being  connected  to  a  metallic  plate  of  suitable  length  so  as  to  cause 


84  WIRELESS  TELEGRAPHY 

the  system  to  resonate  electrically  in  unison  with  the  electrical  oscil- 
lations transmitted  to  it." 

This  part  of  the  apparatus  is,  therefore,  essentially  the  receiving 
apparatus  of  Lodge  shown  in  Fig.  54,  with  a  telegraphic  relay  or 
sounder  substituted  for  the  galvanometer  G.  That  the  substitution 
could  be  made  had  already  been  shown  in  detail  by  Popoff. 

The  specifications  say  further:  •"  When  transmitting  through  the 
air,  and  it  is  desired  that  the  signal  or  electrical  action  should  only 
be  sent  in  one  direction,  or  when  it  is  necessary  to  transmit  electrical 
effects  to  the  greatest  possible  distance  without  wires,  I  place  the 
oscillation  producer  at  the  focus  or  focal  line  of  a  reflector  directed 
to  the  receiving  station,  and  I  place  the  tube  or  imperfect  contact 
at  the  receiving  instrument  in  a  similar  reflector  directed  towards 
the  transmitting  instrument." 

This  part  of  the  specification  provides  for  the  use  of  reflectors  like 
those  of  Hertz.  However,  on  account  of  the  difficulty  of  construct- 
ing reflectors  sufficiently  large,  the  reflectors  were  soon  abandoned 
by  Marconi  in  his  practical  work,  and  have  not  been  subsequently 
revived. 

Grounded  Circuits.  —  He  says  further:  "When  transmitting 
through  the  earth  or  water  I  connect  one  end  of  the  tube  or  contact 


FIG.  56.     Mr.  Marconi's 
1896  transmitter. 


FIG.  57.     Marconi's  1896 
receiver. 


to  earth  and  the  other  end  to  conductors  or  plates,  preferably  similar 
to  each  other,  in  the  air  and  insulated  from  the  earth."  A  diagram 
of  this  receiving  apparatus  is  shown  in  Fig.  57.  The  corresponding 
earthed  sending  apparatus  is  shown  in  Fig.  56. 

The  earthing  of  the  circuits  was  for  a  time  considered  to  be  the 
strongest  feature  of  the  Marconi  apparatus;  but  recent  experiments 


WIRELESS  BY  HERTZIAN  WAVES— MARCONI,  1896-1898     85 

have  shown  that  the  earthing  of  the  circuits,  though  a  convenience 
in  construction,  is  not  essential.1 

It  was  with  these  earthed  circuits  that  Marconi  made  his  first 
great  gains  in  the  distance  of  transmission;  but  as  we  now  look  back 
over  the  experiments,  we  see  that  the  gain  in  distance  came  about 
primarily  through  the  fact  that  with  this  apparatus  his  circuits  were 
placed  vertical  rather  than  horizontal,  and  also  through  the  use  of 
longer  waves  and  more  energy  and  larger  radiating  and  receiving 
antennae,  rather  than  through  the  use  of  the  mere  earth  connections. 
To  this  subject  we  return  in  Chapter  XIV. 

Marconi's  Coherer.  —  In  addition  to  the  practical  introduction 
of  the  vertically  placed  radiator  with  ground  connection  Signer  Mar- 
coni also  made  tremendous  progress  over  other  early  investigators 
in  his  skill  in  constructing  and  using  the  coherer.  A  sketch  of  the 
coherer,  drawn  natural  size  from  Marconi's  specifications,  is  shown  in 
Fig.  58.  The  metal  plugs  PP 
are  of  silver  slightly  amalgamated 
with  mercury,  but  no  excess  of  /^==  /  r  FJ== 

mercury  in  the  form  of  globules 

i  f,  ,  i  rr»i         i         r,  FIG.  58.     Marconi  coherer 

is  left  on  them.     The  plugs  fit  (natural  size). 

accurately  into  a  glass  tube,  and 

are  within  jV  inch  of  each  other.  The  filings  in  the  space  between 
the  plugs  are  preferably  96%  nickel  and  4%  silver,  and  should  not 
be  fine,  but  rather  coarse.  They  should  be  dry  and  free  from  grease 
and  dirt,  and  should  be  uniform  in  size.  The  tube  containing  the 
filings  is  preferably  exhausted  of  air  and  sealed  up.  In  sealing  up  the 
tube  care  should  be  taken  not  to  oxidize  the  filings.  In  order  that 
the  coherer  may  not  be  injured  by  the  current  through  it,  not  more 
than  T^OU  of  an  ampere  of  current  should  be  used  in  the  local  circuit. 
Marconi's  Decohering  Device. — One  of  the  greatest  difficulties  to 
be  overcome  in  operating  a  delicate  coherer  arises  from  the  fact  that 
the  signal  causes  the  coherer  to  become  conductive,  and  if  left  alone, 
the  coherer  perseveres  in  this  conductive  condition.  In  order  to 
restore  it  to  its  high  resistance  so  as  to  be  ready  for  the  next  signal, 
it  is  necessary  to  employ  an  automatic  tapper,  or  trembler,  which  is 
started  into  action  by  the  incoming  impulse,  and  which  stops  the 
signal  and  itself  when  the  incoming  impulse  ceases.  Signor  Marconi 
brought  the  decohering  device  to  a  high  state  of  perfection,  and  as  a 
result  changed  the  capricious  tube  of  filings  into  a  reliable  instrument 
for  practical  use. 

1  See  Chapter  XIV. 


WIRELESS  TELEGRAPHY 

A  diagram  drawn  from  Marconi's  descriptions  is  shown  in 
Fig.  59.  The  continuous  lines  of  the  drawing  show  the  coherer  Co, 
the  relay  R,  the  trembler  T,  and  the  sounder  S,  and  their  circuits. 
These  had  been  used  in  almost  the  same  form  by  Popoff.  The 
dotted  lines  p,  pi,  q,  qf,  and  h  show  the  further  improvements, 
introduced  by  Marconi,  and  consisting  of  resistances  to  prevent 
the  inductive  kick  of  the  instruments  from  acting  on  the  coherer. 
The  action  of  the  decohering  device  and  the  protective  resistances 


FIG.  59.     Receiving  circuit  showing  coherer  and  protective  devices. 

may  be  best  understood  by  following  the  process  as  an  actual 
message  is  sent  from  one  station  and  received  by  another.  This 
is  done  in  the  next  paragraph. 

Action  of  the  Apparatus  in  Sending  and  Receiving.  —  Let  us 
suppose  the  Morse  key  in  the  primary  of  the  Ruhmkorff  coil  at  the 
sending  station  to  be  operated  as  in  ordinary  line  telegraphy. 
While  the  key  is  closed,  the  interrupter  in  the  primary  of  the  coil 
makes  and  breaks  the  circuit  at  the  rate  of,  say,  100  a  second.  At 
each  break  of  the  primary  the  potential  of  the  secondary  rises 
to  a  high  value  and  charges  the  oscillator  sufficiently  to  produce  a 


WIRELESS  BY  HERTZIAN  WAVES— MARCONI,  1896-1898     87 

spark.  We  have  thus  in  our  supposed  case  100  sparks  a  second. 
Each  spark  occurs  with  oscillations  of  very  high  frequency  and 
produces  a  train  of  electric  waves.  With  such  a  sending  station 
we  should  have  arriving  at  the  receiving  station  a  train  consisting 
of  a  few  l  of  these  extremely  rapid  waves,  followed  T£IF  of  a  second 
later  by  a  second  similar  train;  and  thus  at  intervals  of  T<5i>  second 
there  would  arrive  successive  short  trains  of  waves  while  the  send- 
ing key  is  depressed. 

Under  the  action  of  these  trains  of  incoming  waves  the  filings 
in  the  tube  cohere,  so  that  a  current  flows  from  the  battery  Bi 
(Fig.  59)  through  the  coherer  Co  and  the  relay  R.  This  battery 
current  pulls  the  armature  of  the  relay  so  as  to  close  the  gap  at 
A.  When  the  gap  is  closed  a  second  battery  B  sends  a  current 
through  the  coils  of  the  sounder  St  and  also  through  the  coils  of 
the  trembler  T.  The  trembler  is  like  an  electric  bell  (with  a 
somewhat  shorter  striking  arm),  and  makes  a  series  of  strokes 
against  the  tube  of  the  coherer.  This  decoheres  the  filings,  but 
so  long  as  the  key  at  the  sending  station  is  closed,  the  waves 
continue  to  arrive  and  cause  a  repetition  of  the  coherence,  thus 
putting  the  coherer  in  a  state  of  repeated  coherence  and  decoher- 
ence  during  the  arrival  of  the  waves.  The  armature  of  the  relay 
is  adjusted  so  that  the  relay  is  somewhat  sluggish  and  does  not 
open  at  each  decoherence.  Therefore,  the  contact  of  the  relay 
remains  closed,  and  consequently  the  sounder  armature  stays 
down  as  long  as  the  trains  of  waves  continue  to  arrive.  When, 
however,  the  sending  key  is  released,  and  the  waves  cease  to  arrive, 
the  decoherence  due  to  the  tapper  perseveres,  the  relay  contact 
opens,  the  sounder  arm  is  released,  and  at  the  same  time  the 
trembler  stops.  Thus  each  closing  and  opening  of  the  key  at  the 
sending  station  produces  a  corresponding  down  and  up  stroke  of 
the  sounder,  making  a  dash  or  a  dot,  according  as  the  sending 
key  is  depressed  for  a  long  or  a  short  interval  of  time. 

Instead  of  the  sounder  for  translating  the  message  an  ordinary 
Morse  registering  tape-machine  may  be  used  to  give  a  written 
record  of  the  dashes  and  dots. 

Marconi's  Protective  Resistances  and  Inductances.  —  Return- 
ing now  to  diagram  Fig.  59,  let  us  examine  into  the  purpose  of  the 
resistances  p,  p{,  q,  qr,  and  h,  represented  by  the  dotted  lines. 

1  I  have  taken  revolving-mirror  photographs  of  the  spark  of  a  Marconi 
Oscillator  with  a  period  of  TtftfiUffff  second,  and  found  that  there  are  about 
12  waves  in  a  train. 


88  WIRELESS  TELEGRAPHY 

One  of  these  resistances  is  shunted  about  each  of  the  electromagnets 
of  the  circuit  and  one  about  each  make-and-break  contact  in  the 
circuit.  These  resistances  are  for  the  purpose  of  preventing  a 
sudden  rise  of  electromotive  force  in  any  of  the  circuits  due  to  the 
action  of  the  self-inductances  of  the  electromagnet.  We  have 
learned  in  Chapter  III  that  when  a  current  is  flowing  in  a  large 
self-inductance,  —  for  example,  through  the  coherer  and  relay,  — 
and  the  circuit  is  suddenly  interrupted,  —  e.g.,  by  a  stroke  of  the 
tapper  against  the  coherer,  —  the  self-inductance  in  the  circuit 
produces  a  large  electromotive  force  tending  to  cause  the  current 
to  continue.  This  large  e.m.f.  of  inductance  in  the  coherer  cir- 
cuit is  equivalent  to  a  discharge  through  the  coherer,  and  if  not 
prevented  would  affect  the  coherer  and  cause  it  to  cohere  again, 
when  in  fact  the  tapping  was  designed  to  cause  it  to  decohere. 
This  action  is  prevented  by  the  resistance  q  shunted  about  the 
relay.  In  a  similar  manner  the  other  resistances  absorb  the  energy 
sent  into  the  circuit  by  the  inductive  kicks  from  the  other  electro- 
magnets, which  would  otherwise  act  either  directly  or  inductively 
on  the  coherer. 

Mr.  Marconi  gives  the  following  appropriate  values  for  these 
resistances:  p  and  pv  ought  each  to  be  of  resistance  four  times  the 
resistance  of  the  trembler;  the  resistance  of  q  should  be  three  or 
four  times  that  of  the  relay;  q'  should  be  about  20,000  ohms,  or 
should  be  a  water  resistance  offering  polarization  voltage  equal  to 
the  battery  E\\  h  should  have  a  resistance  equal  to  four  times  the 
resistance  of  the  telegraph  instrument  S.  These  resistances 
should  all  be  non-inductive.  The  resistance  of  the  relay  itself 
should  be  more  than  1000  ohms,  and  the  working  current  should 
be  less  than  TITO  o  ampere. 

Two  coils  of  a  few  turns  of  wire  CC  wound  inductively  on  iron 
cores  are  inserted  in  the  relay  circuit  to  prevent  the  electric  oscil- 
lations due  to  the  incoming  waves  from  escaping  the  coherer 
by  going  into  the  relay  circuit.  However,  the  inductance  of  the 
relay  itself  effects  this  purpose  to  a  large  extent. 

The  decohering  and  protective  devices  described  in  Marconi's 
patent  specifications  are  still  a  model  for  the  proper  construction 
of  these  important  accessories  to  the  coherer.  Recently,  however, 
the  coherer  has  been  almost  completely  replaced  by  other  forms  of 
detectors  operating  on  other  principles  and  that  do  not  need  to  be 
decohered.  These  newer  detectors  are  the  subject  matter  of  a 
later  chapter. 


WIRELESS   BY  HERTZIAN  WAVES— MARCONI,  1896-1898     89 


Balloons  or  Kites.  —  Another  important  suggestion  contained 
in  the  1897  specifications  is  the  suggestion  that  "  the  larger  the 
plates  of  the  receiver  and  the  transmitter,  and  the  higher  from  the 
earth  the  plates  are  suspended,  the  greater  is  the  distance  at  which 
it  is  possible  to  communicate  at  parity  with  other  conditions." 
"  Balloons  can  also  be  used  instead  of  plates  on  poles,  provided 
they  carry  up  a  plate  or  are  themselves  made  conductive  by  being 


/°^ 


r — EH 


FIG.  60.     Simple  Marconi  circuits  with  antenna  sustained  by  a  kite. 
Switch  for  "cutting  over"  from  sending  to  receiving. 

covered  with  tinfoil.  As  the  height  to  which  they  may  be  sent  is 
great,  the  distance  at  which  communication  is  possible  becomes 
greatly  multiplied.  Kites  may  also  be  successfully  employed  if 
made  conductive  by  means  of  tinfoil."  This  sentence,  therefore, 
provides  for  the  use  of  antennae  of  great  height.  It  should  be 
noted  here  that  the  plates  or  tinfoil  covering  on  the  balloons  or 
kites,  which  the  inventor  makes  a  necessary  provision  of  the 
apparatus,  are  really  nonessential. 

A  diagram  of  circuits  in  which  kites  are  used  for  suspending  the 
vertical  wires  is  shown  hi  Fig.  60. 


90 


WIRELESS  TELEGRAPHY 


Shifting  from  Sending  to  Receiving. —  In  practice  there  are  both 
a  sending  and  a  receiving  apparatus  at  each  station.  Only  one 
antenna  is  needed,  and  this  is  shifted  from  the  spark  balls  to  the 
coherer,  in  changing  from  sending  to  receiving.  This  shifting  is 
done  by  means  of  a  switch,  as  is  shown  in  Fig.  60,  or  by  means  of 
the  key  itself,  as  is  shown  in  Fig.  61.  In  Fig.  61  the  key,  which  is 
also  a  switch,  is  in  the  position  for  sending,  and  the  coherer,  which 
is  at  R,  is  disconnected  from  the  antenna.  When  the  message  is 
finished,  or,  if  desired,  between  words  of  the  message,  the  key 
is  released,  and  the  arm  b,  which  is  insulated  from  61,  is  allowed  to 


u^ 
FIG.  61.     Marconi's  key  for  changing  from  sending  to  receiving. 


descend  so  that  the  contact  point  b2  rests  on  the  point  b3.  This 
connects  the  antenna  with  the  coherer  and  puts  the  station  in  a 
condition  to  receive. 

The  "  Claims  "  of  Marconi's  1896  Patent.  —  The  "  claims  "  of 
a  patent  are  a  series  of  succinct  statements  at  the  end  of  the  descrip- 
tive matter  and  are  supposed  to  embody  the  invention  in  its  most 
general  form.  Signor  Marconi's  English  patent  of  1896  contains 
nineteen  claims.  The  three  most  comprehensive  of  these  claims  are 
the  following: 

15.  A  receiver  consisting  of  a  sensitive  tube  or  other  imperfect  contact 
inserted  in  a  circuit,  one  end  of  the  sensitive  tube  or  other  imperfect  contact 
being  put  to  earth  whilst  the  other  end  is  connected  to  an  insulated  conductor. 

16.  The  combination  of  a  transmitter  having  one  end  of  its  sparking  appli- 
ance or  poles  connected  to  earth,  and  the  other  to  an  insulated  conductor,  with 
a  receiver  as  is  mentioned  in  claim  15. 

17.  A  receiver  consisting  of  a  sensitive  tube  or  other  imperfect  contact 
inserted  in  a  circuit,  and  earth  connections  to  each  end  of  the  sensitive  con- 
tact or  tube  through  condensers  or  their  equivalent. 


WIRELESS  BY  HERTZIAN  WAVES  -  MARCONI,  1896-1898     91 

Marconi's  Achievements  between  1896  and  1898.  —  In  July,  1896, 
soon  after  arriving  in  England,  Mr.  Marconi  submitted  his  plans  to 
Sir  William  Preece,  director  of  the  postal-telegraph  system  of  Eng- 
land. Preece,  of  whose  activity  in  connection  with  attempts  at 
wireless  telegraphy  we  have  already  learned,  entered  eagerly  into 
the  new  experiments. 

The  first  messages  were  sent  from  a  room  in  the  General  Post 
Office  to  an  impromptu  station  100  yards  distant.  Soon  afterwards, 
at  Salisbury  Plain,  with  parabolic  reflectors  about  the  instruments, 
communication  was  established  at  a  distance  of  two  miles.  In  May, 
1897,  discarding  the  reflectors  and  using  grounded  circuits,  Mr. 
Marconi  covered  a  distance  of  8.7  miles  between  Lavernock  Point 
and  Brean  Down.  Kites  were  employed  in  this  experiment  to  sup- 
port the  vertical  wires. 

In  July,  1897,  important  trials  were  made  at  Spezia,  Italy,  at  the 
request  of  the  Italian  Government,  and  communication  was  estab- 
lished at  a  distance  of  12  miles  between  a  warship  and  a  shore  station. 

In  July,  1898,  the  Marconi  apparatus  was  used  to  report  the  yacht 
races  at  the  Kingston  Regatta,  and  a  large  number  of  correct  mes- 
sages were  ^exchanged  between  a  press  boat  and  the  shore  at  dis- 
tances extending  up  to  20  miles. 

These  various  experiments  constituted  a  complete  demonstration 
of  the  utility  of  the  invention. 


CHAPTER  XIII 
ELECTRIC  WAVE  TELEGRAPHY  BY  RESONANT  CIRCUITS 

A  SIMPLE  radiating  circuit,1  like  that  shown  in  Fig.  60  of  the  pre- 
ceding chapter,  consisting  of  a  spark  gap  with  one  side  grounded 
and  the  other  attached  to  an  antenna,  has  a  definite  period  of  electric 
oscillation.  The  receiving  circuit  has  also  a  characteristic  period  of 
oscillation.  A  maximum  effect  will  be  obtained  when  the  two  have 
the  same  fundamental  period;  that  is  to  say,  when  the  two  circuits 
are  in  resonance.  Marconi  recognized  this  fact,  and  in  his  specifica- 
tions, particularly  with  reference  to  the  use  of  a  receiving  resonator 
of  two  metallic  vanes,  he  provides  a  method  of  experimentally  de- 
termining the  size  of  the  vanes  that  will  give  resonance  with  the 
transmitter. 

In  the  course  of  further  experiments  it  was  soon  found,  however, 
that  the  type  of  simple  receiving  conductor  in  which  the  coherer  is 
inserted  directly  in  the  antenna  circuit  is  not  a  very  discriminating 
resonator.  Also  it  is  usually  not  practicable  to  change  the  size  of 
the  vanes  or  the  length  of  the  vertical  wires  in  order  to  make  changes 
in  the  period  of  the  circuit. 

When  several  wireless  telegraph  stations  are  to  be  operated  at 
once,  it  is  highly  desirable,  in  order  to  be  able  to  avoid  confusion,  to 
have  a  method  of  readily  adjusting  the  receiving  circuit  to  resonance 
with  the  wave  lengths  it  is  desired  to  receive  and  out  of  resonance 
with  undesired  signals  of  a  different  wave  length.  Several  methods 
have  been  devised  for  accomplishing  this  result,  which  though  attain- 
ing only  limited  success,  have  yet  been  of  great  advantage  in  wireless 
telegraphy.  Circuits  capable  of  being  attuned,  or  adjusted  for 
resonance,  are  called  syntonic  circuits,  or  resonant  circuits. 

In  the  present  chapter  it  is  proposed  to  discuss  some  of  the  general 
types  of  resonant  circuits.  Quantitative  experiments  in  regard  to 
resonance,  and  some  of  the  practical  details  of  construction  of  appara- 
tus, will  be  given  later. 

1  We  shall  refer  to  a  rectilinear  oscillator  of  this  character  as  a  circuit, 
since  according  to  the  work  of  Maxwell  a  circuit  need  not  be  conductively 
closed. 

92 


ELECTRIC  WAVE  TELEGRAPHY  BY  RESONANT  CIRCUITS    93 


A  Simple  Variable  Circuit.  —  A  simple  method  of  easily  varying 
the  period  of  a  receiving  circuit  consists  in  the  use  of  a  variable  in- 
ductance L  (Fig.  62),  inserted  between  the  detec- 
tor and  the  antenna  or  between  the  detector  and 
the  ground  at  the  receiving  station.  Such  a  vari- 
able inductance,  or  tuning  coil,  is  made  of  a  single 
layer  of  wire  wound  on  an  insulating  tube  of  glass 
or  ebonite,  and  is  varied  by  a  contact  sliding  along 
the  coil  so  as  to  put  more  or  fewer  turns  of  induct- 
ance into  the  circuit.  A  similar  tuning  coil, 
though  usually  of  larger  wire,  may  be  used  at  the 
sending  station  also.  At  the  sending  station  the 
coil  is  inserted  between  the  spark  gap  and  the  an- 
tenna or  between  the  spark  gap  and  the  ground 
connection. 

Increase  of  inductance  in  either  circuit  increases 
the  time  of  vibrations,  which  brings  a  correspond- 
ing increase  of  wave  length. 

The  use  of  adjustable  inductances  in  both  the 
sending  and  the  receiving  circuits  was  apparently 
first  suggested  by  Sir  Oliver  Lodge  in  a  patent 
application  of  1887,  which  is  reviewed  later  in  the 
present  chapter. 

Coupled  Circuits.  —  Certain  other  methods,  employed  for  adjust- 
ing both  the  sending  station  and  the  receiving  station,  and  found  to 
produce  better  results  both  for  transmitting  with  large  quantities  of 
energy  and  for  receiving  with  comparatively  sharp  resonance,  make 
use  of  coupled  circuits.  The  resonance  relations  in  these  coupled  cir- 
cuits has  been  the  subject  of  much  theoretical  and  experimental 
research.  As  introductory  to  the  description  of  the  coupled  circuits, 
I  shall  recall  to  the  reader  the  familiar  and  interesting  experiments  of 
Mr.  Tesla  and  of  Professor  Thomson  on  the  production  of  electric 
oscillations  of  high  frequency  and  high  potential. 

High-frequency  Transformers  of  Thomson  and  Tesla.  —  The 
high-frequency  transformer  that  was  apparently  independently 
developed  by  Mr.  Nikola  Tesla  and  Professor  Elihu  Thomson  about 
1890  is  shown  in  sketch  in  Fig.  63.  A  primary  coil  P,  consisting  of 
one  or  two  turns  of  heavy  wire,  is  connected  in  series  with  a  bank 
of  Ley  den  jars  C  and  a  spark  gap  G.  A  secondary  coil  S,  consisting  of 
three  or  four  hundred  turns  of  wire  wound  in  a  single  layer  on  a  paper 
or  vulcanite  tube,  is  inserted  axially  within  the  primary.  When  the 


:G.  62.  Simple 
antenna  circuit 
having  a  variable 
inductance  for 
tuning. 


94 


WIRELESS  TELEGRAPHY 


bank  of  jars  is  connected  by  means  of  the  leads  W,  W  with  the  second- 
ary of  a  Ruhmkorff  coil,  or  better  with  the  secondary  of  an  alternating 
current  step-up  transformer,  the  Ley  den  jars  are  repeatedly  charged 
by  the  Ruhmkorff  coil  or  transformer  at  intervals  of,  say,  TTJTF  or 
v^  of  a  second.  When  the  potential  at  each  charge  of  the  jars 
reaches  a  value  high  enough  to  spark  across  the  gap  G,  the  jars  dis- 
charge with  oscillations  of  extremely  high  frequency.  A  group  of 
these  oscillations  occurs  during  each  spark  at  the  gap  G.  These 


FIG.  63.     Tesia  or  Thomson  coil. 


high-frequency  oscillations  in  the  primary  coil  P  act  inductively  on 
the  secondary  coil  $,  and  on  account  of  the  extreme  rapidity  of  change 
of  current  in  the  primary,  the  electromotive  force  induced  in  the 
secondary  is  very  high,  and  produces  a  series  of  sparks  between  the 
terminals  T±  and  T2  of  the  secondary. 

It  should  be  noted  that  the  primary  has  a  period  of  its  own, 
and  that  the  coil  of  wire  S  used  as  a  secondary  has  also  a  period 
of  its  own;  and  in  order  to  get  the  greatest  spark  at  the  secondary 
terminals,  it  is  necessary  to  adjust  the  number  of  jars  C  or  else 
the  number  of  turns  of  wire  on  either  P  or  S,  so  that  the  condenser 


ELECTRIC  WAVE  TELEGRAPHY  BY  RESONANT  CIRCUITS     95 

circuit  and  the  secondary  coil  shall  be  in  resonance  with  each 
other. 

By  the  use  of  apparatus  of  this  character  Mr.  Tesla  has  pro- 
duced enormous  sparks  —  twenty-three  feet  long  and  of  great 
volume  —  graphically  described  as  being  accompanied  by  a  roar 
like  Niagara. 

The  transformer  PS  is  called  a  high-frequency  transformer,  an 
oscillation  transformer,  or  an  air-core  transformer,  to  distinguish  it 
from  an  ordinary  iron-core  transformer,  such  as  is  used  with 
commercial  alternating  currents  of  slow  frequency. 

Oscillation  transformers,  built  on  somewhat  different  lines  from 
the  one  above  described,  have  met  with  application  to  both  the  send- 
ing and  the  receiving  circuits  of  electric- wave  telegraphy,  and  by  the 
use  of  these  transformers  a  considerable  advance  has  been  made, 
both  in  the  greater  distances  attained  and  in  the  diminished  con- 
fusion of  signals  of  different  wave  lengths. 

Two  Systems  of  Coupled  Circuits.  —  The  form  given  to  these 
coupled  circuits  is  considerably  varied  in  practice.  There  are, 
however,  two  important  general  types.  These  are  represented  in 
the  accompanying  figures  (Fig.  64  and  65)  and  are  called  re- 
spectively the  inductively  coupled  and  the  direct  coupled  types. 

The  Inductively  Coupled  Type.  —  This  type  is  shown  in  Fig.  64. 
In  this  system  the  sending  station,  shown  on  the  left,  is  seen  to 
consist  of  a  Tesla  high-frequency  apparatus,  with  one  secondary 
terminal  connected  to  an  antenna  and  the  other  secondary  ter- 
minal connected  to  the  ground.  Power  is  supplied  to  the  circuit  by 
an  alternating  current  transformer  or  a  Ruhmkorff  coil  to  which 
the  wires  W ,  W  lead. 

The  receiving  station  of  this  system,  shown  at  the  right  in 
Fig.  64,  has  also  an  oscillation  transformer  P'  S',  and  is  in  prin- 
ciple like  the  sending  station,  except  that  the  detector  Df  with  its 
accessories  is  usually  put  in  place  of  the  spark  gap  of  the  sending 
apparatus.  The  coils  P'  and  S'  and  condenser  C"  used  with  the 
receiving  apparatus  generally  have  different  inductances  and 
capacity  from  those  of  the  sending  apparatus,  and  not  being 
traversed  by  high-potential  currents  they  are  usually  made  more 
compact. 

In  this  inductively  coupled  system  of  circuits,  oscillatory  cur- 
rents in  the  sending  antenna  are  produced  inductively  by  the 
oscillatory  discharge  of  the  condenser  C  through  the  primary  coil 
P.  These  oscillatory  currents  in  the  sending  antenna  produce 


96 


WIRELESS  TELEGRAPHY 


electric  waves,  which  travel  away  in  all  directions  with  the  velo- 
city of  light  (186,000  miles  per  second).  Arriving  at  the  receiving 
station  these  electric  waves  set  up  oscillations  in  the  antenna  cir- 
cuit A'P'E'.  These  currents  through  Pr  act  inductively  on  S',  and 
produce  oscillatory  currents  in  the  detector  circuit. 


\ 


FIG.  64.     Inductively  coupled  transmitting  and  receiving  circuits. 


\ 


FIG.  65.     Direct  coupled  transmitting  and  receiving  circuits. 

The  Direct  Coupled  Type.  —  Figure  65  shows  the  other  form  of 
coupled  apparatus,  constituting  the  direct  coupled  system.     This 


ELECTRIC  WAVE  TELEGRAPHY  BY  RESONANT  CIRCUITS    97 

system  employs  auto-transformers;  that  is  to  say,  instead  of  hav- 
ing separate  primary  and  secondary  coils  in  the  high-frequency 
transformer,  the  primary  coil  (P  or  P')  at  either  station  is  a  part 
of  the  secondary  coil.  At  the  sending  station  (at  the  left)  the 
condenser  discharges  through  some  of  the  turns  of  the  secondary, 
and  the  discharge  acts  inductively  on  the  whole  of  the  secondary. 
Likewise,  at  the  receiving  station  the  oscillations  in  the  antenna 
pass  through  a  part  P'  of  the  secondary  S'  and  act  inductively  on 
the  whole  of  S'.  Theory  and  experiment  show  that  in  principle 
the  direct  coupled  circuits  differ  very  little  from  the  inductively 
coupled  system. 

Introduction  of  Coupled  Circuits  into  Practice.  —  Postponing 
for  a  time  the  direct  discussion  of  the  principles  involved  in  the 
use  of  the  coupled  circuits,  let  us  take  up  historically  the  matter 
of  the  introduction  of  these  circuits  into  wireless  telegraph  practice. 
The  examination  of  the  question  as  to  the  priority  of  the  different 
claimants  to  this  improvement  is  fraught  with  considerable  diffi- 
culty. Lodge,  and  Marconi  in  England,  and  Braun  in  Germany, 
have  clearly  established  dates  of  publication  by  patent  applica- 
tions. While  examining  the  question  of  priority,  I  shall  also  give 
a  brief  description  of  the  apparatus  of  these  several  inventors,  so 
far  as  pertains  to  the  form  of  circuits  used. 

Sir  Oliver  Lodge's  Apparatus.  —  On  May  10,  1897,  Professor 
Lodge  filed  a  patent  application  in  England  for  improvements  in 


FIG.  66.     Lodge's  transmitter  and  receiver. 

wireless  telegraphy.     The  corresponding  application  in  the  United 
States  was  filed  Feb.  1,  1898.  What  he  claims  to  be  the  most  promi- 


98 


WIRELESS  TELEGRAPHY 


nent  feature  of  his  invention  is  represented  in  Fig.  66.  The 
emitter  and  receiver  consist  preferably  of  two  large  conical  con- 
ductors, called  capacity  areas,  supported  by  poles;  but  horizontally 
placed  conductors  may  also  serve  as  his  capacity  areas,  and  one 
of  these  capacity  areas,  he  says,  may  be  the  earth. 

At  the  sending  station  (left)  he  joins  the  two  capacity  areas  h 
and  hl  to  polished  knobs  h2  and  h3.  Between  either  capacity  area 
and  its  knob  be  places  a  syntonizing  self-inductance  coil.  "  The 
object  of  this  coil,"  Lodge  says,  is  "  to  prolong  the  electric  oscilla- 
tions occurring  in  the  radiator,  so  as  to  constitute  it  a  radiator  of 
definite  frequency  or  pitch  and  obtain  a  succession  of  tone  waves 
emitted,  and  thereby  to  render  syntony  in  a  receiver  possible, 
because  exactitude  of  response  depends  on  the  fact  that  the  total 
number  of  oscillations  in  a  suitably  arranged  circuit  is  very  great." 
He  provides  also  for  varying  the  number  of  active  turns  of  these 
coils  for  the  purpose  of  varying  the  period  of  the  circuits. 

After  having  described  the  ordinary  way  of  charging  the  vanes 
of  the  emitter  by  connecting,  them  directly  or  through  small  spark 
gaps  to  the  secondary  of  a  Ruhmkorff  coil,  as  Hertz  and  Righi  had 


FIG.  67.     Lodge's  apparatus  for  exciting  an  antenna  by  means  of  a 
Leyden  jar  discharge. 

done,  Lodge  proposes  also  the  use  of  Leyden  jars  in  the  manner 
shown  in  Fig.  67.  In  this  diagram  the  jars  are  shown  at  jj.  The 
inner  coatings  of  the  jars  are  connected  to  the  secondary  of  a 
Ruhmkorff  coil,  the  outer  coatings  of  the  jars  are  joined  by  an 
inductance  coil  of  thin  wire  k.  This  coil  is  necessary  in  order  that 
the  jars  may  charge.  When  the  jars  discharge  across  the  gap 
h10hn,  sparks  also  appear  across  the  gaps  h6h7  and  h?h3.  This 
apparatus,  therefore,  shows  the  use  of  a  Leyden  jar  circuit  dis- 


ELECTRIC  WAVE  TELEGRAPHY  BY  RESONANT  CIRCUITS     99 

charging  into  the  antenna  circuit,  and  if  the  coil  k  has  a  large 
inductance,  as  it  seems  to  have  from  the  fact  that  it  is  made  of 
"  fairly  thin  wire,"  this  sending  arrangement  may  be  looked  upon 
as  a  special  and  very  imperfect  form  of  the  direct-coupled  type  of 
sending  circuit  of  Fig.  65.  It  is  imperfect  in  the  use  of  the  mul- 
tiplicity of  spark  gaps,  for  if  all  the  gaps  except  hwhn  had  been 
closed,  the  coil  k,  which  was  put  in  as  a  charging  bridge  across  the 
unnecessary  gaps,  could  then  have  been  omitted,  and  the  apparatus 
would  have  been  a  very  useful  form  of  direct-coupled  emitter. 

While  we  are  accustomed  to  the  use  of  multiple  gaps  in  replace- 
ment of  a  single  gap,  and  while  the  multiple  gap  is  in  some  construc- 
tions a  distinct  advantage  over  the  single  gap,  still  the  introduction 
of  one  of  the  multiplicity  of  gaps  directly  into  the  antenna  circuit 


FIG.  68.     Lodge's  inductively  coupled  receiving  transformer. 

is  certainly  an  annulment  of  the  chief  advantage  accruing  from  the 
coupled  circuits. 

In  the  receiving  apparatus  Lodge  shows  the  use  of  an  oscillation 
transformer.  Reference  is  made  to  Fig.  68.  His  conical  capacity 
areas  or  their  equivalent  are  connected  to  the  primary  coil  h4.  About 
this  a  secondary  coil  u  is  placed,  and  is  connected  with  the  coherer  e, 
a  battery  /,  and  the  telegraphic  receiving  instrument  g.  The  purpose 
of  connecting  the  detector  in  a  secondary  circuit  instead  of  directly 
in  the  antenna,  is,  according  to  the  patentee,  to  "  leave  the  resonator 
freer  to  vibrate  electrically  without  disturbance  from  attached  wires." 
This  is  an  excellent  reason,  but  the  receiving  apparatus,  as  shown  in 
this  diagram,  which  was  taken  from  Lodge's  patent  specifications, 
has  the  fatal  defect  that  no  condenser  is  shown  in  the  secondary 
circuit,  and  that  the  high-frequency  oscillations  have  to  go  through 
the  telegraph  instrument.  Hence,  apart  from  the  suggestion  of  the 


100 


WIRELESS  TELEGRAPHY 


use  of  a  transformer  in  the  receiving  apparatus,  we  cannot  consider 
this  circuit  of  Lodge  to  be  a  clear  disclosure  of  the  inductively  coupled 
receiving  system. 

Later  Lodge  and  Muirhead,  in  a  British  specification  filed  Dec.  8, 
1897,  partially  remedied  the  above  defect,  by  the  arrangement  of 
circuits  shown  in  Fig.  69.  Between  the  two  winged-shaped  capacity 

areas  A  A,  they  inserted  an  induc- 
tance coil  d,  and  a  large  condenser 
C.  About  this  condenser  a  battery 
B  and  a  telegraph  instrument  /  are 
connected;  and  about  both  induc- 
tance and  condenser  is  connected  a 
coherer  Co.  With  proper  values  of 
the  inductance  and  capacity,  this 
arrangement  is  a  special  case  of  the 
direct  coupled  receiving  apparatus. 
These  contrivances  of  Lodge  and 
of  Lodge  and  Muirhead  seem  not 
to  have  been  developed  by  them 
experimentally  to  their  natural  com- 
pletion, which  would  perhaps  have 
led  to  one  or  the  other  of  the  types 
given  in  Figs.  64  and  65.  They  did 
come  to  one  of  these  types  much 
later,  but  only  after  Mr.  Marconi 
and  Professor  Ferdinand  Braun 
had  published  their  descriptions  of 
the  coupled  circuits,  and  by  various 
public  demonstrations  and  polemics 
had  shown  the  great  advantage  of  the  coupled  circuits. 

Just  a  word  as  to  Lodge's  wing-shaped  vanes  at  the  transmitter 
and  receiver.  These  are  an  enlarged  oscillator  and  resonator  pre- 
serving the  symmetry  of  Hertz's  original  apparatus.  In  practice 
they  have  not  been  much  used  in  this  symmetric  form,  probably  on 
account  of  the  difficulty  of  giving  to  the  apparatus  sufficiently  large 
dimensions  when  both  vanes  have  to  be  supported  vertically  in  an 
elevated  position.  In  practice,  Lodge  and  Muirhead  early  replaced 
the  bottom  vane  and  sometimes  also  the  top  one  by  the  alternative, 
horizontally-placed  conductor,  consisting  of  a  sheet  of  wire  netting. 
The  spark  gap,  instead  of  being  halfway  between  these  conductors, 
is  usually  nearer  the  lower  conductor,  as  shown  in  Fig.  70,  with  an 


FIG.  69.  A  method  employed  by 
Lodge  for  connecting  the 
receiving  apparatus  to  the 
antenna. 


ELECTRIC  WAVE  TELEGRAPHY  BY  RESONANT  CIRCUITS    101 


inductance  L  added  below  the  gap, 
for  preserving  approximate  electrical 
symmetry  and  for  tuning. 

In  addition  to  these  various  sugges- 
tions by  Lodge  in  regard  to  the  use  of 
tuning  coils  and  transformers  in  the 
circuits,  and  the  maintenance  by  him 
of  the  possibilities  of  the  ungrounded 
circuits,  Professor  Lodge,  together  with 
Messrs.  Muirhead  and  Robinson,  has 
also  devised  a  new  form  of  coherer. 
This  is  described  in  Chapter  XVI. 

The  Coupled  Circuits  of  Ferdinand 
Braun.  —  Let  us  return  to  the  matter 
of  the  coupled  circuits.  In  a  German 
patent,  No.  111,578,  applied  for  October 
14,  1898,  Professor  Ferdinand  Braun  of 
Strassburg  in  Germany  describes  "a 


(Lodge). 


M 


M 


FIG.  71a,  716,  71c,  7ld.    Professor  Ferdinand  Braun's  methods  of  coupling  a 
condenser  circuit  to  an  antenna. 


102  WIRELESS  TELEGRAPHY 

form  of  connection  for  an  oscillator  coupled  with  an  air-wire  for 
spark  telegraphy."  He  proposes  to  use  in  this  invention,  which 
is  a  sending  apparatus,  the  waves  which  are  produced  by  the 
11  discharge  of  Ley  den  jars  in  the  presence  of  induction  coils." 

The  accompanying  Figures,  71a,  716,  71C,  71d,  are  taken  from 
Braun's  patent  specifications. 

In  71a  "  F  is  a  Ley  den  jar,  a  the  spark  gap,  P  an  inductance  coil 
and  M  the  emitting  wire." 

In  71&  "  the  form  of  connection  is  so  changed  that  a  primary  and  a 
secondary  coil  are  used,  so  that  in  the  circuit  of  the  emitter  M  a  coil 
only  is  present." 

Fig.  71C  "  shows  two  Ley  den  jars  connected  one  behind  the  other 
in  so-called  cascade  connection,  and  " 

Fig.  71d  "  shows  the  same  connection  for  the  use  of  a  transformed 
current." 

In  examining  these  figures  it  should  be  borne  in  mind  that  the 
two  coils  P  and  S,  which  for  simplicity  of  drawing  are  shown  side 
by  side,  are  really  wound  one  around  the  other  so  as  to  form  an 
oscillation  transformer. 

It  should  also  be  borne  in  mind,  while  reading  the  specifications, 
that  Professor  Braun  means  primarily  to  describe  a  method  of  con- 
necting the  Ley  den-jar  circuit  to  what  he  calls  the  air-wire  circuit, 
and  that  the  document  does  not  purport  to  give  a  description  of  a 
complete  sending  apparatus.  This  is  clear  from  his  single  claim  at 
the  end  of  his  description.  His  claim  is  a  "  form  of  connection  of  an 
oscillator  coupled  with  an  air  wire  for  spark  telegraphy,  characterized 
by  an  oscillation  circuit  containing  a  Ley  den  jar  and  a  spark  gap, 
to  which  the  air  wire  for  sending  out  the  waves  is  connected  either 
directly  or  by  means  of  a  transformer,  for  the  purpose  of  bringing 
by  this  means  greater  quantities  of  energy  into  action." 

His  clear  understanding  of  how  this  energy  may  be  increased  is 
evident  from  a  paragraph  of  the  specification  which  says:  "Above 
all,  the  slower  oscillations  have  the  advantage  that  their  energy  may 
be  increased  by  increasing  their  potential  amplitude  (by  transforma- 
tion) as  well  as  by  increase  of  capacity  and  by  the  use  of  powerful 
sources  of  electricity."  The  gain  of  energy  by  increasing  the  poten- 
tial could  not  be  attained  with  an  emitter  having  the  spark  gap 
directly  in  the  antenna,  because  there  is  a  certain  "  active  "  spark 
length  that  cannot  be  exceeded.  This  is  pointed  out  in  the  specifi- 
cations. 

However,  in  neither  the  German  nor  the  corresponding  American 


ELECTRIC  WAVE  TELEGRAPHY  BY  RESONANT  CIRCUITS    103 


patent,  filed  Feb:  6,  1899,  does  Professor  Braun  speak  of  the  neces- 
sity of  properly  attuning  the  secondary  circuit  to  the  period  of  the 
condenser  circuit,  which  is  a  prerequisite  for  attaining  the  high  poten- 
tial in  the  antenna  circuit,  and  without  this  attuning  of  the  secondary 
to  the  primary  circuit,  the  large  capacity  of  the  primary  condenser 
and  the  use  of  powerful  sources  of  electricity  would  not  give  any 
advantage  over  the  simple  Marconi  antenna. 

The  first  mention  by  Braun  of  the  required  tuning,  so  far  as 
I  have  been  able  to  find,  is  in  a  publication  of  the  5th  of  March, 
1901,  in  the  Physikalische  Zeitschrift,  Vol.  2,  p.  373,  and  in  a  book 
by  Braun  entitled  Drahtlose  Telegraphie  durch  Wasser  und  Luft, 
published  in  1901. 

In  examining  Braun's  patent  drawings  one  may  wish  to  know 
whether  the  antenna  circuit  is  to  be  grounded  or  otherwise  bal- 
anced by  a  capacity  at  the  other  end  of  the  secondary.  Nothing 
is  said  on  this  subject  in  the  German  patent,  but  in  the  correspond- 
ing American  patent  he  says,  with  reference  to  Fig.  7 Id,  that 
"  one  end  of  the  secondary  coil  of  the  transformer  S  is  connected 
with  the  transmitting  wire  M,  and  the  other 
end  is  shown  prolonged  and  ending  in  an 
arrow  to  indicate  that  it  may  be  prolonged 
by  adding  a  suitable  length  of  insulated  wire  or 
connected  to  some  other  capacity  area/'  In 
his  book  of  1901  a  corresponding  prolongation 
or  addition  of  capacity  is  indicated  in  his  draw- 
ing of  the  direct  coupled  circuit,  as  is  shown  in 
Fig.  72. 

There  is  nothing  in  these  early  patents  of 
Professor  Braun  relating  to  coupled  circuits  at 
the  receiving  station.  The  coupled  receiving 
circuits  were  undoubtedly  invented  by  Mar- 
coni, and  also  his  description  of  the  induc- 
tively coupled  sending  station,  though  of  pub- 
lished date  a  little  later  than  that  of  Braun,  is 
a  much  fuller  and  a  more  complete  disclosure 
of  the  invention.  The  work  of  Marconi  in  developing  the 
coupled  circuits  will  now  be  discussed. 

Marconi's  Coupled  Circuits.  —  Mr.  Marconi,  in  an  English 
patent  applied  for  June  1,  1898,  clearly  sets  forth  the  transformer 
arrangement  for  a  receiving  station.  This  is  shown  in  Fig.  73,  in 
which  A  leads  to  the  antenna,  and  E  to  earth.  The  coils  JV2, 


oo 


FIG.  72.  Form  of 
direct  coupled 
transmitter  de- 
vised by  Fer- 
dinand Braun. 


104 


WIRELESS  TELEGRAPHY 


which  are  represented  as  side  by  side,  are  the  oscillation  trans- 
former and  are  really  wound  one  around  the  other.  The  primary 
Jl  is  connected  to  the  antenna  and  the  earth;  while  the  secondary 
is  in  circuit  with  the  coherer  T  and  the  condenser  K1.  A  relay  R 
and  battery  B  are  connected  about  the  coherer  through  the 
choking  coils  clc2.  This  is,  therefore,  a  clear  presentation  of  the 
inductively  connected  receiving  station. 


FIG.    73.      Marconi's    inductively 
coupled  receiving  circuit. 


FIG.  74.      Marconi's   inductively 
coupled  transmitter. 


In  1900  Marconi  was  granted  an  English  patent  for  an  induc- 
tively coupled  sending  station  also.  This  is  shown  in  Fig.  74,  and 
is  of  the  typical  form  of  our  Fig.  64,  with,  however,  an  added 
variable  inductance  g  in  the  antenna. 

In  the  1900  description  of  this  apparatus  Mr.  Marconi  clearly 
points  out  the  necessity  of  having  the  primary  and  the  secondary 
circuits  at  the  sending  station  and  the  corresponding  circuits  at 
the  receiving  station  adjusted  to  resonance  with  one  another. 
He  says:  "  The  capacity  and  self-induction  of  the  four  circuits  — 
i.e.,  the  primary  and  secondary  circuits  at  the  transmitting  station 
and  the  primary  and  secondary  circuits  at  any  one  of  the  receiving 
stations  in  a  communicating  system  —  are  each  and  all  to  be  so 
independently  adjusted  as  to  make  the  product  of  the  self-induc- 
tion multiplied  by  the  capacity  the  same  in  each  case  or  multiples 


ELECTRIC  WAVE  TELEGRAPHY  BY  RESONANT  CIRCUITS    105 

of  each  other  —  that  is  to  say,  the  electrical  time  periods  of  the 
four  circuits  are  to  be  the  same  or  octaves  of  each  other." 

The  advantages  of  the  coupled  circuit  at  the  sending  station, 
according  to  Signor  Marconi,  arises  in  "  the  approximately  closed 
circuit  of  the  primary  being  a  good  conserver  and  the  open  cir- 
cuit of  the  secondary  being  a  good  radiator  of  wave  energy." 

The  variable  inductance  g  in  Fig.  74  placed  in  the  antenna  of 
the  sending  circuit  and  a  corresponding  coil  at  the  receiving 
station  were  used  to  aid  in  this  process  of  tuning. 


K 


FIG.  75.     Apparatus  used  by  Marconi  for  sending  two  messages  at  once. 

A  sending  and  a  receiving  station  devised  by  Mr.  Marconi  for 
sending  or  receiving  two  messages  at  once  with  the  use  of  a  single 
antenna  are  shown  in  Fig.  75  and  Fig.  76.  This  was  successfully 
employed  and  exhibited  by  Marconi  in  the  autumn  of  1900. 
Two  operators  at  the  two  keys  K  and  K  of  the  sending  station 
made  the  signals.  The  two  condenser  circuits  having  different 
values  of  capacity  and  self-inductance  were  independently  charged, 
and  discharged  with  different  periods  of  oscillation.  These  two 
periods  were  both  impressed  on  the  antenna  through  circuits 
which  by  means  of  the  antenna  inductances  d  and  d'  were  made  to 
resonate  with  the  respective  condenser  periods.  At  the  receiving 


106 


WIRELESS  TELEGRAPHY 


station,  Fig.  76,  the  waves  constituting  the  double  message  acted 
on  the  receiving  antenna.  The  waves  of  the  shorter  period  in- 
duced oscillations  through  the  antenna  and  through  the  right- 
hand  primary  to  the  earth;  while  the  oscillations  of  longer  period 
passed  through  the  circuit  to  the  left,  which  contained  the  greater 
inductance  in  the  antenna  circuit.  The  two  periodic  disturbances, 
thus  separated,  act  inductively  on  their  properly  tuned  coherer 
circuits,  and  give  up  the  two  messages  without  confusion  to  the 
two  receiving  instruments.  This  duplex  wireless  telegraphy  can 


FIG.  76.     Marconi  duplex  receiving  apparatus. 

be  carried  on  only  provided  the  wave  lengths  of  the  two  sending 
stations  are  not  two  nearly  equal. 

Some  very  notable  achievements  were  made  by  Mr.  Marconi 
with  these  resonant  circuits  in  1901  and  1902. 

A  sending  station  of  great  power  was  completed  at  Poldu  in 
Cornwall,  England,  in  1901.  In  December  of  that  year  signals 
were  reported  to  have  been  sent  across  the  Atlantic  Ocean  from 
Poldu  to  Cape  Race  near  St.  Johns  in  Newfoundland.  The  signals, 
which  consisted  of  the  letter  "S  "  repeated  at  stated  intervals, 
were  said  to  have  been  clearly  received  at  Cape  Race,  by  means 
of  a  receiving  antenna  consisting  of  a  copper  wire  400  feet  long 


ELECTRIC  WAVE  TELEGRAPHY  BY  RESONANT  CIRCUITS    107 

supported  by  a  kite.  The  detector  employed  in  the  transatlantic 
experiments  was  an  instrument  known  as  the  "  Italian  Navy 
Coherer,"  and  consisted  of  a  globule  of  mercury  between  iron 
terminals  in  a  glass  tube.  This  form  of  detector  is  self-restoring, 
and  with  it  a  telephone  receiver  in  series  with  a  battery  is  used  in 
the  local  circuit  in  the  place  of  the  ordinary  telegraph  relay. 

In  March,  1902,  messages  sent  out  from  Poldu  were  received 
by  the  .Marconi  apparatus  on  board  the  Steamer  Philadelphia 
when  the  steamer  was  1550  miles  (2400  kilometers)  from  the  send- 
ing station.  In  December  of  the  same  year  Marconi  announced 
the  transmission  of  three  entire  messages  from  Glace  Bay,  Nova 
Scotia,  to  Poldu  in  England,  a  distance  of  2300  miles. 

On  January  19,  1903,  the  powerful  Marconi  station  at  Well- 
fleet,  Cape  Cod,  Massachusetts,  transmitted  to  Poldu,  England, 
the  following  message  from  the  President  of  the  United  States  to 
the  King  of  England : 

"His  MAJESTY,  EDWARD  VII, 
London,  England. 

In  taking  advantage  of  the  wonderful  triumph  of  scientific  research  and 
ingenuity  which  has  been  achieved  in  perfecting  a  system  of  wireless  teleg- 
raphy, I  extend  on  behalf  of  the  American  people  most  cordial  greetings  and 
good  wishes  to  you  and  to  all  the  people  of  the  British  Empire. 

THEODORE  ROOSEVELT." 
WELLFLEET,  MASS., 
January  19,  1903. 

This  message,  though  intended  to  be  relayed  at  Cape  Race,  was 
received,  according  to  reports  issued  by  the  Marconi  Company, 
direct  at  the  Poldu  station  in  England. 


CHAPTER  XIV 

NATURE  OF  THE  OSCILLATION.       THE  GROUNDING  OF 

CIRCUITS 

IN  the  two  preceding  chapters,  devoted  to  a  period  of  invention 
and  rapid  development  of  wireless  telegraphy,  several  important 
facts  have  been  introduced  with  only  casual  examination.  Among 
the  questions  there  raised  the  most  interesting  is  perhaps  the  ques- 
tion of  the  role  played  by  the  earth.  This  question  has  two  aspects. 

First,  it  has  been  seen  that  both  grounded  and  ungrounded  oscil- 
lators have  been  employed.  What  is,the  relation  between  these  two 
forms  of  oscillator,  and  what  effect  has  the  ground  connection  on  the 
nature  of  the  vibration? 

Second,  it  has  been  apparent  from  the  great  distances  attained, 
in  the  transmission  of  messages  entirely  across  the  Atlantic  Ocean 
that  the  electric  waves  are  not  lost  to  the  receiver  by  reason  of  the 
curvature  of  the  earth,  even  when  the  two  stations  are  separated 
by  a  distance  that  is  a  considerable  fraction  of  the  earth's  whole 
circumference.  How  is  it  .that  the  electric  waves  are  propagated 
from  one  station  to  the  other,  and  how  does  the  earth  contribute 
to  the  process? 

These  two  questions,  dealing  with  the  nature  of  the  vibration  and 
the  manner  of  the  propagation  of  the  waves,  will  be  considered  respec- 
tively in  this  chapter  and  in  the  next  chapter.  As  introductory,  we 
shall  need  first  to  consider  the  oscillations  occurring  in  a  simple 
ungrounded  Hertz  oscillator. 

Current  and  Potential  in  a  Hertz  Oscillator.  —  Suppose  the  Hertz 
oscillator  to  consist  of  two  metallic  rods  or  wires  with  a  spark  gap 
between,  and  suppose  the  two  halves  of  the  oscillator  to  be  charged, 
the  one  positive  and  the  other  negative,  as  shown  in  the  first  diagram, 
(a),  of  Fig.  77,  in  which  the  shaded  area  attached  to  the  upper  half 
of  the  oscillator  is  taken  to  indicate  positive  electricity,  while  the 
unshaded  area  attached  to  the  lower  rod  indicates  negative  electric- 
ity. These  areas  are  made  rectangular  to  show  that  there  is  at  the 
beginning  a  uniform  distribution  of  the  two  electricities  respectively 
on  the  two  rods. 

108 


NATURE  OF  THE  OSCILLATION 


109 


If  the  electrostatic  capacity  per  unit  of  length  of  the  rods  is  uni- 
form throughout  both  rods,  which  is  approximately  true,  when  the 
rods  are  not  too  short  the  potential  of  the  conductor  at  any  point  of 
its  length  will  be  proportional  to  the  charge,  so  that  the  shaded  area 
representing  a  distribution  of  positive  charge  may  also  be  looked 
upon  as  showing  the  distribution  of  positive  potential,  while  the 
unshaded  area  represents  negative  potential.  Thus,  in  the  condition 
depicted  in  diagram  (a),  there  is  a  uniform  positive  potential  over 


t  =  o 


W 

Charged 
top  + 


Neutral 


Neutral        Charged 
top -4- 


(O 

}urre] 


ent 

o 


DQ 

Current 
Max  .down 


CO 

Current 


FlG.  77. 


Current  Current 

«=O  Max.up  =o 

Potential  and  current  distribution. 


the  top  rod,  and  a  corresponding  negative  potential  over  the  bottom 
rod.     This  is  before  the  spark  begins. 

Suppose,  now,  the  spark  to  start  between  the  rods;  the  gap  between 
the  rods  becomes  conductive,  and  a  current  begins  to  flow  between 
the  rods.  There  is  a  flow  of  positive  electricity  from  the  top  rod 
and  a  flow  of  negative  electricity  from  the  bottom  rod.  The  elec- 
tricity to  flow  first  across  the  gap  is  that  in  the  neighborhood  of  the 
spark  gap,  because  it  is  there  that  the  potential  gradient  is  greatest. 
After  a  short  time  —  one-fourth  the  period  of  a  complete  oscillation 
—  the  condition  of  the  charge,  and  likewise  the  potential,  of  the  rod 


WIRELESS  TELEGRAPHY 

will  be  that  condition  represented  in  diagram  (6),  in  which  one-half 
of  the  positive  electricity  has  gone  into  the  lower  rod,  and  half  the 
negative  electricity  has  gone  into  the  upper  rod,  giving  both  rods 
equal  quantities  of  positive  and  negative  electricity,  so  that  both 
rods  are  neutral.  Why,  then,  does  not  the  action  cease? 

In  order  to  be  able  to  see  why  the  action  does  not  cease  when  the 
charge  has  become  neutral  throughout  the  oscillator,  we  must  take 
into  consideration  the  current  in  the  conductor  as  well  as  the  charges. 
The  second  row  of  diagrams  of  Fig.  77  represents  the  current  at  the 
epochs  corresponding  to  the  potential  representations  of  the  first 
row. 

At  the  beginning  the  potential,  or  charge  distribution,  is  shown  by 
diagram  (a)  of  the  top  row.  At  the  same  time  no  current  is  flowing, 
which  fact  is  represented  by  the  inactive  oscillator  shown  at  (a')  of 
the  second  row.  The  current  is  now  supposed  to  begin.  It  cannot 
spring  to  its  final  value  at  once,  because  the  increase  of  the  current 
builds  up  a  magnetic  field  surrounding  the  oscillator,  and  this  grow- 
ing magnetic  field  produces  an  electromotive  force  in  the  conductor 
opposing  the  growth  of  the  current.  Time  is  thus  required  for  the 
current  to  become  established.  At  a  time  equal  to  one-fourth  the 
period  of  complete  oscillation,  t  =  Tr/4,  the  current  has  grown  to  its 
maximum  value,  which  is  represented  at  (&')•  The  shading  to  the 
right  of  the  conductor  is  meant  to  represent  the  magnitude  of  the 
current  at  each  point  of  the  conductor,  though  the  current  is  along 
the  conductor  and  not  perpendicular  to  it,  as  the  shading  is.  It  is 
interesting  to  note  that  the  current  is  not  uniform  throughout  the 
length  of  the  conductor.  The  current  is  greatest  near  the  middle 
of  the  conductor,  and  is  zero  at  each  end.  The  reason  that  it  is 
greatest  at  the  middle  is  that  the  current  flowing  out  front  the  center 
toward  either  end  decreases  by  reason  of  the  charge  that  it  leaves 
along  the  conductor  en  route.  At  the  very  end  of  the  conductor  the 
current  is  zero,  because  no  electricity  flows  out  beyond  the  end  and 
none  flows  in  from  beyond  the  end.  We  have  thus  in  the  conductor 
a  distribution  of  current  like  that  of  (6')  —  large  in  the  middle  and 
zero  at  both  ends. 

Thus,  at  the  time  t  =  T/£,  the  conductor  is  in  a  neutral  condition 
with  respect  to  charge,  but  is  being  traversed  by  a  current  in  a  down- 
ward direction.  This  current  is  a  maximum  with  respect  to  time, 
for  the  next  instant  the  positive  electricity  in  the  lower  rod  begins  to 
be  in  excess.  This  calls  into  play  an  opposing  electromotive  force, 
and  diminishes  the  current,  which,  however,  cannot  cease  at  once, 


NATURE  OF  THE  OSCILLATION  111 

because  any  diminution  of  the  current  diminishes  the  surrounding 
magnetic  field,  and  gives  an  electromotive  force  tending  to  preserve 
the  current.  The  current  thus  continues  to  pile  up  a  positive  charge 
on  the  lower  rod,  in  spite  of  the  fact  that  this  piled-up  charge  is 
exerting  a  restoring  force. 

Presently,  however,  this  restoring  force,  which  has  gone  on  increas- 
ing, brings  the  current  to  a  stop.  Then  when  there  is  no  current, 
there  is  no  magnetic  field,  and  the  accumulated  positive  electricity 
on  the  lower  rod  starts  the  current  upward.  This  reversal  of  the 
current  occurs  at  a  time  t  =  T/2;  and  the  condition  of  the  charge 
and  current  is  represented  at  (c)  and  (c7).  The  upward  current 
continues  to  flow,  and  produces  successively  the  conditions  (d)  and 
(d'),  at  t  =  3774,  and  (e)  and  (ef)  at  t  =  T. 

In  the  last  named  state  the  upper  rod  is  entirely  positive,  while 
the  lower  rod  is  entirely  negative.  This  resembles  the  initial 
state  of  the  rod,  but  is  not  identical  with  it,  because  the  initial 
state  was  brought  about  by  an  extraneous  slow  charging  source 
(Holtz  machine  or  Ruhmkorff  coil)  instead  of  by  the  very  rapid 
surging  that  is  going  on  in  the  oscillator  when  it  is  oscillating  with 
its  own  natural  period. 

From  the  condition  of  initial  uniform  distribution  we  have 
followed  the  charge  and  current,  by  rather  large  stages  of  a  quarter 
of  a  period  each,  through  a  single  oscillation.  The  charge  on  the 
conductor  will  continue  to  oscillate,  going  through  the  successive 
steps  several  times  —  the  accumulation  of  electricity  becoming 
less  and  less  at  each  oscillation  until  the  spark  extinguishes. 

Nodes  and  Loops  of  Potential  and  Current.  —  From  the  pre- 
ceding discussion  it  is  apparent  that  the  two  ends  of  the  Hertz 
oscillator  undergo  maximum  fluctuations  of  potential,  and  are, 
therefore,  loops  of  potential.  The  middle  of  the  conductor  during 
the  oscillation  has  no  accumulation  of  charge  on  it;  the  potential 
of  the  middle,  therefore,  never  rises  above  zero  (after  the  start), 
and  is  a  node  of  potential. 

On  the  contrary,  the  nodes  of  current  are  at  the  ends  of  the  oscil- 
lator, while  a  loop  of  current  is  at  the  middle  of  the  oscillator. 

EXPERIMENTS   ON  THE   DISTRIBUTION   OF  CURRENT  IN  AN 
UNGROUNDED    HERTZ    OSCILLATOR 

In  the  preceding  dicussion  there  was  given  a  theoretical  exami- 
nation of  the  nature  of  the  potential  and  the  current  distribution 
occurring  in  a  Hertz  oscillator.  I  have  recently  made  a  simple 


112 


WIRELESS  TELEGRAPHY 


experiment   that  approximately   confirms   the   deductions   there 
given  in  regard  to  the  current. 

The  Principle  of  the  Experiment  and  a  Description  of  the  Oscil- 
lator. —  The  principle  of  the  experiment  is  illustrated  in  Fig.  78. 
Instead  of  making  a  breach  at  various  points  in  the  oscillator  and 
inserting  therein  an  instrument  for  determining  the  current,  this 
current  at  different  points  in  the  oscillator  was  studied  by  means 
of  its  inductive  action  on  a  small  neighboring  circuit  WM  placed 
successively  at  different  positions  along  the  oscillator,  as  indicated 
by  the  dotted  squares  in  Fig.  78.  The  spark  gap  of  the  oscillator 
is  shown  at  G.  The  two  conductors  of  the  oscillator  00,  were 
each  a  wire  1  mm.  in  diameter  and  9  meters  long,  supported  hori- 
zontally 1  meter  above  the  wooden  floor  of  a  long  room  in  the  third 
story  of  the  laboratory.  The  oscillating  system  was  thus  at  a 
height  of  about  10  meters  above  the  surface  of  the  earth,  and  was 
probably  very  little  disturbed  by  the  capacity  of  the  earth.  The 
oscillator  was  supported  by  three  insulating  stands,  —  one  at  the 
spark  gap  and  one  at  each  end  of  the  wire.  The  central  stand 
for  supporting  the  spark  gap  carried  also  a  storage  battery  and 
a  small  Ruhmkorff  coil  for  charging  the  oscillator. 


]° 


FIG.  78.    Plan  of  apparatus  for  explor- 
ing current  distribution. 


FIG.  79.     Detail  of  exploring  circuit. 


The  Exploring  Circuit.  —  An  enlarged  view  of  the  apparatus, 
showing  details  of  the  exploring  circuit  by  which  the  measurements 
were  made,  is  given  in  Fig.  79.  This  circuit,  shown  at  the  left 
of  the  oscillator,  consists  of  a  square  loop  of  heavy  copper  wire, 


NATURE  OF  THE  OSCILLATION 


113 


L,  30  cm.  on  a  side,  and  having  in  series  with  it  a  variable  con- 
denser C  and  a  high-frequency  current-reading  instrument  at  7. 
I  shall  now  describe  the  instrument  /  and  the  condenser  C. 

Description  of  the  Instrument.  —  The  instrument  at  /  as  is 
shown  in  Fig.  80  consists  of  a  disc  of  silver,  suspended  by  a  fine 
fiber  of  spun  quartz  so  as  to  hang  near  a  small  coil  of  a  few 
turns  of  wire,  with  which  the  disc  made  an  angle  of  45  degrees. 
The  disc  is  at  M ,  and  the  coil,  which  in  this  experiment  consisted 
of  five  turns  of  wire  wound  on  a  vulcanite  tube,  is  shown  at  C, 
Fig.  80.  The  two  ends  of  the  coil  are  connected  to  binding  posts, 
by  which  the  coil  is  put  into  the  circuit. 
The  front  of  the  disc  carries  a  small 
mirror,  enabling  the  deflections  of  the 
disc  to  be  measured  by  means  of  a 
telescope  and  scale  such  as  is  used  with 
delicate  galvanometers. 

The  mounting  of  the  instrument  is  also 
shown  in  Fig.  80. 
The  disc  is  sus- 
pended in  the 
vertical  vulcan- 
ite tube,  which 
is  mounted  on 
leveling  screws; 
the  support  of 
the  coil  is  in- 

FIG.  80.  High-frequency  dynamometer.  Mounting  shown  serted  in  the  Side 
at  left,  suspension  at  right.  of    the    vertical 

tube,  and  is  arranged  to  be  moved  in  and  out  by  a  micrometer 
screw.  This  delicate  motion  of  the  coil  in  or  out  brings  the 
coil  nearer  to  or  farther  from  the  suspended  silver  disc  so  as  to 
vary  the  sensitiveness  of  the  instrument,  to  make  it  suitable  for 
measuring  small  or  large  oscillating  currents. 

The  action  of  the  instrument,  which  we  shall  call  a  "  high- 
frequency  dynamometer,"  is  as  follows:  oscillations  in  the  coil 
induce  oscillations  in  the  disc.  Between  these  two  sets  of  oscilla- 
tions there  is  a  force  which  causes  the  disc  to  tend  to  set  itself 
at  right  angles  to  the  coil.1  The  deflections  of  the  dynamometer 
are  proportional  to  the  square  of  the  current  through  it.2 

1  The  principle  of  this  instrument  was  independently  discovered  by  Dr. 
Elihu  Thomson  and  by  Professor  Fleming.  The  instrument  was  first  shown 


114  WIRELESS  TELEGRAPHY 

The  Variable  Condenser.  —  Returning  to  Fig.  79,  the  loop  L 
contains,  besides  the  high-frequency  dynamometer  /,  a  variable 
condenser  C.  This  condenser  is  of  a  form  much  used  in  wireless 
telegraphic  apparatus,  and  is  described  by  Korda  in  German 
Patent  No.  72447,  issued  Dec.  13,  1893.  It  consists  of  two  sets 
of  semicircular  plates,  —  one  set  F  being  connected  together  and 
fixed  in  position,  and  the  other  set  H  being  also  connected  together 


FIG.  81.    Korda  air  condenser. 

and  capable  of  rotation  about  a  central  axis.  By  rotating  the 
plates  H  so  as  to  bring  a  greater  or  less  area  of  the  two  sets  of 
plates  into  interlapping  position,  the  capacity  of  the  condenser 
can  be  varied.  The  position  of  the  plates  H  with  respect  to  F 
can  be  read  on  a  scale  attached  to  the  top  plate  of  H  and 
passing  under  a  fixed  pointer.  A  photograph  of  a  condenser  of 

to  be  applicable  to  the  measurement  of  oscillating  currents  of  high  frequency 
by  Messrs.  Northrup,  Pierce  and  Reichmann,  and  has  been  used  in  the  present 
improved  form  by  the  author  in  a  large  number  of  resonance  experiments, 
some  of  which  are  later  to  be  described  in  this  volume. 

2  A  theoretical  and  experimental  proof  of  this  proposition  is  given  by  the 
author  in  Phys.  Review,  Vol.  20,  p.  226,  1905. 


NATURE  OF  THE  OSCILLATION 


115 


this  character,  with  capacity  somewhat  larger  than  that  of   the 
condenser  employed  in  these  experiments,  is  shown  in  Fig.  81. 

Large  Current  at  Resonance.  —  Variations  of  the  capacity  of 
C  varies  the  natural  period  of  oscillation  of  the  condenser  circuit, 
and  when  this  period  is  made  equal  to  that  of  the  Hertz  oscillator 
OGO,  a  maximum  deflection  of  the  instrument  /  is  obtained,  under 
the  action  of  the  oscillation. 

The  resonant  condenser  circuit  when  calibrated  in  terms  of 
wave  length  is  a  form  of  "  wave  meter."  How  this  calibration 
is  effected  will  be  shown  later. 

Exploration  of  Current  Distribution.  —  Since  the  wave  meter 
in  this  form,  on  account  of  the  instrument  /,  is  not  conveniently 
movable,  it  was  necessary  to  move  the  oscil- 
lator in  order  to  explore  the  distribution  of 
current  in  the  oscillator.  The  oscillator,  with 
its  exciting  induction  coil  and  storage  bat- 
tery, was  moved  lengthwise,  keeping  it  al- 
ways the  same  distance  from  the  wave  meter, 
by  means  of  the  vulcanite  guides  DD  of 
Fig.  79.  Readings  of  the  dynamometer 
were  taken  for  various  positions  of  the 
oscillator  with  respect  to  the  wave  meter. 
This  was  equivalent  to  moving  the  wave 
meter  along  the  oscillator,  and  the  readings 
of  the  dynamometer  were  proportional  to 
the  square  of  the  current  in  the  wave  meter, 
and  therefore  proportional  to  the  square  of 
the  current  at  different  points  of  the  oscil- 
lator; because  the  induced  current,  keeping 
everything  else  the  same,  is  proportional  to 
the  inducing  current. 

The  results  obtained  for  the  distribution  2         e    8  10 

of  the  current  in  the  oscillator  are  plotted  Relative  current 

in  Fig.  82.     The  curve  of  Fig.  82  shows  that    FIG    82.      Distribution 

of    current    along   a 


y 
8 
7 
6 

!• 

I4 

02  3 

£2 

gi 

1° 

I1 

§2 

S3 
4 
5 
6 

7 
8 

*•»», 

, 

I 

N 

\. 

\ 

\ 

\ 

V 

\ 

\l 

~] 

2 

i 

/ 

1 

1 

/ 

/ 

/ 

,,'• 

/' 

1 

Hertz  oscillator,  as 
determined  by  ex- 
periment. 


the  current  in  the  oscillator  is  greatest  near 

the  gap  and  falls  off  to  zero  at  the  ends  of 

the  oscillator  in  a  manner  not  very  different 

from  that  shown  in  the  theoretical  drawings  of  Fig.  77.     There 

is  a  loop  of  current   in  the  middle  and  a  node  at  each  end  of 

the  oscillator. 


116 


WIRELESS  TELEGRAPHY 


EXPERIMENTS    ON    THE    WAVE     LENGTH    OF    THE    UNGROUNDED 
HERTZ     OSCILLATOR 

Wave  Length  of  the  Hertz  Oscillator.  —  Having  investigated 
the  distribution  of  current  and  potential  in  an  ungrounded  os- 
cillator, let  us  next  inquire  what  is  the  wave  length  of  the  electric 
wave  emitted  by  such  an  oscillator. 

With  an  oscillator  of  two  parallel  wires  near  together,  as  shown 
in  Fig.  83,  the  length  of  the  wave  is  very  approximately  four  times 
the  length  of  one  of  the  wires,  GBC.  If  now  we  take  the  two 
parallel  wires,  separate  them,  and  extend  them  out  oppositely  to 
each  other  from  the  gap  so  as  to  form  the  Hertz  oscillator,  does 
the  wave  length  remain  the  same;  namely,  X  =  4  I,  where  X  is 
the  wave  length  and  I  is  the  length  of  the  half-oscillator?  Some 
theoretical  writers  (for  example,  Abraham  x)  say  that  it  does 
remain  equal  to  4  Z;  while  Macdonald  2  has  computed  X  in  this 
case  to  be  5.06  X  I. 

Recently  Messrs.  Webb  and  Woodman,3  for  very  short  oscilla- 
tors, with  a  half  length  I  between  1  and  5  cm.,  have  obtained 
experimentally  the  relation  X  =  4.8  I. 

For  oscillators  of  half  length  between  1  and  3  meters,  F.  Con- 
rad 4  has  obtained  the  values  presented  in  the  accompanying  table, 
with  the  average  relation  X  =  4.24  I. 

CONRAD'S  TABL'E  FOR  RELATION  OF  X  TO  I 


I 

Half  length  of 
oscillator  in 
meters. 

X 

Wave  length  in 
meters. 

X 
j 

1.00 

4.20 

4.20 

1.92 

8.00 

4.17 

2.00 

8.40 

4.20 

2.75 

12.0 

4.37 

3.15 

13.4 

4.25 

Average            4  .  24 

The  experiments  of  Conrad  and  those  of  Messrs.  Webb  and 
Woodman  both  give  evidence  of  being  very  careful  experiments, 

1  M.  Abraham,  Wied.  Ann.,  Vol.  66,  p.  435,  1898. 

2  Macdonald,  Electric  Waves,  p.  111. 

3  Webb  and  Woodman,  Phys.  Review,  Vol.  29,  p.  89,  1909. 

4  F.  Conrad,  Drude's  Ann.,  Vol.  22,  p.  670,  1907. 


NATURE  OF  THE  OSCILLATION 


117 


and  we  must  therefore  conclude  that  the  ratio  of  \/l  for  very  short 
oscillators  is  greater  than  for  the  long  oscillator. 

We  are  primarily  interested  in  the  long  oscillators,  and  in  order  to 
extend  the  experimental  records  to  the  case  of  longer  oscillators 
than  those  studied  by  Conrad,  I  have  made  a 
series  of  measurements  with  the  apparatus  of 
Figs.  78,  79,  80. 

Calibration  of  the  Wave  Meter.  —  The  wave 
meter  was  calibrated  for  various  adjustments 
of  the  condenser  C  by  setting  it  to  resonance 
with  various  lengths  of  the  two  parallel  wires 
of  Fig.  83,  as  had  been  previously  done  by 
Drude.  With  the  wave  meter  calibrated  to  read 
directly  in  wave  lengths,  the  parallel  calibrating 
wires  were  removed,  and  the  Hertz  oscillator, 
consisting  of  two  oppositely  extending  wires  FIG.  83.  Parallel- 
of  various  lengths  (1  mm.  in  diameter),  was 
brought  up  near  the  wave  meter,  and  the  wave 
length  produced  by  the  oscillator  was  deter- 
mined. The  results  obtained  are  given  in  the  following  table: 

AUTHOR'S  TABLE  OF  RESULTS  FOR  RELATION  OF  X  TO  I 


wire  oscillator  for 
calibrating  wave- 
meter  for  short 
wave-lengths. 


I 

Half  length  of 
oscillator  in 
meters. 

X 

Wave  length  in 
meters. 

X 

I 

4.0 

16.9 

4.22 

4.5 

18.9 

4.20 

5. 

21.2 

4.23 

5.5 

23.2 

4.22 

6. 

24.9 

4.15 

7. 

29.5 

4.21 

8. 

33.6 

4.20 

9. 

38.7 

4.23 

10. 

41.6 

4.16 

11. 

46.1 

4.22 

12. 

49.5 

4.13 

13. 

53.9 

4.14 

14. 

57.5 

4.11 

15. 

63.0 

4.19 

Average               4.19 

The  average  of  the  results  obtained  by  the  author  for  the  ratio 
of  X  to  I,  namely,  \/l  =  4.19,  for  wave  lengths  between  17  and 
63  meters,  is  a  little  less  than  the  corresponding  ratio,  4.24,  ob- 


118  WIRELESS  TELEGRAPHY 

tained  by  Conrad  for  wave  lengths  between  4  and  13  meters. 
The  difference  is  only  1%. 

From  these  results  we  may  conclude  that  the  wave  length 
produced  by  a  Hertz  rectilinear  oscillator  is  very  approximately 
4.20  times  the  length  of  one  limb  of  the  oscillator,  provided  this 
limb  is  greater  than  1  meter  long  and  of  comparatively  small 
diameter. 

Let  us  next  see  how  the  vibration  of  a  conductor  is  modified 
when  one  end  is  connected  to  earth. 

ON    GROUNDED    CIRCUITS 

Grounded  Circuits.  Image  Theory.  —  Suppose  now  the  lower 
limb  of  a  vertical  Hertz  oscillator  to  be  cut  away  close  up  to  the 
spark  gap  and  be  replaced  by  a  connection  to  earth.  According 
to  electrical  theories,  if  the  earth  were  a  perfect  conductor,  the 
electrical  wave  length  of  the  earthed  system  would  be  the  same 
as  that  of  the  Hertz  oscillator, —  the  earth  merely  taking  the  place 
of  the  other  half  of  the  Hertz  oscillator.  The  earthed  system, 
which  is  a  simple  Marconi  emitter,  would  have  the  same  distri- 
bution of  current  and  potential  in  the  antenna  as  the  upper  half 
of  the  Hertz  oscillater  had  before  removing  the  lower  limb. 

In  order  to  examine  this  theory,  let  us  confine  our  attention 
in  the  beginning  to  a  receiving  station,  and  suppose  that  we  have 
there  simply  an  ungrounded  rectilinear  conductor  isolated  in  space, 
and  placed  parallel  to  the  electric  force  of  the  incoming  waves. 
Let  the  length  of  this  straight-line  conductor  be  so  chosen  that  its 
natural  period  of  electric  oscillation  is  equal  to  the  period  of  the 
waves.  The  distribution  of  current  in  the  conductor  would  re- 
semble that  shown  in  Fig.  82. 

It  we  could  introduce  a  current  reading  detector  into  the  circuit 
without  disturbing  the  conditions,  the  instrument  would  give  a 
maximum  reading  when  placed  at  the  center  of  the  receiving 
conductor;  this  is  the  point  at  which  the  fluctuation  of  potential 
is  zero. 

Suppose  with  such  an  instrument  in  the  circuit  we  should  cut 
away  the  lower  half  of  the  conductor;  the  reading  would  become 
zero,  because  there  would  be  no  capacity  out  beyond  the  instru- 
ment into  which  the  current  could  flow.  If  now  a  capacity  is 
attached  to  the  instrument  in  the  place  of  the  removed  conductor, 
some  current  would  flow  between  the  straight  wire  and  the  capacity 
and  register  in  the  instrument. 


NATURE  OF  THE  OSCILLATION 


119 


If  the  capacity  attached  were  very  large  (e.g.  the  earth),  the 
point  of  zero  fluctuation  of  potential  would  again  be  brought  near 
the  instrument,  because  a  large  fluctuation  of  potential  cannot 
occur  in  a  very  large  capacity  under  the  action  of  the  currents 
with  which  we  are  concerned.  We  should,  therefore,  have  the 
same  current  as  when  the  conductor  was  made  up  of  two  parts 
symmetrical  about  the  instrument. 

In  actual  systems,  the  grounding  may  be  imperfect.  In  that 
case  the  symmetrical  image  would  give  only  approximately  an 
equivalent  system. 

I  have  made  some  experiments  to  test  this  image  theory  of  the 
action  of  the  ground  connection.  The  experiments  consisted  in 
comparing  resonance  curves  taken  with  various  forms  of  grounded 
circuits  with  the  corresponding  resonance  curves  taken  with  an 
image  circuit  in  the  place  of  the  ground.  Two  of  these  experi- 
ments are  here  briefly  described. 


EXPERIMENTS  TO  TEST  IMAGE  THEORY  OF  THE  GROUND 

Experiment  I.  The  Aerial  Circuit  and  its  Image  Tuned  by 
Variable  Inductances.  —  In  testing  the  image  theory  of  the  action 
of  the  ground  at  the  receiving  sta- 
tion the  form  of  circuit  shown  in 


t 

-in  mm 


FIG.  84.  Circuit  employed  in 
study  of  the  image  theory  of 
the  ground. 


1234567 
Inductance  x!0'5Henry 

FIG.  85.  Resonance  curves  in  study  of  the 
image  theory  of  the  ground.  Curve  H 
was  obtained  with  horizontal  duplicate 
of  antenna;  curve  G,  with  ground. 


Fig.  84  was  employed.  The  high-frequency  dynamometer  de- 
scribed on  p.  113  was  used  for  detecting  and  measuring  the  minute 
oscillating  currents  at  the  receiving  station,  and  was  placed  at  / 


120  WIRELESS  TELEGRAPHY 

in  series  with  a  variable  inductance  and  a  vertical  antenna  23.2 
meters  long.  This  aerial  system,  by  means  of  a  switch  at  S,  could 
be  connected  to  the  ground  G,  or  the  ground  could  be  thrown  off 
and  replaced  by  metallic  parts  duplicating  the  aerial  system. 
The  duplicate  was,  however,  not  an  exact  theoretical  image  of 
the  aerial  system,  because  the  second  antenna  had  to  run  off 
horizontally  instead  of  straight  down. 

The  horizontal  wire  was  made  equal  in  length  to  the  vertical 
antenna,  23.2  meters,  and  was  supported  about  1  meter  from  the 
ground  by  cords  attached  to  posts.  In  series  with  the  horizontal 
wire  was  a  variable  inductance  Lf  duplicating  the  tuning  coil  L, 
and  a  small  coil  /'  of  fine  wire  duplicating  the  coil  of  the  receiving 
instrument. 

Curves  giving  the  results  of  the  experiment  are  shown  in  Fig.  85. 
For  curve  G  the  grounded  circuit  was  used,  and  deflections  of  the 
receiving  instrument  were  taken  for  various  values  of  the  inductance 
of  the  tuning  coil  L;  the  deflections  are  plotted  against  values  of  L. 

The  switch  S  was  then  thrown  so  as  to  connect  the  receiving 
circuit  to  the  horizontal  system  instead  of  to  the  ground.  With 
this  arrangement  the  curve  H  was  obtained.  In  taking  this  curve, 
the  tuning  coil  L  and  its  image  L'  were  kept  identical  and  varied 
together.  The  curve  H,  therefore,  shows  the  deflections  of  the  re- 
ceiving instrument  plotted  against  the  common  values  of  L  and  L'. 

Discussion  of  Results  'in  Experiment  I.  —  The  two  curves  G 
and  H  of  Fig.  85  are  seen  to  have  their  maxima  for  the  same 
value  of  inductance.  That  is,  a  given  value  of  inductance,  2.1  X 
10  ~5  henries,  gives  a  maximum  deflection  in  the  case  of  the 
grounded  circuit.  To  obtain  a  maximum  with  the  duplicated 
system  the  same  inductance  2.1  X  10 ~5  henries  must  be  used  in 
both  the  vertical  circuit  and  in  its  horizontal  duplicate.  The  result 
is  a  confirmation  of  the  image  theory  of  the  grounded  circuit. 
The  earthing  of  the  circuit  gives  it  the  same  period  of  vibration  as 
the  duplication  of  the  aerial  system  gives. 

It  is  interesting  to  note  that  the  deflection  (current  square)  is  about 
20%  larger  with  the  duplicated  system  than  with  the  grounded  system 
—  a  fact  that  may  be  accounted  for  by  supposing  a  higher  resist- 
ance with  the  grounded  system  than  in  the  wholly  metallic  system. 

Some  other  facts  in  regard  to  the  experiment  are  discussed  in 
the  original  publication.1 

1  G.  W.  Pierce:  Resonance  in  Wireless  Telegraph  Circuits.  Part  IV,  Physi- 
cal Review,  Vol.  22,  p.  174,  1906. 


NATURE  OF  THE  OSCILLATION 


121 


The  curve  F,  with  which  we  are  not  here  concerned,  was  obtained 
with  the  duplicate  antenna  wound  around  the  house  of  the  receiv- 
ing station. 

Experiment  II.  Quarter- Wave  Ground.  —  What  was  perhaps 
a  more  interesting  experiment  confirmatory  of  the  image  theory  of 
the  ground  was  made  by  replacing  the  ground  by  a  horizontal  wire 
of  which  the  length  could  be  varied.  The  relative  amounts  of 
energy  received  (deflections)  for  different  lengths  of  the  horizontal 
wire  are  shown  in  the  curve  A  of  Fig.  86.  Resonance  was  obtained 
when  this  wire  had  the  length  of  38 
meters,  which  was  very  close  to  one- 
fourth  the  wave  length  (155  meters). 
The  ground  gives  the  system  the 
same  period  as  an  added  quarter- 
wave  wire  gives  the  system.  Curve 
B  obtained  with  different  conditions 
leads  to  the  same  results. 

Conclusion  from  the  Experiments. 
—  These  experiments  I  and  II  show 
that  the  effect  of  the  ground,  so  far 
as  concerns  the  vibration  in  the  an- 
tenna, is  to  introduce  into  the  circuit 
at  the  ground  a  point  of  zero  fluctu- 
ation of  potential,  —  an  effect  that 
can  also  be  obtained  with  an  arti- 
ficial ground  consisting  of  a  sym- 
metrical duplicate  of  the  aerial 
system  or  consisting  of  a  horizontal 
wire  not  far  from  the  earth  and  of 
length  equal  to  one-quarter  of  the  wave  length  to  be  received. 

Professor  Ferdinand  Braun  at  a  date  earlier  than  that  of  my 
experiments  has  suggested  the  use  of  horizontal  wire  in  replace- 
ment of  the  ground  and  also  the  use  of  a  capacity  consisting  of  a 
large  cylindrical  conductor  in  the  place  of  the  ground.  He  has 
not,  however,  so  far  as  I  know  published  any  quantitative  results 
on  the  subject. 


30  40  50 

Length  of  Horizontal      Meters 

FIG.  86.  Showing  that  the 
ground  may  be  replaced  by  a 
quarter-wave  wire. 


CHAPTER  XV 


PROPAGATION    OVER    THE    EARTH 

The  Propagation  of  the  Waves  over  the  Surface  of  the  Earth.  — 

In  the  preceding  chapter  it  was  shown  that  so  far  as  concerns  the 
wave  length  and  the  distribution  of  current  and  potential,  the 
grounding  of  an  antenna  was  equivalent  to  attaching  it  to  its 
image. 

Let  us  discuss  further  this  idea  with  reference  now  to  the  man- 
ner of  propagation  of  electric  waves  over  the  surface  of  the  earth. 
Does  the  earth  contribute  to  the  propagation  of  the  waves  in  an 
advantageous  or  only  in  a  detrimental  way  ? 

We  shall  begin  this  discussion  with  the  assumption  that  the 
earth  is  a  perfect  conductor.  With  this  assumption  we  can  apply 
to  the  problem  further  reasoning  based  on  Sir  William  Thomson's 
theory  of  electrical  images. 

Theory  of  Images.  —  Suppose  we  have  two  small  bodies  A  and 
B  with  equal  charges  of  electricity  of  opposite  signs.  The  direc- 
tion of  the  electric  force  between  A  and  B  is  represented  by  the 

curved  lines  in  Fig.  87. 
A  plane  P  drawn  every- 
where equally  distant  from 
A  and  B  will  be  a  surface  of 
zero  potential. 

The  proof  that  P  is  a 
surface  of  zero  potential  is 
as  follows:  The  potential 
of  a  point  is  the  work  re- 
quired to  be  done  in  order 
to  bring  a  unit  positive 
charge  up  to  the  point  from  an  infinite  distance.  Now  a  unit 
charge  of  positive  electricity  can  be  brought  up  to  any  point 
of  the  plane  P  without  doing  any  work;  because  the  force  at  any 
point  of  the  plane,  being  made  up  of  an  attraction  F  due  to  B  and 
an  equal  repulsion  Ff  due  to  A,  exerts  a  force  perpendicular  to  the 
plane,  but  no  force  along  the  plane.  The  force  is  therefore  per- 

122 


FIG.  87.  Lines  of  electric  force  between 
two  oppositely  charged  bodies  A  and  B. 
The  plane  P  is  at  zero  potential. 


PROPAGATION  OVER  THE  EARTH         123 

pendicular  to  the  direction  of  motion  when  the  charge  is  brought 
up  along  the  plane,  and  the  work  done  is  therefore  zero.  For 
further  details  in  regard  to  work  and  potential  see  Appendix  I. 

Having  shown  that  the  plane  P  is  everywhere  at  zero  potential, 
let  us  next  introduce  the  idea  well  established  in  treatises  on  elec- 
tricity, that  so  long  as  we  keep  the  potential  of  the  plane  P  equal 
to  zero  the  electric  force  in  the  region  between  A  and  the  plane  P 
is  completely  fixed,  no  matter  what  changes  we  may  introduce 
below  the  plane.  If,  then,  the  lower  half  of  the  diagram  is  removed 
and  the  plane  is  in  some  other  way  kept  at  zero  potential,  the 
electric  force  between  A  and  the  plane  will  be  the  same  as  before; 
namely,  that  represented  in  Fig.  88,  which  is  the  upper  half  of 


zot.   o 


FIG.  88.  Lines  of  electric  force  between  a 
charged  body  A  and  an  infinite  conducting 
plane  kept  at  zero  potential. 

Fig.  87.  We  may  keep  the  plane  at  zero  potential  by  grounding 
it  so  that  it  comes  into  coincidence  with  the  surface  of  the  earth; 
or  the  surface  of  the  earth  itself  may  take  the  place  of  the  plane, 
provided  the  earth  for  a  considerable  area  around  the  charged  body 
A  is  a  good  conductor. 

That  is  to  say,  if  the  earth's  surface  is  a  good  conducting  plane 
for  a  considerable  extent,  and  a  charged  body  A  be  placed  above 
the  surface  of  the  earth,  the  field  of  electric  force  between  A  and 
the  plane  surface  of  the  earth  will  be  the  same  as  the  upper  half 
of  the  field  between  A  and  a  body  B,  which  has  a  charge  equal  to 
A  and  opposite  in  sign,  —  B  being  at  the  distance  below  the  plane 
that  A  is  above  it.  This  equal  opposite  charge  symmetrically 
placed  in  regard  to  the  plane  is  called  the  electrical  image  of  A  in 
the  plane. 

Similar  Theory  Applied  to  the  Oscillator.  —  If  we  next  consider 
the  case  of  the  electric  oscillator,  the  field  of  electric  force  for  the 
symmetrical  oscillator,  as  we  have  seen  in  Chapter  VIII,  is  roughly 
that  represented  in  Fig.  89.  The  ideal,  nonmaterial  plane  PP 
through  the  figure  is  at  zero  potential,  so  that  the  lower  half  of 
the  diagram  could  be  replaced  by  the  surface  of  the  earth,  if  it 
were  plane  and  perfectly  conductive,  without  disturbing  the  upper 


124 


WIRELESS  TELEGRAPHY 


half  of  the  figure.  Whence  it  follows  that  the  oscillation  and  radia- 
tion from  an  oscillator  grounded  to  an  infinite,  plane,  perfect  conductor 
is  the  same  as  the  oscillation  and  radiation  of  the  upper  half  of  a 
symmetrical  Hertz  oscillator.  The  nature  of  the  wave  sent  out 
from  an  oscillator  so  grounded  is  represented  in  Fig.  90. 


FIG.  89.     Lines  of  electric  force  about  a  Hertz  oscillator. 


FIG.  90. 


Lines  of  electric  force  about  a  half-oscillator  discharging 
to  a  perfectly  conductive  ground. 


Guided  Electric  Waves.  —  Figure  90  shows  approximately  the 
theoretical  mode  of  propagation  of  the  electric  wave  over  those 
parts  of  the  surface  of  the  earth  where  the  earth  is  a  good  conduc- 
tor. The  loops  there  shown  receding  from  the  oscillator  are  lines 
of  electric  force.  Now  a  line  of  electric  force  must  be  either  a 
closed  line,  as  in  Fig.  89,  or  must  terminate  at  one  end  on  a  positive 
charge  and  at  the  other  end  on  an  equal  negative  charge.  There 
is,  therefore,  a  series  of  successive  positive  and  negative  charges 
induced  in  the  surface  of  the  earth,  and  these  positive  and  negative 
charges  move  with  the  wave  with  the  velocity  of  light.  The  earth 
thus  serves  as  a  guiding  conductor  and  causes  the  loops  of  electric 
force  terminating  on  it  to  follow  the  surface  of  the  earth.  This 
accounts  for  the  fact  that  communication  is  possible  between  sta- 
tions which  on  account  of  the  intervening  curvature  of  the  earth 


PROPAGATION  OVER  THE  EARTH         125 

are  not  visible  from  each  other.  We  have  here  a  simple  view  of 
the  matter,  obtained  on  the  assumption  that  the  earth  is  a  perfect 
conductor. 

The  Earth  not  a  Perfect  Conductor.  —  The  surface  of  the  earth 
is,  however,  not  everywhere  a  good  conductor  of  electricity.  The 
sea  and  moist  soil  are  better  conductors  than  dry  stone.  In  some 
places  the  surface  materials  of  the  earth  are  in  fact  good  insulators. 

The  attenuation  of  the  electric  wave  is  on  this  account  very 
different  over  different  parts  of  the  surface  of  the  earth,  —  condi- 
tioned on  the  fact  that  there  is  a  greater  or  less  penetration  into 
the  insulating  portions  and  a  greater  or  less  absorption  of  energy 
at  the  poorly  conducting  portions.  This  subject  has  been  sub- 
mitted to  a  very  remarkable  mathematical  treatment  by  Dr.  Zen- 
neck.  The  mathematical  reader  is  referred  to  Dr.  Zenneck's 
paper  1  or  to  Professor  Fleming's  2  translation  and  "  free  para- 
phrase "  of  it,  for  a  beautiful  discussion  of  this  interesting  question. 
I  shall  attempt  to  give  here  a  brief  statement  of  some  of  Dr. 
Zenneck's  results  without  attempting  to  present  his  reasoning.  In 
doing  this  I  wish  to  acknowledge  the  assistance  afforded  by  Pro- 
fessor Fleming's  excellent  commentary  on  Zenneck's  paper. 

In  order  to  simplify  the  matter,  Dr.  Zenneck  at'  first  considers 
only  the  case  of  a  plane  electric  wave  traveling  without  divergence 
over  a  flat  surface.  He  is  thus  at  first  leaving  out  of  account  the 
spreading  out  of  the  wave  and  the  consequent  diminution  of  ampli- 
tude by  mere  distance;  and  he  is  also  omitting  the  attenuation  of 
the  wave  due  to  the  curvature  of  the  surface. 

Instead  of  considering  the  earth  to  be  a  perfect  conductor,  as 
has  usually  been  done  before,  Zenneck  looks  upon  the  boundary 
between  the  earth  and  the  air  as  the  boundary  between  two  media 
of  different  conductivities  and  different  dielectric  constants;  and 
he  transforms  Maxwell's  equations  so  as  to  take  account  of  the 
two  media. 

He  arrives  at  the  conclusion  that  where  the  earth  is  a  good  con- 
ductor (for  example,  sea  water),  the  electric  force  (at  the  surface) 
is  perpendicular  to  the  surface.  For  waves  of  wave  length  600 
meters,  which  is  the  wave  length  used  in  most  of  the  calculations, 
sea  water  acts  as  a  good  conductor,  and  the  electric  force  at  the 
surface  of  the  sea  is  perpendicular  to  the  surface,  as  is  shown  in 

1  J.  Zenneck:  Annalen  der  Physik,  Vol.  23,  1907. 

2  Fleming:  Engineering  (London),  June  4  and  11,  1909. 


126 


WIRELESS  TELEGRAPHY 


diagram  (a),  Fig.  91.  This  figure  represents  merely  how  one  side 
of  one  loop  of  electric  force  of  our  Fig.  90  comes  down  to  the  sur- 
face. The  other  side  of  the  loop  would  likewise  be  perpendicular 
to  the  surface  of  the  sea,  but  would  have  an  opposite  sign. 

There  would  thus  arrive  at  a  station  at  sea  a  train  of  electric 
waves,  and  the  electric  force  would  be  vertical,  and  would  go 
through  a  series  of  continuous  oscillations  between  positive  and 
negative  values,  with  the  frequency  of  the  waves;  that  is,  a  train 


Air 


FIG.  91.     Diagrams  taken  from  Professor  Fleming's  paper  in  the 
Electrician,  illustrating  Dr.  Zenneck's  Theory. 


FIG.  92.     Diagram  of  the  electric  force  in  a  wave  train. 

like  that  of  Fig.  92  would  come  along  near  the  surface  in  the 
medium  above  the  surface,  and  would  affect  the  antenna  first  in 
one  direction  and  then  in  the  opposite  direction.  In  the  sea 
water  itself  beneath  the  surface,  the  forces  would  be  zero. 

Still  confining  our  attention  to  a  wave  of  wave  length  600  meters, 
let  us  next  suppose  the  wave  to  be  traveling  along  a  surface  of 
dry  rock  of  resistance  100,000  ohms  for  a  meter  cube  and  of  dielec- 
tric constant  k  =  2  to  3.  Zenneck  finds  for  this  case  that  the 
electric  force  in  the  air  above  the  earth  is  by  no  means  perpendicu- 
lar, but  leans  forward  in  the  direction  of  travel;  and  that  not 
only  the  magnitude  of  the  force  changes,  but  the  inclination  also 
changes  as  the  wave  progresses.  There  is  a  similar  force, 
although  differently  inclined  and  of  different  magnitude,  below 
the  surface  in  the  rock  itself.  This  condition  Zenneck  finds  to  be 
represented  by  the  semiellipses  of  (6),  Fig.  91.  The  electric  force 
is  obtained  in  magnitude  and  direction  from  this  diagram  (6)  by 


PROPAGATION  OVER  THE  EARTH 


127 


considering  a  radius  drawn  from  the  center  of  the  ellipse  to  a 
particle  moving  around  the  ellipse  with  the  frequency  of  the  wave. 
The  length  and  the  direction  of  the  radius  so  drawn  would  repre- 
sent the  changing  magnitude  and  direction  of  the  electric  force. 
Such  an  electric  wave,  oscillating  both  in  magnitude  and  direction 
is  equivalent  to  two  waves,  one  tending  to  produce  vertical  cur- 
rents and  the  other  tending  to  produce  horizontal  currents  (the 
two  effects  being  also  out  of  phase  with  each  other).  The  hori- 
zontal oscillating  force  induces  currents  in  the  earth's  surface,  and 
diminishes  the  energy  of  the  progressing  wave,  so  that  in  this  case 
the  distance  to  which  signals  can  be  sent  is  less  than  in  the  case  of 
the  good  conductor. 

In  the  case  of  propagation  over  very  dry  soil,  which  is  not  so  good 
an  insulator  as  the  rock  (r  =  10,000  ohms  per  meter  cube,  k  =  1 
to  3)  Zenneck  finds  the  result  represented  in  diagram  (c),  Fig.  91. 


X=Distance  in  Kilometeres  at  which  the  "Wave  .Amplitu 
is  reduced  to  0.367  =*ye  of  .Amplitudejlt  Origi 

_p  H-  o  8  o  S  •< 

\ 

/ 

M  10  01  g  g  g 

BielectEic  Constant 

\ 

~7 

\ 

/ 

/ 

X 

\ 

^^    ^s 

^^         ^ 

k 

'</, 

/s 

/ 

' 

)1          1           10         100        1000      10,000    100 
Sea      Eresh        Bamp              Dry 
Water    Water          Seal                Soil 

Sneoifin  Rpsist.anop  in  Ohms  ner  Mete 

000  1,000,000 
Insulators 
rCuhe 

FIG.  93.     Curves  taken  from  Professor  Fleming's  commentary  on  Zenneck's 
theory,  from  the  Electrician. 

Although  the  conductivity  in  this  case  is  between  that  of  (a)  and 
(6),  the  form  of  the  ellipses  is  not  intermediate  between  (a)  and  (6). 
The  relation  is  not  a  simple  one,  involving  resistance  alone ;  because, 
in  fact,  a  perfect  conductor  and  a  perfect  insulator  give  in  the  region 
above  the  surface  the  same  form  of  unabsorbed,  vertical  wave;  and 
there  is  an  intermediate  case  of  conductivity  and  dielectric  con- 


128  WIRELESS  TELEGRAPHY 

stant  that  gives  the  most  distorted  and  most  absorbed  wave. 
Equations  are  given  by  Zenneck  for  computing  any  particular  case, 
and  he  presents  the  result  in  the  form  of  a  set  of  interesting  curves. 

Let  us  examine  first  his  result  for  the  loss  in  intensity  due  to 
absorption  of  energy  by  the  surface  as  the  wave  travels  along  over 
it.  For  a  wave  of  600  meters  wave  length,  this  is  shown  in  the 
curves  of  Fig.  93,  which  is  Zenneck's  diagram  with  added  verbal 
margins  by  Professor  Fleming. 

In  examining  Fig.  93,  it  should  be  borne  in  mind  that  this  dia- 
gram takes  account  of  the  reduction  of  intensity  by  the  action  of 
the  underlying  medium  alone,  and  shows  nothing  in  regard  to  the 
law  of  the  diminution  of  amplitude  of  the  wave  by  its  spreading 
out  in  all  directions.  It  is  seen  from  the  diagram  that  except  in 
the  case  of  fairly  good  conductors,  both  the  resistance  and  the 
dielectric  constant  of  the  body,  over  which  the  wave  travels,  need 
to  be  taken  into  account  and  that  insulators  produce  nearly  the 
same  attenuation  as  conductors,  and  that  the  worst  surface  over 
which  to  send  the  waves  is  dry  soil  of  small  dielectric  constant. 
The  best  surface,  so  far  as  concerns  absorption  alone,  is  either  a 
good  conductor  or  a  good  insulator. 

But  this  absorption  alone  is  not  all  that  is  to  be  reckoned  with. 
To  get  complete  information  as  to  the  propagation  it  is  necessary 
to  take  into  account 

(1)  The  effect  of  the  curvature  of  the  earth,  and 

(2)  The  effect  of  the  spreading  of  the  wave  with  the  distance 
(divergence) . 

Effect  of  Curvature  of  Earth.  —  Although  Zenneck's  mathemati- 
cal discussion  does  not  take  into  account  the  curvature  of  the 
earth,  he  makes  the  following  important  observation  in  regard  to 
the  action  of  the  curvature:  "  For  a  good  conducting  earth's  sur- 
face with  not  too  small  a  dielectric  constant  (for  example,  sea 
water)  it  is  highly  probable  that  the  curvature  of  the  earth  does 
not  materially  modify  the  conditions.  Since  sea  water,  for  the 
waves  of  wireless  telegraphy,  behaves  in  all  essential  points  like  a 
metal,  it  must  be  assumed  that  the  waves  use  the  sea  water  surface 
as  guides  in  the  way  that  waves  on  wires  are  guided  by  the  wires 
of  a  Lecher  system,  and  like  these  follow  the  curvature  of  the 
conductor." 

For  poorly  conducting  earth,  the  curvature  plays  a  more  detri- 
mental role,  and  for  a  good  insulating  surface  of  small  dielectric 
constant  it  is  certain  that  the  waves  would  be  like  those  in  free 


PROPAGATION  OVER  THE  EARTH         129 

space,  and  would  not  be  constrained  at  all  to  follow  the  curvature 
of  the  surface. 

From  this  it  is  clear  that  for  the  easy  transmission  of  the  electric 
waves  between  stations  sufficiently  separated  to  have  a  large  por- 
tion of  the  earth's  curved  surface  between,  what  is  required  is  a 
good  conducting  and  not  an  insulating  expanse  for  the  waves  to 
travel  over.  In  the  succeeding  sections  we  shall  compare  the  dis- 
tance of  transmission  over  poor  conductors  with  that  over  a  good 
conducting  expanse.  To  do  this  we  must  take  into  account  the 
divergence  of  the  waves  with  distance  to  see  whether  or  not  the 
absorption  is  important  in  any  particular  case. 

Diminution  of  Amplitude  by  Divergence  with  Distance.  —  On 
account  of  the  divergence  of  the  waves  from  the  sending  station, 
the  amplitude  of  the  electric  force  in  the  wave  is  approximately 
inversely  proportional  to  the  distance  from  the  oscillator,  provided 
there  is  no  absorption  and  provided  the  distance  is  not  too  small. 
This  has  been  shown  theoretically  to  be  true  in  the  case  of  the 
propagation  of  the  waves  in  free  space.  This  law  has  also  been 
approximately  verified  for  wireless  telegraph  waves  traveling  over 
sea  water  for  distances  up  to  60  miles,  in  a  very  beautiful  set 
of  experiments  performed  on  the  Irish  Channel  by  Messrs.  W. 
Duddell  and  J.  E.  Taylor.1 

Messrs.  Duddell  and  Taylor's  experiments  consisted  in  receiving 
and  measuring  the  current  set  up  in  the  antenna  of  a  shore  station 
by  electric  waves  sent  out  from  the  British  telegraph  repair  ship 
Monarch,  while  the  ship  was  at  various  distances  from  the  receiving 
station.  The  very  minute  currents  received  were  measured  by 
Duddell's  thermogalvanometer,  of  which  the  following  is  a  brief 
description : 

The  thermogalvanometer  invented  by  Mr.  W.  Duddell2  is  in 
principle  the  Radiomicrometer  of  Professor  C.  V.  Boys,  with  a 
modification  required  to  adapt  it  to  measuring  oscillatory  electric 
currents  instead  of  heat  radiation,  for  which  Boys'  instrument  was 
designed.  A  diagram  of  the  essential  parts  of  the  instrument  is 
shown  in  Fig.  94.  Between  the  poles  NS  of  a  strong  permanent 
magnet  is  hung  a  small  loop  of  one  turn  of  wire  L,  by  means  of  a 
very  fine  quartz  fiber  F.  The  loop  is  closed  below  by  a  thermal 
junction  of  bismuth  Bi  and  antimony  Sb.  Heat  applied  in  any 

1  Duddell  and  Taylor:  Journal  of  the  Institution  of  Electrical  Engineers, 
Vol.  35,  pp.  321-352,  1905. 
2  W.  Duddell:  Phil.  Mag.,  Vol.  8,  p.  91,  1904. 


130 


WIRELESS  TELEGRAPHY 


way  to  the  thermal  junction  produces  an  electric  current  in  the 
loop,  which  being  in  a  magnetic  field  tends  to  rotate  so  as  to  be  at 
right  angles  to  the  field.  A  diminutive  mirror  M  fastened  to  a 
vertical  glass  rod  at  the  top  of  the  loop  and 
rotating  with  the  loop  permits  the  deflections 
of  the  loop  to  be  read  by  means  of  a  tele- 
scope and  scale.  This  part  of  the  appara- 
tus is  the  radiomicrometer  of  Professor 
Boys,  and  was  used  by  Boys  to  measure 
small  quantities  of  radiant  heat,  which  was 
allowed  to  fall  on  the  thermal  junction. 
Professor  Boys  estimated  that  with  a  lens 
18  inches  in  diameter  for  concentrating 
the  radiant  heat  upon  the  thermal  junction, 
he  could  measure  the  heat  received  from  a 
candle  three  miles  away.  Mr.  DuddelPs 
very  ingenious  modification  of  this  delicate 
instrument  so  as  to  adapt  it  to  the  meas- 
urement of  oscillatory  electric  currents, 

FIG.  94.    Duddell  ther-  consisted  in  placing,  in  the  case  of  the  sus- 
mogalvanometer. 

pended  system  and  very  near  to  the  ther- 
mal junction,  a  "  heater  "  of  fine  wire,  as  shown  in  the  figure. 
Electric  oscillations  conducted  through  this  "  heater  "  heated  it, 
and  a  part  of  the  heat  so  produced  was  communicated  by  radia- 


Heater 


Microamperes 
Received  X  Miles 

' 

V 

(' 

1     ••'    —. 

—      — 

•         — 

1 

12     16     20     24     28      32     36     40     44      48 
Distance  between  Transmitter  an'd  Receiver 


52 


56     60 

Miles 


FIG.  95.  Results  of  Messrs.  Duddell  and  Taylor's  experiments  on  distance  law. 

tion  and  convection  to  the  suspended  thermal  junction.  By  the 
use  of  a  set  of  interchangeable  heaters  the  instrument  could  be 
given  a  wide  range  of  sensitiveness. 

Using  this  thermogalvanometer  for  measuring  the  received  cur- 
rent Messrs.  Duddell  and  Taylor  found  that  within  certain  limits 
the  current  received  is  approximately  inversely  proportional  to 
the  distance  from  the  sending  station;  that  is  to  say,  the  current 
multiplied  by  the  distance  is  approximately  constant.  Figure  95 
shows  graphically  the  results  obtained  during  three  cruises  of  the 


PROPAGATION  OVER  THE  EARTH 


131 


Monarch.  In  these  curves  the  product  of  received  current  times 
distance  is  plotted  against  the  distance.  If  this  product  were  a 
constant,  the  curves  should  each  be  a  straight  line  parallel  to  the 
horizontal  axis.  It  is  seen  that  between  16  and  60  miles  each  of 
the  three  curves  is  approximately  horizontal.  Messrs.  Duddell 
and  Taylor's  measurements  will  therefore  be  seen  to  show  that 
the  received  current  from  a  given  constant  sending  station  is 

g  3800 

2 

13600 

^o 

5  3400 


1000  2000  3000  4000  5000  6000  7000 

Transmission  distance  over  perfectly  conductive  expanse  Kilometers 

FIG.  96.     Comparison  of  transmission  distances. 

somewhat  nearly  inversely  proportional  to  the  distance.  In  view 
of  the  great  difficulty  of  keeping  the  conditions  at  the  sending 
station  constant  throughout  each  of  the  experiments,  and  in  view 
of  the  difficulty  of  measuring  the  small  currents  received,  Messrs. 
Duddell  and  Taylor  deserve  much  praise  for  this  laborious  and 


132 


WIRELESS  TELEGRAPHY 


careful  piece  of  work,  which  was  performed  at  a  time  when  quan- 
titative experiments  in  wireless  telegraphy  were  few. 

Diminution  of  Amplitude  by  Divergence  Together  with  Absorp- 
tion. —  Assuming  the  inverse  first-power  law  to  represent  the  effect 
of  divergence,  let  us  now  combine  with  this  effect  the  effect  of 
absorption  of  the  waves  in  passing  over  various  terrestrial  surfaces. 
To  do  this  we  shall  make  use  of  Zenneck's  theoretical  treatment 
of  the  question  of  absorption.  Following  Zenneck's  equations  and 
data,  I  have  constructed  the  chart  of  Fig.  96,  showing  the  equiva- 
lent distance  of  transmission  over  various  terrestrial  materials  in 
comparison  with  the  transmission  distance  over  a  nonabsorbing 
good-conducting  surface.  The  wave  length  assumed  in  this  calcu- 
lation is  600  meters.  The  results  shown  by  the  curves  of  Fig.  96 
are  also  presented  numerically  in  the  following  table: 

TABLE  I. 

GIVING  EQUIVALENT  DISTANCES  OF  TRANSMISSION  OVER  VARIOUS 
TERRESTRIAL  MATERIALS.  FROM  ZENNECK'S  EQUATIONS  AND  DATA. 
WAVE  LENGTH  600  METERS.  THE  DISTANCES  ARE  IN  KILOMETERS. 


Equivalent  Distances  of  Transmission  over 

A  Perfectly 
Conductive 
Expanse 

Sea 
Water. 

Fresh  Water 
or  Very  Wet 
Soil. 

Wet 
Soil. 

Damp 

Soil. 

Dry 

Soil. 

Very  Dry 
Soil. 

100 

99 

98 

97 

80 

70 

30 

200 

195 

170 

165 

115 

85 

35 

300 

290 

260 

215 

140 

105 

40 

400 

385 

350 

295 

170 

120 

43 

500 

480 

400 

340 

190 

130 

48 

1000 

920 

700 

560 

270 

150 

55 

1500 

1320 

940 

720 

320 

175 

60 

2000 

1680 

1140 

850 

360 

185 

63 

2500 

2030 

1300 

950 

380 

200 

68 

3000 

2360 

1450 

1050 

400 

215 

70 

3500 

2680 

1580 

1140 

420 

225 

75 

4000 

2970 

1700 

1220 

430 

240 

80 

4500 

3230 

1820 

1300 

440 

255 

82 

5000 

3490 

1915 

1370 

460 

270 

85 

5500 

3750 

2040 

1440 

475 

280 

87 

6000 

3960 

2140 

1520 

495 

295 

90 

6500 

4200 

2240 

1580 

500 

310 

92 

7000 

4450 

2340 

1640 

520 

320 

95 

From  this  table  it  will  be  seen  that  the  effects  of  absorption 
show  up  more  and  more  with  increasing  distance  of  transmission. 
As  an  example  of  the  meaning  of  the  table,  take  the  case  where 
3000  stands  in  the  first  column.  The  table  shows  that  a  station 
that  could  send  waves  capable  of  being  read  at  a  distance  of  3000 


PROPAGATION  OVER  THE  EARTH         133 

kilometers  over  a  perfectly  conductive  expanse  could  be  read  at 
a  distance  of  2360  kilometers  over  the  sea;  1450  kilometers  over 
fresh  water  or  a  rain-soaked  soil;  400  kilometers  over  damp  soil, 
and  only  70  kilometers  over  some  kinds  of  very  dry  soil.  Although 
exact  quantitative  experiments  are  lacking  in  regard  to  the  equiva- 
lence of  these  various  distances  in  a  practical  case,  yet  these 
figures  do  not  seem  to  be  very  different  from  the  reports  of  wireless 
telegraph  engineers  as  to  the  comparative  ease  of  attaining  great 
distances  over  sea  and  over  various  kinds  of  land.1 

A  deduction  of  the  numerical  results  shown  in  the  above  table 
by  straightforward  reasoning  from  Maxwell's  theory  of  electric 
waves,  and  the  agreement  of  these  results  with  the  facts  of  experi- 
ence, ought  to  be  sufficient  to  satisfy  us  that  we  are  dealing  with 
true  Maxwellian  electric  waves  and  not  with  some  new  kind  of 
electrical  manifestation,  as  some  writers  have  occasionally  intimated. 

Absorption  Conditioned  on  Wave  Lengths.  —  In  discussing 
Zenneck's  results  we  have  confined  our  attention  to  a  wave  length 
of  600  meters.  Zenneck  has,  however,  shown  how  to  modify  his 
formulas  in  order  to  apply  them  to  other  wave  lengths;  and  Pro- 
fessor Fleming  has  carried  the  calculations  through  for  several 
other  wave  lengths,  and  draws  the  following  conclusions: 

"  1.  In  the  case  of  transmission  over  sea,  the  absorption  for 
waves  of  300  meters  wave  length  is  not  very  large ;  but,  neverthe- 
less, increasing  the  wave  length  to  3000  meters  is  an  advantage. 

2.  In  transmission  over  land  the  absorption  of  waves  300  meters 
long  is  very  sensible,  and  increasing  the  wave  length  to  3000  meters 
produces  a  very  beneficial  effect. 

3.  In  the  case  of  extremely  dry  soil  the  terrestrial  absorption 
is  very  large,  and  increasing  the  wave  length  from  300  meters  to 
3000  meters  produces  no  marked  improvement." 

Effect  of  Bodies  of  Water  below  the  Earth's  Surface.  —  For 
information  on  this  subject  the  mathematical  reader  is  referred  to 
an  article  by  Dr.  F.  Hack,  Annalen  der  Phy&ik,  Vol.  27,  p.  43,  1908. 

The  Effect  of  Light  and  Darkness  on  Transmission.  —  Another 
important  subject  connected  with  the  long  distance  transmission 
of  wireless  telegraph  signals  is  the  effect  of  light  and  darkness 
on  transmission  distance.  In  experiments  conducted  between 

1  See  on  this  subject,  Capt.  H.  B.  Jackson,  R.N.,  F.R.S.,  "On  Some 
Phenomena  affecting  the  Transmission  of  Electric  Waves  over  the  Surface 
of  Sea  and  Earth,"  Proc.  Roy.  Soc.  London,  1902,  Vol.  70,  p.  254.  Also 
Fleming,  The  Principles  of  Elec.  Wave  Telegraphy,  1906,  p.  606. 


134  WIRELESS  TELEGRAPHY 

Poldu  and  the  steamer  Philadelphia  in  March,  1902,  Mr.  Marconi 
found  that  the  messages  could  be  received  at  much  greater  dis- 
tances at  night  than  in  the  daytime.  Messages  that  could  be 
received  at  a  distance  of  1600  miles  at  night  could  be  received  only 
at  a  distance  of  700  miles  in  the  daytime.  This  difference  between 
the  distance  of  transmission  in  darkness  and  in  daylight  is  now  a 
matter  of  common  experience  in  wireless  telegraphy.  The  differ- 
ence does  not  manifest  itself  at  short  distances.  Messrs.  Duddell 
and  Taylor,  in  their  classical  experiments  above  described,  could 
not  find  any  difference  between  the  intensity  of  signals  received 
at  night  and  those  received  by  day,  when  the  distance  between 
the  sending  station  and  the  receiving  station  was  60  miles  over  sea. 
At  distances  of  150  miles  the  difference  is  distinctly  noticeable, 
and  for  greater  distances  the  difference  between  night  and  day 
transmission  is  correspondingly  greater.  Recently  Mr.  Marconi 
has  pointed  out  that  the  difficulties  of  transmission  to  long  dis- 
tances are  especially  marked  at  dawn  and  at  sunset. 

Pickard's  Experiments  on  Effect  of  Light  and  Darkness.  —  In 
January  and  July,  1909,  Mr.  Greenleaf  Whittier  Pickard  made 
some  quantitative  experiments  on  this  subject,  and  he  has  very 
kindly  given  me  permission  to  use  his  data,  although  they  have  not 
as  yet  been  published  by  him  elsewhere.  For  the  purposes  of 
these  experiments  Mr.  Pickard  utilized  the  signals  sent  out  from 
the  Marconi  station  at  Glace  Bay,  in  the  course  of  their  regular 
transatlantic  wireless  telegraph  experiments,  and  he  measured  the 
relative  strength  of  the  signals  received  at  Amesbury,  Massachu- 
setts, at  different  hours  of  the  day  and  night.  The  distance 
between  Glace  Bay  and  the  receiving  station  at  Amesbury  is 
about  600  miles.  Mr  Pickard  had  to  take  his  observations  at 
any  time  when  the  Glace  Bay  station  was  in  action,  and  since  he 
had  no  control  over  the  activities  of  this  station,  it  was  necessary 
to  combine  observations  extending  over  two  or  three  days  in 
order  to  cover  fairly  well  the  whole  of  the  24  hours. 

A  set  of  the  observations  taken  by  Mr.  Pickard  in  the  month 
of  July,  1909,  is  plotted  in  Figure  97.  In  this  diagram  the  hour 
of  the  day  or  night  is  plotted  horizontally.  The  values  plotted 
vertically,  which  I  have  called  relative  intensity  of  received  sig- 
nals, are  values  obtained  by  the  use  of  a  crystal  detector  consist- 
ing of  a  crystal  of  bornite  in  contact  with  a  crystal  of  zincite. 
Such  a  high-resistance  crystal  contact  acts  as  a  rectifier  of  the 
oscillatory  currents  generated  in  the  antenna  by  the  incoming 


PROPAGATION  OVER  THE  EARTH 


135 


waves,  so  that  these  high-frequency  currents  are  given  a  unidi- 
rectional character  and  may  be  measured  on  a  galvanometer  by 
reading  its  deflections,  or  they  may  also  be  measured  on  a  telephone 
receiver  by  determining  what  shunt  is  necessary  about  the  tele- 
phone to  reduce  its  sound  to  inaudibility.  The  telephone  method 
is  the  more  convenient  and  this  was  usually  employed  by  Pickard, 
who,  however,  reduced  his  observations  to  galvanometer  readings 


10 


Night 


6 
A.M. 


10 


12 
Noon 


6 
P.M. 


10         12 

Night 

FIG.  97.     Observations  taken  by  Mr.  Pickard  on  the  relative  intensity  of 
signals  received  at  different  hours  of  day  and  night. 

by  calibration  and  by  control  experiments.  The  relative  intensi- 
ties of  received  signals,  plotted  in  the  diagrams,  are  the  rectified 
currents  produced  by  the  electric  waves  in  terms  of  that  rectified 
current  which  will  produce  just  audible  sounds  in  the  telephone. 

We  have  not  yet  had  a  discussion  of  these  crystal  rectifiers 
as  used  to  detect  or  measure  electric  waves,  but  it  should  be  said 
in  passing  that  on  account  of  the  characteristics  of  these  detectors 
the  relative  intensities  here  plotted  are  not  proportional  to  the 
energy  or  to  the  alternating  current  generated  by  the  received 
signals.  We  must  therefore  look  upon  the  intensity  values  of 
Mr.  Pickard's  curves  as  conditioned  by  the  form  of  detector  used. 
Since,  however,  the  detector  employed  was  one  of  high  sensitive- 
ness and  one  much  used  in  commercial  wireless  telegraphy,  these 
curves  obtained  under  actual  working  conditions  are  highly 
instructive.  As  a  precaution  against  changes  that  might  occur 
in  the  detector,  Mr.  Pickard  repeatedly  tested  the  detector  by 
throwing  it  into  a  circuit  containing  a  constant  small  alternating 
electromotive  force  and  a  galvanometer,  and  when  necessary  the 
detector  was  readjusted  so  as  to  give  a  fixed  rectified  current 
under  the  fixed  e.m.f. 

By  a  reference  to  the  curves  of  Fig.  97  we  see  that  for  the  partic- 
ular crystal  detector,  used  with  a  2000-ohm  telephone  receiver  as  in 
actual  practice,  there  was  obtained  in  the  telephone  receiver  about 
30  times  as  much  current  near  midnight  as  during  the  daytime. 


136  WIRELESS  TELEGRAPHY 

The  wave  length  of  the  Glace  Bay  station,  from  which  the  signals 
originated,  was  4000  meters;  so  that  in  spite  of  the  fact  that  the 
use  of  such  great  wave  lengths  has  been  reported  to  diminish 
the  discrepancy  between  night  and  day  transmission,  Mr.  Pickard's 
measurements  show  that  there  still  remains  a  great  weakness  of 
the  daytime  signals  as  compared  with  signals  transmitted  at  night. 

Mr.  Pickard  has  called  my  attention  to  the  very  striking  de- 
pression in  the  intensity  curve  at  dawn.  This  depression  occurs 
between  the  time  of  sunrise  at  Glace  Bay  and  the  time  of  sunrise 
at  Amesbury  (3.40  and  4.31  A.  M.  respectively  on  July  28,  both 
reduced  to  Eastern  Standard  Time  *).  Mr.  Pickard  says:  "  Al- 
though this  effect  is  small,  it  is  too  large  to  be  accounted  for  by 
observational  errors  even  in  a  single  series,  and,  as  a  matter  of 
fact,  I  find  it  running  through  all  my  dawn  measurements,  — 
about  a  dozen,  all  told."  (Quotation  from  a  letter  of  Mr.  Pickard.) 
This  is  in  agreement  with  Marconi's  observation  in  regard  to  the 
difficulty  of  signaling  at  dawn.  A  similar  depression  was  not 
found  by  Pickard  at  sunset,  possibly,  he  thinks,  on  account  of  a 
paucity  of  observations  at  sunset  due  to  the  fact  that  the  Glace 
Bay  station  was  seldom  operating  at  sunset. 

It  is  very  noticeable  that  the  daylight  absorption  persists  for 
some  time  after  sunset  and  begins  some  time  before  sunrise. 
Whence  it  appears  that,  in  summer  at  least,  the  best  working 
between  the  two  stations  examined  in  the  experiment  lasts  for 
but  a  few  hours  each  night;  perhaps  about  four  hours.  This 
time  of  good  working  ought  to  be  somewhat  longer  for  two  stations 
having  the  same  hour  of  sunrise  and  sunset.  On  the  other  hand, 
in  the  case  of  two  transatlantic  stations  which  are  situated  nearly 
east  and  west  of  each  other,  and  which  have  a  difference  of  time 
of  about  5  hours,  if  the  weakening  of  the  signals  begins  before 
sunrise  at  the  eastern  station  and  continues  after  sunset  at  the 
western  station,  the  communication  would  be  at  its  best  between 
the  two  stations  for  only  a  very  short  time,  perhaps  two  or  three 
hours  each  night,  particularly  in  summer.  Thus  we  see  that  in 
the  case  of  wireless  telegraphy,  in  addition  to  a  commercial  reason, 
there  is  also  a  physical  reason  for  "  night  messages  at  reduced 
rates." 

Efforts  to  Explain  Action  of  Daylight.  —  When  the  inequalities 

1  For  an  accurate  computation  of  the  time  of  sunrise  and  sunset  of  the 
Amesbury  and  the  Glace  Bay  stations  I  am  indebted  to  Professor  Robert  W. 
Willson,  Professor  of  Astronomy  at  Harvard  University. 


PROPAGATION  OVER  THE  EARTH         137 

of  day  and  night  transmission  of  electric  waves  were  first  observed, 
the  theory  was  at  once  advanced  that  the  effect  was  due  to  the 
action  of  the  daylight  in  rendering  the  air  conductive  for  electricity. 
We  have  noticed  in  Chapter  II  that  light,  especially  ultraviolet 
light,  is  one  of  those  agencies  that  ionizes  the  air  by  breaking  it 
up  into  charged  positive  and  negative  particles,  and  that  air  so 
ionized  will  conduct  electricity  in  a  manner  known  as  convection; 
that  is,  if  the  ionized  air  is  brought  between  two  plates  which  are 
charged  to  different  potential,  the  positively  charged  particles  in 
the  ionized  air  will  be  driven  from  the  plate  of  higher  potential  to 
the  plate  of  lower  potential,  while  the*  negatively  charged  particles 
will  be  driven  in  the  opposite  direction.  This  motion  of  the  charged 
particles  constitutes  an  electric  current  flowing  between  the  plates. 
Inadequacy  of  Explanation  Based  on  Conductivity  of  Air  Near 
the  Surface  of  the  Earth.  —  This,  suggests  two  ways  in  which  the 
effect  of  the  light  would  act  to  decrease  the  distance  of  transmis- 
sion by  daylight,  assuming  that  the  air  near  the  earth  is  more 
conductive  in  the  daytime  than  at  night. 

(1)  The  conductivity  of  the  air  in  the  daytime  in  the  neighborhood 
of  the  sending  antenna  would  cause  the  charge  to  leak  off  the  anten- 
na so  that  it  would  not  be  charged  to  so  high  a  potential  and  would 
therefore  not  produce  so  large  an  oscillating  current  as  at  night. 

(2)  The  air  in  the  interval  between  the  sending  and  the  receiv- 
ing station,  being  more  conductive  in  the  daytime,  would  absorb 
more  of  the  energy  of  the  waves  than  at  night. 

Both  of  these  explanations,  based  on  the  conduction  of  the  air 
near  the  earth,  seem  entirely  inadequate  to  explain  the  phenome- 
non. The  first  explanation  is  untenable  because  the  effects 
of  the  daylight  do  not  manifest  themselves  when  the  stations  are 
separated  by  short  distances,  and  can,  therefore,  not  be  localized 
at  the  sending  station.  As  to  the  effect  of  absorption,  if  we  take 
the  average  experimentally  determined  value  for  the  conductivity 
of  the  air  near  the  surface  of  the  earth  as  2  X  10  ~25  electromagnetic 
units  for  a  centimeter  cube  of  air,1  and  substitute  this  value  in  the 
formula2  A  =  A0e~*x, 

where  for  small  conductivity 

£=  27T(7  X  3  X  1010; 

1  This  value  is  taken,  following  Zenneck,  from  Gerdien,  Physikaeische 
Zeitschrift,  Vol.  6,  p.  647,  1905. 

2  This  formula  is  derived  in  Boltzmann's  Vorlesungen  ueber  Maxwells 
Theorie,  §96  (Leipzig,  1891). 


138 


WIRELESS  TELEGRAPHY 


in  which  Ao  is  the  amplitude  if  there  were  no  absorption,  A  the 
amplitude  of  the  absorbed  wave,  x  the  distance  in  centimeters, 
cr  the  conductivity  in  e.  m.  u.,  we  arrive  at  the  result  that  the 
absorption  due  to  the  conductivity  of  the  air  is  entirely  negligible, 
even  for  very  large  distances.  In  order  that  the  absorption  of  the 
air  should  reduce  the  amplitude  of  the  wave  to  one-third  its  value 
in  3000  kilometers  distance  the  conductivity  of  the  air  would  have 
to  be  100,000  times  as  great  as  it  really  is. 

We  therefore  cannot  look  upon  the  attenuation  of  the  electric 
waves  in  daylight  as  due  to  a  periodic  variation  of  the  conductivity 
of  the  air  in  the  regions  near  the  earth's  surface,  because  these 
variations  of  conductivity,  according  to  measurements  that  have 
been  made  of  this  quantity,  are  entirely  too  small. 

It  is  also  interesting  to  note  that  the  fluctuations  of  the  conduc- 
tivity of  the  air  from  maxima  to  minima  do  not  coincide  in  time 
with  the  fluctuations  of  intensity  of  transmitted  waves.  The  aver- 
age daily  variation  of  the  conductivity  of  the  air  as  determined  by 
Zoelss  from  2864  observations  extending  over  two  years  is  shown 
in  the  curves  of  Fig.  98.  These  curves  were  obtained  by  deter- 
mining the  rate  of  leak  of  a  charged  body.  The  curves  a  +  and 


g 

|fl«fiO 

*~ 

—  .^ 

.^ 

/^ 

'  

>x 

A 

a- 

ya+ 

5  s1-00 

33 
S'S.so 

a 

^ 

^== 

^^ 

^x 

^ 

^ 

&     2i:30      5:      7:30     10:   12.:30      3:     5:30      8:     10:30 
Hour  A..M.                            Hour  P.M. 

FIG.  98.     Rate  of  dissipation  of  electricity  at  different  hours  of  day 
and  night  (Zoelss). 

a  —  were  obtained  when  the  body  was  charged  positive  or  negative 
respectively.  Curves  of  this  character,  although  they  differ  at 
different  places  on  the  earth,  usually  show  a  minimum  dissipation 
of  electric  charge  at  sunrise  and  a  little  after  sunset,  a  maximum 
near  noon,  and  a  second  maximum  near  midnight.  This  would 
correspond  to  good  transmission  at  sunrise  and  sunset  and  poor 
transmission  at  noon  and  at  midnight,  which  does  not  accord 
with  the  facts. 

Effect  of  lonization  of  Upper  Strata.  —  The  action  of  the  sun's 
light  in  ionizing  the  air  ought  to  be  much  greater  in  the  upper 


PROPAGATION  OVER  THE  EARTH  139 

regions  of  the  atmosphere  than  at  the  surface  of  the  earth,  because 
the  chief  ionizing  rays  of  light  are  those  of  very  short  wave  length 
(the  ultraviolet),  and  these  short  waves  of  light  are  strongly 
absorbed  by  the  air,  and  therefore  do  not  penetrate  to  a  very 
great  depth  in  the  earth's  atmosphere.  The  stratum  of  upper 
atmosphere,  rendered  conductive  by  the  sunlight,  may  serve  to 
some  extent  as  a  reflector  of  the  electric  waves  so  as  to  assist  in 
confining  the  waves  to  the  surface  of  the  earth.  If  this  effect 
were  appreciable,  the  waves  would  be  more  strongly  confined  to 
the  surface  of  the  earth  in  the  daytime  than  in  the  night,  and  trans- 
mission would  be  easier  in  the  daytime  than  at  night,  except  for  a 
possible  interference  between  the  direct  and  the  reflected  wave. 
This  interference,  if  it  should  exist,  would  intensify  waves  of  some 
wave  lengths  and  partially  annul  waves  of  a  different  wave  length, 
so  that  by  changing  the  wave  length  through  a  range  correspond- 
ing to  a  half  period  it  ought  to  be  possible  to  turn  the  interference 
to  advantage.  No  such  effects  have  been  found,  and  the  increase 
of  the  conductivity  of  the  upper  air  by  ionization  in  daylight  when 
looked  upon  as  a  reflector  does  not  act  in  the  proper  direction  to  be 
the  determining  factor  in  explaining  the  inequality  of  transmis- 
sion of  electric  waves  by  day  and  by  night.  Professor  A.  E.  Ken- 
nelly  has  called  my  attention  to  the  fact,  however,  that  there  may 
exist  in  the  upper  strata,  as  we  pass  upward,  a  gradual  change 
from  insulating  to  good  conducting  strata,  which,  coupled  with 
irregularly  distributed  conducting  areas,  might  result  in  a  general 
deflection  upward  of  the  waves,  and  a  consequent  loss  of  received 
energy,  and  that  this  effect  might  be  greater  in  daylight  than  at 
night.  This  theory  has  not  yet  been  given  exact  mathematical 
expression,  so  that  up  to  the  present  we  seem  not  to  have  found 
an  adequate  explanation  of  the  difficulties  of  daytime  transmission 
in  comparison  with  night  transmission  of  electric  waves  to  great 
distances.  The  question  is  one  of  great  importance  from  a  theo- 
retical standpoint,  and  if  the  discovery  of  the  explanation  of  the 
phenomenon  should  bring  with  it  the  discovery  of  a  means  for 
bringing  the  distance  of  communication  by  daytime  up  to  that  by 
night,  it  would  remove  a  very  exasperating  limitation  to  electric 
wave  telegraphy. 

Experiments  with  the  use  of  very  long  electric  waves  are  under 
way  by  the  National  Electric  Signaling  Company  and  by  the 
Marconi  Company,  and  it  is  reported  that  some  approach  toward 
uniformity  of  day  and  night  transmission  has  been  made. 


CHAPTER  XVI 
ON   DETECTORS 

HAVING  examined  at  some  length  various  problems  in  connection 
with  the  propagation  of  electric  waves  to  great  distances  over  the 
surface  of  the  earth,  let  us  take  up  next  a  description  and  exami- 
nation of  some  of  the  instruments  used  in  receiving  the  oscillations 
of  wireless  telegraphy  and  translating  them  into  audible  or  visible 
signals. 

The  instruments  employed  are  the  indicating  instrument  (relay, 
galvanometer,  telephone,  etc.),  by  which  the  signals  are  read,  and 
the  detector,  by  which  the  high-frequency  oscillations  are  put  into 
a  condition  to  affect  the  indicating  instrument. 

INDICATING    INSTRUMENTS 

Classification  of  Indicating  Instruments.  —  The  indicating  in- 
strument now  usually  employed  is  (1)  a  sensitive  telephone 
receiver,  but  (2)  a  relay,  in  connection  with  a  sounder  or  ordi- 
nary telegraphic  recording  instrument,  (3)  a  galvanometer,  or  (4) 
an  electrometer,  may  serve  as  indicating  instrument. 

Sensitiveness  of  Relay.  —  The  most  sensitive  relay  will  trip 
with  about  one  one-thousandth  of  a  volt  e.m.f .  applied  to  its  ter- 
minals. With  the  restoring  spring  of  the  instrument  set  under 
sufficient  tension  to  act  reliably  and  rapidly  enough  to  receive 
messages,  a  relay  (even  when  constructed  to  have  high  sensitiveness) 
would  require  perhaps  one  two-hundredth  of  a  volt  to  operate  it. 

Sensitiveness  of  Telephone  Receiver.  —  Dr.  L.  W.  Austin  l  has 
recently  made  some  experiments  on  the  volt  sensitiveness  of  a 
pair  of  800-ohm  Schmidt- Wilkes  head  telephone  receivers,  such  as 
have  been  very  much  employed  in  recent  electric- wave  telegraphy. 
In  stating  the  sensitiveness  of  a  telephone  receiver  it  is  necessary 
to  specify  the  frequency,  because  the  sensitiveness  depends  very 
markedly  on  the  frequency  of  the  e.m.f.  applied  to  the  circuit. 
This  is  no  doubt  largely  due  to  the  possession  by  the  diaphragm 

1  Bulletin  of  the  Bureau  of  Standards,  Vol.  5,  p.  149,  1908. 
140 


ON  DETECTORS 


141 


of  a  natural  period  of  vibration.  The  following  table  (Table  II) 
taken  from  Dr.  Austin's  paper  gives  the  number  of  volts  required 
to  produce  just  audible  sounds  in  the  pair  of  telephone  receivers 
under  the  application  of  sinusoidal  electromotive  forces  of  various 
numbers  of  complete  cycles  per  second. 

*        TABLE   II. 

VOLT    SENSITIVENESS    OF   A    PAIR    OF    SCHMIDT-WILKES    800-OHM 
TELEPHONES. 


No.  of  cycles 

Volts    to    produce    audible 

per  second. 

sound. 

60 

620     millionths  of  a  volt. 

120 

290 

180 

170 

300 

60 

420 

17 

540 

8 

660 

3 

780 

1.1 

900 

0.6 

Sensitiveness  of  Galvanometers.  —  A  very  sensitive  galvano- 
meter of  ordinary  construction  and  of  about  1000-ohms  resistance 
will  give  a  visible  deflection  with  less  than  one  ten-millionth  of  a 
volt,  but  such  an  instrument  has  too  slow  a  period  (ten  seconds) 
to  use  in  indicating  wireless  telegraph  messages.  In  1903  Profes- 
sor Einthoven  1  designed  a  new  form  of  galvanometer  that  has 
a  very  rapid  period  and  at  the  same  time  a  high  sensitiveness. 
Einthoven's  instrument  consists  of  a  very  fine  silvered  or  platinized 
quartz  fiber  hung  between  the  poles  of  a  strong  magnet.  The 
current  to  be  measured  is  sent  through  the  silver  or  the  platinum 
coating  on  the  fiber,  and  the  fiber  tends  to  move  out  of  the  mag- 
netic field.  The  deflections  of  this  fiber  may  be  observed  with  a 
microscope,  or  may  be  photographed  on  a  rotating  drum  carrying 
a  photographic  film.  The  direction  of  the  deflection  of  this  galva- 
nometer, like  that  of  the  ordinary  galvanometers,  reverses  with 
reversal  of  the  current.  In  one  one-hundredth  of  a  second  Ein- 
thoven's instrument  will  give  a  deflection  sufficiently  large  to  be 
registered  on  the  photographic  plate,  under  application  of  an  e.m.f. 
of  one  ten-thousandth  of  a  volt.  Used  in  connection  with  a  suitable 


1  Annalen  der  Physik,  Vol.  12,  p.  1059,  1903. 


142  WIRELESS  TELEGRAPHY 

detector  it  is,  therefore,  adapted  to  the  photographic  registration 
of  wireless  telegraph  messages,  and  has  been  employed  for  this 
purpose. 

Sensitiveness  of  the  Capillary  Electrometer.  —  A  very  minute 
column  of  mercury  in  a  capillary  glass  tube  and  in  contact  with 
sulphuric  acid  is  employed  in  the  construction  of  a  capillary  elec- 
trometer. Under  the  action  of  a  current,  the  electrolytic  polariza- 
tion of  the  contact  causes  a  change  of  the  surface  tension  of  the 
mercury  and  causes  the  column  of  mercury  to  rise  or  fall  in  the 
glass  tube.  This  minute  motion  of  the  mercury  column  is  observed 
with  a  low-power  microscope.  A  delicate  capillary  electrometer 
will  give  a  readable  deflection  with  an  applied  electromotive  force 
of  one  ten-thousandth  of  a  volt,  and  is  capable  of  use  as  an 
indicating  instrument. 

In  what  follows  I  shall  describe  the  method  of  employing 
some  of  these  indicators  in  connection  with  detectors  for  rapid 
oscillations. 

Why  a  Detector  in  Addition  to  the  Indicating  Instruments 
Must  be  Employed.  —  Some  misconception  exists  as  to  why  a 
detector  must  be  employed  with  these  various  indicating  instru- 
ments in  order  to  receive  and  read  the  messages.  The  misconcep- 
tion is  that  the  detectors  are  more  sensitive  to  electrical  energy  than 
the  telephone  receiver  or  galvanometer  is.  This  is  not  the  case. 
But  in  the  reception  of  the  electric  waves  the  electrical  energy 
received,  being  in  the  form  of  rapid  oscillations,  cannot  affect  the 
telephone  or  the  galvanometer.  These  rapid  oscillations  cannot 
affect  the  galvanometer  because  the  deflections  of  the  galvanometer 
reverse  with  reversals  of  the  current,  so  that  the  deflecting  impulses, 
if  applied  directly  to  the  galvanometer,  would  be  first  in  one  direc- 
tion and  then  in  the  other,  with  a  frequency  of  the  order  of  a  mil- 
lionth of  a  second,  and  motion  of  a  mass  as  light  even  as  the  fiber 
of  the  Einthoven  galvanometer  could  not  result  from  these  rapidly 
reversing  impulses.  Likewise,  a  telephone  diaphragm  could  not  be 
made  to  move  with  such  rapidity.  In  the  case  of  the  telephone, 
on  account  of  the  large  self -inductance  of  the  instrument,  the  high- 
frequency  e.m.f .  generated  by  the  waves  would  produce  in  a  circuit 
containing  a  telephone  receiver  only  extremely  weak  currents. 

The  use  of  the  detectors  is  to  transform  these  rapid  oscillations 
into  effects  that  can  be  manifested  by  the  indicating  instruments. 
How  this  transformation  is  accomplished  will  be  explained  in  the 
subsequent  discussion. 


ON  DETECTORS  143 

CLASSIFICATION    OF    DETECTORS 

We  shall  describe  the  detectors  under  the  following  more  or  less 
arbitrary  titles: 

Coherers. 

Magnetic  Detectors. 
Thermal  Detectors. 
Crystal  Rectifiers. 
Electrolytic  Detectors. 
Vacuum  Detectors. 

In  illustrating  the  manner  of  introducing  these  various  detectors 
into  the  receiving  system  a  diagram  of  only  a  simple  form  of  receiv- 
ing circuit  will  be  exhibited  with  the  descriptions.  It  is  to  be 
understood,  however,  that  all  the  detectors  can  also  be  used  in 
various  forms  of  direct  and  inductively  connected  circuits  as  well 
as  in  the  simple  circuits. 

COHERERS 

As  coherers,  we  shall  include  only  those  detectors  which  employ 
a  loose  contact  and  require  to  be  shaken,  tapped,  or  otherwise 
moved  to  restore  the  contact  to  its  sensitive  condition  after  the 
receipt  of  a  signal.  We  have  already  described  the  filings-tube 
coherer  of  Branly  and  Marconi.  A  great  many  modifications  of 
this  instrument  have  been  made,  including  the  use  of  a  single 
contact  or  a  few  contacts  in  series  or  parallel,  between  metallic 
balls  or  points,  to  take  the  place  of  the  filings.  Also  a  great  many 
variations  in  the  method  of  decohering  the  contacts  have  been 
made.  These  will  not  be  described  here. 

These  various  forms  of  coherer  have  their  importance  in  the 
fact  that,  on  the  receipt  of  electric  waves,  a  sufficiently  large  cur- 
rent is  started  in  the  local  circuit  to  operate  a  relay,  ring  a  bell, 
or  give  other  form  of  alarm  that  can  be  heard  at  a  distance  from 
the  operator's  desk.  Also  the  current  permitted  to  flow  in  the 
local  circuit  of  the  coherers  during  the  receipt  of  electric  waves  is 
sufficiently  large  to  start  machinery  and  control  a  mechanism  (for 
example,  a  torpedo  or  dirigible  craft)  at  a  distance.  This  kind  of 
result  is  not  easily  attained  with  the  other  form  of  detectors  listed 
above,  which  do  not  permit  of  the  use  of  sufficiently  large  currents 
in  the  local  circuit  to  sound  an  alarm  or  start  electrical  machinery. 


144 


WIRELESS  TELEGRAPHY 


Thus  the  coherer,  though  lacking  in  sensitiveness  to  feeble  waves 
and  not  now  generally  employed  in  the  receipt  of  messages,  has 
still  a  field  of  usefulness. 

Besides  the  filings  coherer  described  in  Chapter  XII,  we  shall 
describe  here  another  interesting  form  of  coherer, —  that  devised 
in  1902  by  Lodge,  Muirhead  and  Robinson. 

The  Lodge-Muirhead  Coherer.  —  This  instrument  consists  of 
a  small  steel  disc  A  (Fig.  99),  rotated  by  a  clockwork,  so  that  the 
disc  is  just  separated  from  a  column  of  mercury  B  by  a  thin  film 
of  mineral  oil  on  the  surface  of  the  mercury.  One  electrical  con- 
nection is  made  to  the  wheel  through  a  brush  E,  the  other  con- 
nection is  made  to  the  mercury  well 
through  the  binding  post  H. 

The  impulse  of  the  electric  oscillations 
breaks  down  the  oil  film  and  establishes 
momentary  cohesion  between  the  steel 
disc  and  the  mercury.  A  current  from 
a  local  battery  passes  through  the  disc 
and  mercury  contact,  and  operates  a 
siphon  recorder,  which  is  used  in  series 
with  the  battery  and  the  coherer. 
After  the  impulse  ceases  the  motion  of 
the  disc  brings  continuously  a  fresh  oil 
film  into  the  contact  and  causes  de- 
coherence.  The  siphon  recorder  gives 
a  written  record  of  the  dots  and  dashes 
of  the  message.  A  .felt  brush  at  K 
serves  to  keep  the  rotating  disc  free  from 
dust  before  and  after  contact  with  the 


FIG.  99.    The  Lodge-Muir- 
head-Robinson  coherer. 


mercury. 

Concerning  the  Theoretical  Explanation  of  the  Action  of  the 
Coherers.  —  A  generally  accepted  theory  as  to  the  reason  for  the 
coherence  of  the  filings,  or  other  form  of  imperfect  contact  used 
in  the  coherers,  has  not  been  established.  I  shall  state  briefly 
some  of  the  views  presented  in  explanation  of  the  phenomenon. 
Before  the  arrival  of  the  waves,  the  high  resistance  of  the  contact 
is  generally  supposed  to  be  due  to  the  presence  of  some  kind  of 
poorly  conductive  film  at  the  contact.  In  the  case  of  the  Lodge- 
Muirhead  coherer,  the  insulating  film  is  evidently  present  in  the 
form  of  a  film  of  oil.  In  many  of  the  other  coherers  a  poorly 
conductive  film  is  present  in  the  form  of  an  oxide  of  the  metal. 


ON  DETECTORS  %          145 

This  is  evident  from  the  fact  that  in  some  cases  the  metallic  par- 
ticles (e.g.,  iron  or  steel)  are  artificially  prepared  by  oxidizing 
them  in  order  to  make  of  them  a  good  coherer.  The  poorly 
conductive  film  may  also  be  present  in  some  cases  in  the  form  of  a 
sulphide  of  the  metal.  On  account  of  the  readiness  with  which 
many  metals  (called  the  "  baser  metals  ")  enter  into  combination 
with  the  oxygen  or  sulphur  dioxide  of  the  air,  a  thin  film  of  oxide 
or  sulphide  is  always  present  on  the  surface  of  most  of  the  baser 
metals,  unless  special  care  is  taken  to  remoVe  it. 

Apart,  however,  from  the  existence  of  such  films  of  foreign  matter 
at  the  contact,  it  seems  not  impossible  that  the  high  resistance 
before  the  arrival  of  the  waves  may  be  a  property  of  the  surfaces 
of  even  pure  metals  when  these  surfaces  touch  only  very  lightly. 

If  we  assume  the  presence  of  the  poorly  conductive  film  at  the 
contacts  of  the  coherer,  we  may  suppose  that,  on  the  arrival  of 
the  electric  waves,  the  poorly  conductive  film  is  removed  by  the 
heat  developed  by  the  oscillatory  currents.  This  starts  the  local 
current,  which,  developing  further  heat,  still  further  improves  the 
contact  and  permits  the  passage  of  further  current.  Instead  of 
heat  being  the  chief  agency  in  removing  the  oxide  or  other  poorly 
conductive  film,  or  in  bringing  together  the  loose  contacts,  it  may 
be  that  this  is  done  by  the  electric  attraction  between  the  filings, 
which  before  the  current  starts  will  be  charged  with  opposite  signs 
of  electricity,  and  which  under  the  added  e.m.f.  produced  by  the 
electric  oscillations  may  attract  each  other  strongly  enough  to  pull 
the  contacts  together. 

We  shall  learn  more  about  the  electrical  properties  of  high  resist- 
ance contacts  when  we  come  to  the  study  of  crystal  rectifiers.  It 
is  therefore  proposed  to  omit  further  discussion  of  the  specific 
action  of  the  coherers,  because  of  the  more  general  character  of 
the  information  to  be  presented  later. 

In  the  meanwhile  some  of  the  other  detectors  which  do  not 
depend  on  the  properties  of  a  loose  contact  are  discussed. 

MAGNETIC    DETECTORS 

Rutherford's  Magnetic  Detector.  —  In  1895  and  1896  Pro- 
fessor E.  Rutherford1  discovered  a  sensitive  method  of  detecting 
electric  waves  by  causing  the  electric  oscillations  set  up  by  the 

1  E.  Rutherford,  "A  Magnetic  Detector  of  Electrical  Waves  and  Some  of 
Its  Applications."  Phil.  Trans.  Roy.  Soc.  London,  1897,  Vol.  189,  A.,  p.l; 
also  Proc.  Roy.  Soc.  London,  1896,  Vol.  60,  p.  184. 


146  WIRELESS  TELEGRAPHY 

waves  to  demagnetize  a  bundle  of  fine  steel  wires.  This  bundle 
of  steel  wires  consisted  of  about  twenty  pieces,  each  1  cm.  long 
and  .007  cm.  in  diameter.  The  individual  wires  were  insulated 
from  one  another  by  shellac  varnish,  and  the  bundle  was  placed 
within  a  small  coil  of  about  80  turns  of  insulated  copper  wire. 
The  bundle  of  steel  wires  was  magnetized  by  the  use  of  a  magnet, 
and  was  then  brought  up  near  a  magnetometer,  consisting  of  a 
small  compass  needle  suspended  by  a  fine  fiber  and  carrying 
a  small  mirror  by  which  its  deflections  could  be  read.  The 
needle  of  the  magnetometer  was  deflected  by  the  magnetized 
bundle  of  steel  wires.  If  now  electric  oscillations  were  passed 
through  the  coil  surrounding  the  bundle  of  steel  wires,  these  wires 
lost  some  of  their  magnetism,  which  was  shown  by  a  diminished 
deflection  of  the  neighboring  magnetometer.  Rutherford  found 
that  by  connecting  the  coil  around  the  wire  bundle  to  a  resonator, 
electric  waves  from  a  small  Hertz  oscillator  placed  at  a  distance 
of  a  half  mile  across  the  city  (Cambridge,  England)  could  be 
detected.  With  this  instrument  Rutherford  performed  many 
interesting  experiments  and  carried  out  an  important  research 
on  the  damping  of  electric  oscillations. 

Marconi's  Continuous  Band  Magnetic  Detector.  —  In  1902 
Marconi  devised  two  other  forms  of  magnetic  detector,  one  of 
which  has  met  with  extensive  use  in  practical  wireless  telegraphy, 
and  is  here  described.  Reference  is  made  to  Fig.  100.  A  band 
made  up  of  a  bundle  of  fine,  hard-drawn  iron  wires,  insulated  from 
one  another  to  prevent  eddy  currents,  is  carried  on  the  periphery 
of  two  wooden  discs,  one  of  which  is  turned  by  a  clockwork  or  a 
motor,  so  that  the  band  moves  at  the  rate  of  7  or  8  cm.  per  second. 
This  endless  band  of  iron  wire  passes  axially  through  a  small  glass 
tube  g,  around  which  two  coils  are  wound.  One  of  these  coils,  b, 
is  connected  into  the  oscillation  circuit.  In  the  example  shown, 
the  receiving  circuit  is  of  the  simple  type  consisting  of  antenna, 
detector  and  ground.  In  this  case  the  coil  b  is  put  directly  into 
the  antenna  circuit,  so  that  electric  oscillations  from  the  antenna, 
A,  pass  through  this  coil  of  the  detector.  We  shall  call  the  coil 
b  the  oscillation  coil  of  the  detector.  Around  the  oscillation  coil 
is  a  second  coil,  (7,  connected  in  series  with  a  telephone  receiver. 

To  produce  a  state  of  magnetization  in  the  moving  band,  two 
permanent  horseshoe  magnets  are  placed  near  it.  Two  like 
poles,  NN,  of  the  magnets  are  placed  above  the  center  of  the 
oscillation  coil,  and  the  other  two  poles,  SS,  are  placed  near  the 


ON  DETECTORS 


147 


band  where  it  approaches  and  leaves  the  coils.  These  magnets 
induce  magnetic  poles  in  the  moving  band.  One  of  these  induced 
poles,  say  the  South  pole,  is  within  the  coils,  and  the  two  other 


FIG.  100.     Marconi  magnetic  detector. 

consequent  poles  (North  poles  in  our  illustration)  are  near  the 
point  where  the  band  enters  and  leaves  the  coils. 

General  Facts  in  Regard  to  the  Explanation  of  the  Action  of 
the  Marconi  Magnetic  Detector.  —  If  we  confine  our  attention 
to  a  point  on  the  moving  band,  it  is  seen  that,  as  the  band  moves 
forward,  this  point  becomes  a  North  pole  outside  the  coils,  changes 
to  a  South  pole  within  the  coils,  and  becomes  again  a  North  pole 
after  issuing  from  the  coils.  There  is,  however, -within  the  coils, 
a  steady  state  of  magnetization,  for  although  the  band  is  in  motion, 
every  particle  of  the  band,  as  it  passes  a  particular  point  within 
the  coils,  comes  to  a  particular  state  of  magnetization,  so  that  the 
magnetic  condition  is  fixed  with  respect  to  the  magnetizing  mag- 
nets. This  gives  a  steady  state  of  magnetization  within  the  coils 
and  produces  no  inductive  effect  in  the  form  of  currents  in  the 
telephone  circuit. 

If  now  a  train  of  electric  oscillations  passes  through  the  oscilla- 
tion coil  b,  the  magnetization  of  the  part  of  the  band  within  the 


148  WIRELESS  TELEGRAPHY 

coil  is  changed,  and  this  change  of  the  magnetization  produces  a 
transient  current  in  the  coil  C,  and  the  telephone  gives  a  click.  A 
whole  series  of  trains  of  electric  oscillations  gives  a  series  of  clicks, 
producing  a  musical  note  with  a  pitch  depending  on  the  frequency 
of  arrival  of  the  trains;  and  this  is  the  frequency  of  the  sparks 
at  the  sending  station.  So  that  one  hears,  when  listening  into 
the  telephone  attached  to  the  magnetic  detector,  a  sound  like  that 
produced  by  the  spark  at  the  sending  station.  The  pitch  of  this 
sound  is  determined  by  the  period  of  the  vibrator  of  the  sending 
induction  coil;  or,  in  case  an  alternating  current  transformer  is 
used  to  charge  the  sending  antenna,  the  fundamental  pitch  of  the 
spark,  and  consequently  the  note  that  one  hears  at  the  receiving 
station,  is  determined  by  the  number  of  reversals  per  second  of 
the  alternating  current  supply  at  the  sending  station,  although 
other  notes  may  be  superposed  on  this  fundamental  note,  due  to 
the  fact  that  with  some  adjustments  more  than  one  spark  at  the 
sending  station  occurs  at  each  reversal  of  the  alternating  source. 

We  shall  now  discuss  the  nature  of  the  change  occurring  in  the 
magnetization  of  the  iron  band  of  the  detector  under  the  action 
of  the  oscillations  set  up  by  the  incoming  waves.  The  very  rapid 
oscillations  produced  by  the  electric  waves  used  in  wireless  teleg- 
raphy cannot  produce  a  sound  in  a  telephone  either  when  applied 
to  it  directly  or  inductively,  because,  on  account  of  the  self- 
inductance  that  is  necessary  to  the  telephone,  these  very  rapid 
oscillatory  currents  cannot  traverse  its  circuit.  If  they  could  tra- 
verse its  circuit,  the  diaphragm  of  the  telephone  could  not  take 
up  such  rapid  vibrations,  and  if  it  did  we  could  not  hear  them,  for 
the  highest  note  audible  to  the  human  ear  makes  only  35,000 
vibrations  per  second.  Our  wireless  telegraph  detectors  must  be 
so  constructed  that  the  rapid  oscillations  of  a  train  of  waves  act 
integratively  upon  it,  so  that  the  train  produces  a  single  response 
in  the  telephone;1  and  a  series  of  trains  produce  a  series  of  responses. 
This  series  of  responses  we  can  hear  in  the  telephone,  because  the 
series  of  trains  of  waves  follow  each  other  with  a  periodicity  that 
is  audible. 

In  regard  to  the  manner  in  which  a  train  of  oscillations  act 
integratively  upon  the  magnetized  moving  iron  band  of  Marconi's 
form  of  the  magnetic  detector,  I  shall  present  a  few  paragraphs  of 
explanation. 

Explanation  Assuming  a  Suppression  of  Hysteresis  by  the 
Oscillations.  —  Many  experiments  have  been  made  in  the  effort 


ON  DETECTORS  149 

to  discover  just  what  is  the  effect  produced  on  the  magnetization 
of  the  bundle  of  iron  wires  by  the  oscillations  within  the  coil 
surrounding  the  bundle.  A  steady  current  in  the  coil  would 
magnetize  the  iron  wires  of  the  bundle.  An  oscillatory  current, 
according  to  the  experiments  of  C.  Maurain, l  produces  a  suppres- 
sion of  hysteresis  in  the  iron. 

In  explanation  of  the  term  "  hysteresis,"  reference  is  made  to 
Fig.  101,  in  which  magnetizing  force  is  plotted  horizontally  and 
the  magnetization  produced  by  it  is  plotted  'vertically.  This  curve 
represents  the  hysteresis  in  a  specimen  of  hard-drawn  iron  wire 
such  as  is  used  in  the  magnetic  detectors.  If  we  start  with  the 
magnetizing  force  equal  zero,  and  increase  it  to  OL,  the  magnetiza- 
tion follows  the  curve  OA.  If  now  we  reduce  the  magnetizing 
force  gradually  to  zero,  the  magnetization  follows  the  curve  AC. 
That  is,  the  state  of  magnetization  produced  by  the  magnetizing 
force  when  it  is  decreasing  is  not  the  same  as  the  state  of  magneti- 
zation produced  by  the  force  when  it  is  increasing,  and  after  the 
force  is  removed,  some  magnetization  represented  by  OC  is  left 
in  the  specimen.  In  order  to  reduce  this  magnetization  to  zero, 
it  is  necessary  to  apply  a  reversed  magnetizing  force  OD.  If  we 
go  on  increasing  the  reversed  magnetizing  force  to  OM,  the  mag- 
netization follows  the  branch  DE  of  the  curve.  On  decreasing 
and  again  reversing  the  force,  the  magnetization  traces  out  the 
branch  EFGA.  The  complete  diagram  is  called  a  hysteresis  cycle. 

Hysteresis  is  the  property  of  iron,  steel  and  other  magnetizable 
metals  characterized  by  the  fact  that  the  change  in  magnetization 
due  to  the  application  of  a  magnetizing  force  depends  on  the  pre- 
vious state  of  magnetization  of  the  specimen.  The  state  of  mag- 
netization assumed  by  a  specimen  when  the  magnetizing  force  is 
gradually  removed  is  not  the  same  as  the  state  of  magnetization 
assumed  by  the  specimen  when  the  force  is  gradually  applied. 
The  magnetization  produced  by  a  given  magnetizing  force  is  not 
completely  annulled  by  withdrawing  the  magnetizing  force.  The 
hysteresis  effect  is  small  in  very  soft  iron,  is  increased  by  harden- 
ing the  iron,  and  is  very  great  in  glass-hard  steel. 

According  to  the  experiments  of  C.  Maurain,  which  we  are  now 
discussing  in  their  application  to  the  magnetic  detector,  the  super- 
position of  a  sufficiently  strong  oscillatory  magnetizing  force  upon 
a  slowly  varying  magnetizing  force  causes  a  suppression  of  the 
hysteresis  in  the  specimen.  If  the  oscillatory  force  is  weak,  the 
1  C.  Maurain,  Comptes  Rendus,  Vol.  137,  p.  914-916,  1903. 


150 


WIRELESS   TELEGRAPHY 


suppression  is  only  partial,  giving  for  the  specimen  characterized 
in  Fig.  101  a  diminished  hysteresis,  such  as  is  represented  in 
Fig.  102. 


M 


7 


rnetising  Force 


agnetising  Force 


FIG.  101.     Hysteresis  curve. 


FIG.  102.     Hysteresis  curve. 


In  terms  of  this  result  we  have  a  possible  explanation  of  the 
magnetic  detector.    Reference  is  made  to  Fig.  103.  With  the  poles 


FIG.  103.     Diagram  in  explanation  of  Marconi  magnetic  detector. 

of  the  permanent  magnet  in  the  positions  SNNS,  the  mag- 
netizing force  acting  on  the  band  will  be  positive  under  the 


ON   DETECTORS  151 

South  poles  and  negative  under  the  North  poles;  and  following 
our  usual  method  of  plotting,  the  magnetizing  force  can  be  repre- 
sented approximately  by  the  dotted  wavy  curve  H  of  Fig.  103. 
Now  if  we  suppose  the  band  to  be  moving  in  the  direction  of  the 
arrows,  the  North  magnetization  under  the  first  South  pole  will 
not  follow  the  curve  of  force,  but  will  persist,  and  follow  approxi- 
mately the  continuous  curve  B.  If  now  oscillations  produced  by 
the  electric  waves  are  allowed  to  flow  around  the  oscillation  coil, 
the  hysteresis  in  the  band  is  suppressed,  so  that  the  curve  of 
magnetization  B  falls  back  into  the  position  B',  which  is  nearer 
the  curve  of  magnetizing  force  H  of  Fig.  103.  This  change  from 
the  condition  B  to  B1  is  equivalent  to  a  motion  toward  the  left 
of  the  magnetic  distribution  in  the  coil,  and  therefore  induces 
a  current  in  the  coil  containing  the  telephone  in  circuit.  When 
the  waves  cease,  the  state  of  magnetization  returns  to  that  repre- 
sented by  the  curve  B.  We  have  thus  with  each  train  of  waves 
a  back  and  forth  shift  of  magnetization  of  the  band,  and  conse- 
quently a  to  and  fro  motion  of  the  telephone  diaphragm. 

While  this  description  of  the  process  seems  a  very  reasonable 
explanation  of  the  action  of  the  detector,  yet,  for  the  benefit  of 
those  readers  who  may  wish  a  little  more  insight  into  the  processes 
occurring  in  iron  or  steel  submitted  to  an  oscillatory  field,  I  beg 
leave  to  present  a  brief  account  of  some  experiments  by  E.  Made- 
lung,  in  which  he  made  direct  observations  of  the  effect  of  electric 
oscillation  on  the  magnetization  of  iron  and  steel. 

Experiments  of  E.  Madelung.  —  A  very  comprehensive  and 
beautiful  series  of  experiments  On  Magnetization  by  Rapid  Oscilla- 
tions, and  on  the  Operation  of  the  Rutherford-Marconi  Magnetic 
Detector  has  been  made  by  E.  Madelung,  and  described  in  his 
Gottingen  Dissertation.1 

By  means  of  a  very  ingeniously  devised  application  of  Braun's 
cathode  tube,  Madelung  was  able  to  obtain  on  a  fluorescent 
screen  the  hysteresis  cycle  produced  by  a  slowly  varying  magnetic 
force,  and  to  obtain  also  the  effect  produced  on  this  hysteresis 
cycle  by  superposing  the  rapidly  oscillating  magnetic  force  pro- 
duced by  sending  a  condenser  discharge  through  the  magnetizing 
coils.  , 

Reference  is  made  to  Fig.  104.  I.  With  a  slowly  varying 
magnetizing  force  the  hysteresis  cycle  EAKFGE  was  described. 
II.  Upon  slowly  applying  and  withdrawing  a  magnetizing  force 
1  E.  Madelung:  Drude's  Annalen,  1905,  Vol.  17,  p.  861. 


152 


WIRELESS  TELEGRAPHY 


OM  the  curve  AKC  was  obtained.  III.  On  applying  in  the  coil 
surrounding  the  specimen  a  rapidly  oscillating  electric  current, 
giving  a  magnetizing  force  of  initially  the  same  amplitude  OM 
and  falling  off  in  amplitude  by  damping,  the  spiral  curve  AD  was 
described.  IV.  Applying  a  second  oscillation  gave  a  similar 
spiral  starting  with  the  arc  DJ.  The  complete  spiral  for  this  case 
is  not  drawn;  it  is  like  that  of  AD,  but  is  somewhat  lower  down. 
V.  Applying  more  of  these  oscillations  brought  the  spiral  down 
into  the  position  L,  after  which  further  oscillations  simply  caused 
the  magnetization  to  describe  over  and  over  the  closed  spiral 


FIG.  104.     Dr.  Madelung'e  curve  showing  effect  of  rapid  oscillations 
on  magnetic  hysteresis. 

path  L.  The  path  L  is  thus  the  limit  of  the  condition  attained 
by  the  specimen  when  several  oscillations  are  applied. 

Thus  a  series  of  oscillations  applied  to  the  specimen  originally 
in  the  state  A  reduced  its  magnetization  to  the  state  L. 

The  jump  from  A  to  L  is  the  demagnetization  effect  of  the 
oscillation,  which  was  first  utilized  in  the  construction  of  a  detector 
for  electric  waves  by  Rutherford. 

Suppose  now  that  these  oscillations  be  applied  to  the  specimen 
when  it  is  in  various  different  states  of  magnetization;  Madelung 
found  the  effect  shown  in  Fig.  105.  Applied  at  A,  the  effect  was 
a  change  from  A  to  B]  applied  at  C,  the  specimen,  after  the  oscil- 
lation, was  left  almost  in  the  state  C  unchanged;  applied  at  D,  the 
effect  was  a  change  from  D  to  E.  The  effect  of  the  oscillating 
field  is  thus  a  hastening  of  the  progress  of  the  cycle  in  the  direction 
it  was  already  going  under  the  action  of  the  slowly  varying  field. 


ON  DETECTORS 


153 


A  suppression  of  hysteresis  would  attain  the  same  end  results,  but 
instead  of  being  contented  with  calling  the  effect  "  suppression  of 
hysteresis,"  which  is  a  purely  negative  account  of  the  phenomenon, 
Madelung,  by  his  delineation  of  the  spiral  course  taken  by  the 
magnetization  during  the  application  of  the  oscillating  magnetic 
force,  has  given  us  a  very  distinct  picture  of  the  active  processes 
occurring  in  the  specimen.  He  has  shown  that  the  magnetic  state 
of  the  iron  has  been  violently  agitated  by  the  oscillating  mag- 
netic force,  and  in  this  way  the  sluggishness  of  the  specimen  in 
following  the  slowly  changing  magnetic  force  has  been  overcome. 


FIG.  105. 


High  frequency  oscillations  superposed  on  different  parts 
of  cycle  (Madelung). 


Applying  this  process  to  our  Fig.  103,  we  must  think  of  the  curve 
B  as  going  through  a  set  of  vibratory  tremors  back  and.  forth 
horizontally  as  it  settles  down  toward  the  curve  H.  These  tre- 
mors are  of  too  high  frequency  to  act  on  the  telephone,  which 
therefore  responds  only  to  the  general  displacement  of  the  mag- 
netization from  the  curve  B  toward  the  curve  H. 

Sensitiveness  of  the  Magnetic  Detectors.  —  The  magnetic 
detectors  are  more  sensitive  than  the  coherer,  but  seem  to  be  less 
sensitive  than  the  electrolytic  detector  and  some  of  the  solid  con- 
tact detectors  (the  crystal  detectors). 


THERMAL    DETECTORS 


There  are  two  general  classes  of  detectors  in  which  the  heat 
developed  by  the  electric  waves  is  made  to  manifest  itself  at  the 
receiving  station.  In  one  of  these  classes,  including  the  bolometer 


154 


WIRELESS  TELEGRAPHY 


and  the  barretter,  a  change  of  electrical  resistance  under  the  heat 
developed  is  observed;  and  in  the  other  class  of  thermal  detectors, 
the  thermoelectric  detectors,  it  is  the  thermoelectromotive  force  called 
into  play  by  heating  the  junction  of  two  dissimilar  metals  that  is 
observed. 

Bolometer.  —  The  Bolometer,  which  was  applied  by  Paalzow 
and  Rubens  to  measurements  with  electric  waves,  has  been  de- 
scribed in  Chapter  X  (see  Fig.  47).  Briefly,  the  bolometer  consists 
of  an  accurately  balanced  Wheatstone-s  bridge  of  which  one  arm 
is  composed  of  a  very  fine  wire.  When  electric  waves  are  passed 
through  this  fine  wire,  it  is  heated.  The  heat  developed  changes 
the  resistance  of  the  fine  wire  and  throws  the  bridge  out  of  balance, 
so  that  the  galvanometer  in  circuit  with  the  bridge  gives  a  deflec- 
tion. This  apparatus  has  been  applied  by  Tisot  to  measurements 
of  the  energy  received  in  a  wireless  telegraphic  receiving  station. 
The  action  of  the  bolometer  is  not  sufficiently  rapid  for  use  in 
practical  wireless  telegraphy,  unless  one  should  use  with  it  the 
newly  developed  Einthoven  galvanometer,  which  has  a  very  small 
period. 

Barretter.  —  In  a  United  States  patent,  for  which  application 
was  filed  June  6,  1902,  Professor  R.  A.  Fessenden  has  described  a 
detector  operating  on  the  same  prin- 
ciple of  change  of  resistance  with  heat, 
but  capable  of  being  used  with  a  tele- 
phone receiver.  He  calls  the  appar- 
In  the  construction 

of  the  barretter  Pro- 

fessor    Fessenden 

made  use  of  a  Wol- 

laston  wire,  which  is 

obtained  by  casting 

an    ingot    of    silver 

with  a  platinum  core 

and    drawing    down 

the  ingot.    This  pro- 
duces a  wire  with  a 

silver  exterior  and  a 

fine  platinum  thread 
running  through  the  center.  Then  by  etching  off  the  silver  with 
nitric  acid  from  a  short  length  of  this  wire  the  very  fine  platinum 
core  is  left.  Fessenden's  barretter  consists  of  a  small  loop  of 


atus  a  barretter. 


FIG.  106.  Profes- 
sor Fessenden 's 
barretter. 


FIG.  107.     Diagram  of  circuit 
with  barretter  as  detector. 


ON   DETECTORS  155 

this  fine  platinum  wire,  which  may  be  as  small  as  one  or  two 
ten-thousandths  of  an  inch  in  diameter.  In  the  finished  instru- 
ment this  fine  loop  of  wire  is  inclosed  in  a  glass  or  metal  bulb, 
as  shown  in  Fig.  106.  The  method  of  using  the  detector  is 
shown  in  Fig.  107,  which  contains  the  detector  D  in  series  with 
the  antenna  A  and  ground  G  of  a  receiving  station.  In  the  local 
circuit  about  the  detector  is  a  battery  B  and  a  telephone  re- 
ceiver T.  Oscillations  in  the  antenna  circuit  passing  through 
the  detector  heat  the  fine  loop  of  wire.  This  changes  the  resist- 
ance of  the  little  loop,  and  consequently  modifies  the  current  in 
the  local  circuit,  and  produces  a  sound  in  the  telephone  receiver. 
When  the  waves  cease  the  little  loop  rapidly  cools,  restoring  the 
current  to  its  original  value.  The  adaptability  of  the  instrument 
to  the  receipt  of  signals  is  due  to  the  very  small  heat  capacity  of 
the  fine  wire,  by  reason  of  which  it  heats  and  cools  with  sufficient 
rapidity  to  respond  with  the  train-frequency  of  waves.  The  diffi- 
culty with  the  use  of  this  instrument  arises  in  its  liability  to  be 
burned  out  when  the  signals  become  too  strong. 

In  sensitiveness  the  barretter  falls  far  below  the  sensitiveness 
of  the  electrolytic  and  crystal  detectors  to  be  described  later,  and 
its  use,  except  for  the  purposes  of  laboratory  measurements,  has 
been  practically  discontinued. 

Thermoelectric  Detectors.  —  We  have  already  described  two 
thermoelectric  detectors:  Klemencic's  thermal  junction  (Chapter 
IX)  and  DuddelFs  thermogalvanometer  (Chapter  XV).  These 
instruments  change  the  energy  of  the  electric  waves  into  heat 
localized  in  a  small  amount  of  metal.  The  heat  developed,  in  the 
case  of  Klemencic's  thermal  junction,  is  developed  at  the  thermal 
junction  itself;  while  in  Duddell's  instrument  the  heat  developed 
in  the  "  heater  "  is  conveyed  by  radiation  and  convection  to  the 
thermal  junction.  The  heating  of  the  thermal  junction  produces 
an  electromotive  force,  which  gives  rise  to  a  unidirectional  electric 
current  in  the  local  circuit  and  produces  a  galvanometer  deflection. 
We  have  in  these  instruments,  first,  a  change  of  the  energy  of  the 
electric  oscillation  into  heat,  and  then  a  change  of  this  heat  energy 
again  into  electric  energy.  The  instruments  of  Klemencic  and 
Duddell,  though  very  useful  for  the  purposes  of  measurements,  are 
not  sufficiently  rapid  or  sufficiently  sensitive  for  use  in  the  recep- 
tion of  actual  messages. 

It  has  been  found,  however,  that  a  high  resistance  contact 
between  a  common  metal  and  certain  crystalline  substances,  or 


156  WIRELESS  TELEGRAPHY 

between  two  crystal  substances,  when  connected  with  a  telephone 
receiver,  are  highly  sensitive  to  electric  waves,  and  are  among  the 
most  sensitive  detectors  known.  These  have  been  described  in 
many  cases  by  the  patentees  or  by  writers  on  the  subject  as  ther- 
moelectric detectors.  It  has  been  found,  however,  that  in  a  great 
many  cases,  at  least,  the  thermoelectric  explanation  of  the  phe- 
nomenon is  not  the  correct  explanation;  and  these  detectors  are 
described  and  discussed  in  the  next  chapter,  under  the  head  of 
Crystal  Rectifiers. 


CHAPTER  XVII 


ON  DETECTORS    (Continued) .  —  CRYSTAL  RECTIFIERS 

WE  come  now  to  a  very  sensitive  and  interesting  class  of  de- 
tectors for  receiving  the  signals  of  wireless  telegraphy  and  wireless 
telephony.  These  are  the  detectors  consisting  of  a  self-restoring 
high-resistance  contact  between  solid  bodies,  and  since  one  of  the 
bodies  is  usually  crystalline  in  character,  I  have  given  to  this  class 
of  detectors  the,  name  Crystal  Rectifiers. 

The  crystal  rectifiers  are  self-restoring,  and  are  usually  employed 
with  a  telephone  receiver;  but  a  capillary  electrometer  or  galva- 
nometer can  be  used  in  the  place  of  the  telephone  receiver.  Many 
of  the  detectors  of  this  type  will  give  a  very  strong  response  without 
a  battery  in  the  local  circuit,  but  most  of  them  require  the  battery 
of  small  e.m.f.  for  the  best  sensitiveness. 

Fig.  108  shows  the  connections  for  use  of  a  self-restoring  de- 
tector with  a  battery  B  in  the  local  circuit.  Fig.  109  shows  the 


U^fi 
pi 


FIG.  108.    Crystal  contact  detector 
with  battery  in  local  circuit. 


FIG.  109.     Contact  detector 
without  battery. 


detector  without  a  battery.     The  detector  D  is  shown  attached  to 
the  antenna  and  ground  in  a  very  simple  form  of  receiving  circuit. 

157 


158  WIRELESS  TELEGRAPHY 

Electric  oscillations  in  the  antenna  pass  through  the  detector,  and 
a  response  is  obtained  in  the  telephone. 

In  the  case  of  the  use  of  the  battery  in  the  local  circuit,  we  might 
explain  the  action  by  supposing  that  the  resistance  of  the  detector 
is  changed  by  the  electric  oscillations  through  it  (perhaps  by  the 
heat  developed),  and  that  in  consequence  of  the  change  of  resist- 
ance a  larger  or  smaller  amount  of  current  is  sent  through  the 
telephone  receiver  T. 

In  the  case  where  no  battery  is  employed  (Fig.  109),  we  might 
explain  the  response  in  the  telephone  by  supposing  that  the  oscil- 
lations heat  the  contact  D,  and  that  the  heat  developed  at  the 
contact  (which  consists  of  two  dissimilar  bodies  E  and  F)  gives 
rise  to  thermoelectric  currents  in  the  circuit  containing  the 
telephone. 

These  are  explanations  that  are  apparently  simple,  and  that 
apparently  accord  with  many  of*  the  known  facts  about  thermo- 
electricity. We  shall  see,  however,  in  what  follows,  that  a  careful 
experimental  study  of  the  subject  has  led  to  the  rejection  of  the 
thermoelectric  explanation,  and  has  brought  us  to  regard  the 
action  of  these  detectors,  as  a  case  of  the  easier  passage  of  elec- 
tricity in  one  direction  than  in  the  other  through  the  contact; 
that  is  to  say,  we  are  dealing  with  a  newly  discovered  case  of  rectifica- 
tion of  an  alternating  electric  current  at  a  contact  between  solid  bodies, 
and  in  this  process  of  rectification  heat  plays  only  a  negligible 
role. 

Before  entering  into  a  discussion  of  this  view,  let  us  describe 
some  of  these  detectors  of  the  self-restoring  contact  type.  We 
shall  begin  with  a  rather  poor  representative,  the  carbon  microphone. 

Microphonic  Detector.  —  In  1879,  Professor  D.  E.  Hughes,  the 
inventor  of  the  microphone,  accidentally  found  that  the  contact 
of  a  piece  of  carbon  with  bright  steel,  when  used  with  a  telephone 
receiver,  was  responsive  to  the  inductive  effect  produced  by  the 
make  and  break  of  the  primary  current  of  an  induction  coil. 
Hughes  did  not  publish  his  results  until  interviewed  on  the  subject 
by  Mr.  Fahie  in  1899.  He  then  wrote  Mr.  Fahie  a  letter,  which 
was  published  in  the  London  Electrician,  May  5,  1899.  In  look- 
ing over  the  description  of  Professor  Hughes's  experiments  we  now 
see  that  in  1879  he  was  producing  and  receiving  electric  waves, 
and  had  discovered  in  the  microphone  a  self-restoring  contact 
detector.  A  diagram  of  one  form  of  Hughes's  microphonic 
detector  is  shown  in  Fig.  110,  which  is  redrawn  from  a  sketch  in 


DETECTORS  —  CRYSTAL  RECTIFIERS 


159 


Mr.  Fahie's  History.1  In  this  diagram.  C  is  a  carbon  pencil 
touching  a  steel  needle  N;  S  is  a  brass  spring  by  which  the  pres- 
sure of  the  contact  can  be  regulated.  The  adjustment  of  the 
spring  is  regulated  by  means  of 
the  disc  D. 

Professor  Hughes  used  the  mi- 
crophone with  or  without  a  bat- 
tery in  the  local  circuit  ;  and 
when  the  battery  was  omitted, 
he  attributed  the  sound  in  the 
telephone  to  the  thermoelectro- 
motive  force  developed  at  the 
carbon-steel  junction.  The  de- 
tector was  more  sensitive  with  a  battery  in  the  local  circuit  than 
without  it. 

Various  modifications  of  this  microphonic  detector  of  Hughes 
have  been  employed  in  practical  wireless  telegraphy.  One  modi- 
fication, which  had  a  considerable  application  a  few  years  ago, 


FIG.  110.     Hughes's  microphonic 
steel-carbon  detector. 


FIG.  111.     Steel-carbon  detector. 


FIG.  112.     Detector  of  carbon  gran- 
ule between  metallic  plugs. 


is  obtained  by  placing  a  steel  needle  across  two  blocks  of  carbon, 
as  shown  in  Fig.  111.  Another  is  made  by  placing  a  granule  of 
carbon  between  metallic  plugs  in  a  tube,  as  shown  in  Fig.  112. 

The  microphone  is  more  sensitive  than  the  filings  coherers. 
It  is,  however,  somewhat  troublesome  on  account  of  sensitiveness 
to  mechanical  vibrations  and  on  account  of  liability  to  cohere 
under  strong  signals,  and  it  is  surpassed  in  sensitiveness  to  electric 

1  Fahie,  History  of  Wireless  Telegraphy,  1902,  Dodd,  Mead  &  Co. 


160  WIRELESS  TELEGRAPHY 

waves  by  the  crystal  detectors,  in  which  the  carbon  of  Hughes's 
microphone  is  replaced  by  certain  crystalline  mineral  substances. 

Dunwoody 's  Carborundum  Detector.  —  In  1906  General  H.  H. 
C.  Dun  woody  x  of  the  United  States  Army  (retired)  discovered 
that  a  fragment  of  carborundum,  when  provided  with  suitable 
electrodes  for  connecting  it  into  the  circuit,  will  act  as  a  receiver 
for  electric  waves.  Carborundum  is  a  carbide  of  silicon,  manu- 
factured in  the  electric  furnaces  at  Niagara;  it  is  a  comparatively 
poor  conductor  of  electricity,  is  crystalline  in  character,  and  is  next 
to  the  diamond  in  hardness.  In  Dunwoody's  description  of  the 
detector  the  connection  of  the  carborundum  into  the  circuit  was 
made  by  twisting  wires  around  the  carborundum  or  by  holding 
it  in  a  clamp  between  metallic  jaws  supported  on  an  insulating 
base.  General  Dunwoody  found  that  when  the  carborundum 
detector  was  placed  in  a  wireless  telegraph  receiving  circuit,  and 
a  telephone  was  connected  about  the  detector,  responses  were 
obtained  in  the  telephone  with  or  without  a  battery  on  the  local 
circuit.  The  detector  was,  however,  more  sensitive  with  the 
battery  than  without  it.  A  number  of  experiments  on  this  form 
of  detector  have  been  described  by  the  author  in  publications  in 
the  Physical  Review,  and  are  abstracted  later  in  the  present 
chapter. 

The  carborundum  detector  is  not  highly  sensitive. 

Austin's  Tellurium-Aluminium  and  Tellurium-Silicon  De- 
tectors. —  In  1906  Dr.  L*.  W.  Austin  found  that  tellurium  in 
contact  with  aluminium  or  in  contact  with  silicon  is  a  sensitive 
detector  for  electric  waves  with  or  without  a  battery  in  the  local 
circuit.  He  attributed  the  action  to  thermoelectricity  in  his 
patent  applications  and  early  writings2  on  the  subject,  but 
afterwards  found  that  this  was  not  the  true  explanation  of  the 
phenomenon.3 

1  Dunwoody:  U.S.  Patent,  No.  837,616,  filed  March  23,  1906,  issued 
Dec.  4,  1906. 

2  L.   W.  Austin:    Letter   to  the  Electrical  World,  1906,  Vol.  48,  p.  924; 
U.    S.   Patent,    No.    846,081,    filed    Oct.    27,    1906,  issued   March   5,   1907; 
"The  High   Resistance    Contact    Thermo-Electric    Detector  for  Electrical 
Waves/'  Physical  Review,  1907,  Vol.  24,  p.  508. 

3  After  the    publication  of  the    author's    research    on   the   Carborundum 
Detector  (See  Pierce:  "Crystal  Rectifiers  for  Electric  Currents  and  Electric 
Oscillations,  Part  I,  Carborundum."  Physical  Review,  1907,  Vol.  25,  p.  31),  in 
which  it  was  pointed  out  that  thermo-electricity  could  not  explain  the  phe- 
nomenon,   Austin  came  to  the  opinion   that  the   action  in  the  case  of  his 
tellurium    detector   and   other  detectors    of    a  similar  type  was  also  not 
the  rmoelect  ric. 


DETECTORS  —  CRYSTAL  RECTIFIERS 


161 


Pickard's  Crystal  Detectors.  —  Mr.  Greenleaf  W.  Pickard  has 
been  very  prolific  in  the  discovery  of  materials  of  a  crystalline 
character  that  can  be  used  as  a  member  of  contact  detectors. 
Among  the  substances  used  and  patented  by  him  in  this  connec- 
tion are  silicon,1  zincite,2  chalcopyrite,3  bornite  and  molybdenite.4 

The  mounting  of  Mr.  Pickard's  silicon  detector,  which  is  repre- 
sentative of  a  favorable  method  of  .constructing  the  detectors  of 
this  class,  is  shown  in  Fig.  113.  A  rod  of  brass  A  is  pressed  down 
by  a  spring  S  into  contact  with  a  mass  of  polished  silicon  B, 


H  i  D- 

FIG.  113.     Pickard's  silicon  detector. 


embedded  in  an  easily  fusible  solder  of  Wood's  metal,  M .  The 
solder  in  which  the  silicon  is  embedded  is  contained  in  a  metallic 
cup  P,  which  rests  upon  a  metallic  plate  K.  Connection  to  the 
rod  A  is  made  by  means  of  the  binding  post  E.  Connection  to 

1  G.  W.  Pickard:  Electrical  World,  Vol.  48,  p.  1003,  1906;  U.S.  Patent, 
No.  836,531,  filed  Aug.  30,   1906,  issued  Nov.  20,  1906;  U.  S.  Patent,  No. 
888,119,  filed  Nov.  9,  1907,  issued  May  19,  1908. 

2  G.  W.  Pickard:  U.S.  Patent,  No.  886,154,  filed  Sept.  30,  1907,  issued 
April  28,  1908. 

3  G.  W.  Pickard :  U.  S.  Patent,  No.  912,726,  filed  Oct.  15,  1908,  issued 
Feb.  16,  1909. 

4  G.  W.  Pickard:  U.S.  Patent,  No.  904,222,  filed  Mch.  11,  1907,  issued 
Nov.  17,  1908. 


162  WIRELESS  TELEGRAPHY 

the  silicon  is  made  by  means  of  a  binding  post  not  shown,  which 
connects  with  the  plate  K.  The  ability  to  move  the  cup  contain- 
ing the  embedded  silicon  is  an  advantage,  because  not  all  parts  of 
the  silicon  surface  are  equally  sensitive,  and  this  motion  permits 
the  selection  of  a  sensitive  place  on  the  silicon  as  the  point  of  con- 
tact. Mr.  Pickard  sometimes  uses  two  of  these  active  materials 
in  the  same  detector.  For  example,  a  contact  of  zincite  with 
bornite  is  one  of  the  most  sensitive  electric  wave  detectors  known. 
The  action  of  these  detectors  was  at  first  attributed  by  Mr. 
Pickard  to  thermoelectric  effects,  but  after  I  had  published  the 
opinion  that  the  action  was  not  thermoelectric,  Mr.  Pickard 
amended  many  of  his  patents  to  comply  with  this  latter  view.1 

EXPERIMENTS    CONCERNING    THE    ACTION    OF    THE    CARBORUNDUM 
DETECTOR  AND  THE  OTHER  CRYSTAL-CONTACT  DETECTORS 

Soon  after  the  discovery  by  General  Dunwoody  that  a  crystal- 
line mass  of  carborundum  when  supplied  with  a  contact  electrode 
acts  as  a  detector  for  electric  waves,  I  began  a  series  of  experiments 
to  determine,  if  possible,  the  nature  of  the  phenomenon.  The 
experiments  were  extended  to  other  crystal  detectors.  The  results 
of  these  experiments  have  been  published  in  the  Physical  Review 
in  a  series  of  papers  entitled  "  Crystal  Rectifiers  for  Electric  Cur- 
rents and  Electric  Oscillations."  2  The  method  of  experimenting 
consisted  — 

1  It  is  not  safe  to  take  the  date  of  application  for  a  patent  as  the  date  of 
the  discovery  of  all  the  facts  contained  in  the  patent,  because  there  is  nothing 
in  the  published  patent  to  show  whether  the  matter  of  the  specifications  and 
claims  was  introduced  at  the  date  of  the  application  or  much  later,  as  amend- 
ments. The  actual  date  of  the  amendments  can  be  obtained  from  the  file 
records  in  the  patent  office. 

Sometimes  a  patentee,  without  any  intention  of  obtaining  undue  credit  for 
priority,  but  in  accordance  with  ordinary  United  States  Patent  Office  prac- 
tice, and  by  reason  of  interference  with  another  inventor,  has  had  discoveries 
put  into  his  patent  application  that  were  not  there  when  the  application  was 
made. 

2  G.  W.  Pierce:  Crystal  Rectifiers,  etc.  Part  I.  Carborundum,  Physical 
Review,  Vol.  25,  p.  31, 1907.  Part  II.  Carborundum,  Molybdenite,  Anatase, 
Brookite,  Physical  Review,  Vol.  28,  p.  153,  1909;  and  Proc.  Am.  Acad.  of  Arts 
and  Sciences,  Vol.  45,  p.  317,  1909.  Part  III.  Iron  Pyrites,  Physical  Review, 
Vol.  29,  1909.  See  also  G.  W.  Pierce:  A  Simple  Method  of  Measuring  the 
Intensity  of  Sound,  Proc.  Am.  Acad.  of  Arts  and  Sciences,  Vol.  43,  p.  377, 
February,  1908, 


DETECTORS  —  CRYSTAL  RECTIFIERS 


163 


(1)  In  determining  what  currents  would  flow  through  the  detec- 
tor under  a  given  steady  electromotive  force; 

(2)  In  an  oscillographic  study  of  the  instantaneous  values  of 
the  current  through  the  detector  under  the  action  of  an  alter- 
nating e.m.f.; 

(3)  In  measuring  the  thermoelectric  properties  of  some  of  the 
specimens  and  comparing  the  thermoelectromotive  force  with  the 
rectified  current. 

Some  of  the  facts  obtained  in  these  experiments  are  presented 
in  this  and  the  next  chapter. 

Apparatus  for  Current-voltage  Measurements.  —  Figure  114 
shows  a  sketch  of  a  form  of  circuit  employed  in  studying  the  con- 
ductivity of  crystal  contact  under  various  conditions,  by  means  of 


FIG.  114.     Circuit  for  studying  current-voltage  characteristic  of 
crystal  rectifiers. 


current  and  voltage  measurements.  The  crystal,  held  in  a  clamp, 
is  shown  at  Cr;  B  is  a  storage  battery;  XYZ  is  a  potentiometer 
consisting  of  two  fixed  plates  of  zinc  X  and  Z,  and  one  movable 
plate  Y,  immersed  in  a  zinc  sulphate  solution.  By  means  of  the 
voltmeter  V  the  difference  of  potential  between  the  plates  Y  and 
Z  could  be  read,  and  the  resulting  current  through  the  crystal  was 
given  by  a  galvanometer  or  milliammeter  at  A.  The  resistance 
of  the  galvanometer  was  so  small  in  comparison  with  the  resistance 
of  the  crystal  that  the  reading  of  the  voltmeter  was  practically  the 
drop  of  voltage  in  the  crystal. 

The  switch  Ss  enables  the  observer  to  reverse  the  current  in  the 
crystal  under  examination  without  reversing  the  galvanometer. 
A  known  resistance  at  R  could  be  thrown  into  circuit  with  the 
galvanometer  for  the  purpose  of  calibrating  it. 


164 


WIRELESS  TELEGRAPHY 


Current-voltage    Curve   for   the    Carborundum    Contact.  —  A 

curve  obtained  by  plotting  the  current  against  voltage  in  an  experi- 
ment with  carborundum  is  shown  in  Fig.  115.  It  is  seen  that  the 
current  through  the  carborundum  is  not  proportional  to  the  vol- 
tage impressed  upon  it;  the  apparent  resistance  of  the  carborun- 
dum or  its  contact  diminishes  with  increasing  current. 

Experiments  were  made  by  the  writer  on  a  great  many  speci- 
mens of  carborundum  and  other  crystal  detectors,  and  curves  of 
approximately  the  shape  shown  in  Fig.  115  were  obtained  in  all 
the  cases. 

On  reversing  the  electromotive  force  so  as  to  send  the  current 
in  the  opposite  direction  through  the  contacts  a  most  interesting 


32 


7 


Volts 


FIG.  115.     Current- voltage  curve  of  a  carborundum  contact. 


property  was  discovered;  namely,  the  property  of  unilateral  con- 
ductivity. 

Unilateral  Conductivity  of  the  Carborundum  Contact.  —  The 
current  through  the  crystal  in  one  direction  under  a  given  electro- 
motive force  was  found  to  be  different  from  the  current  in  the 
opposite  direction  under  the  same  electromotive  force;  that  is  to 
say,  the  heterogeneous  conductor  formed  of  the  crystal  and  its 
contacts  is  unilaterally  conductive.  This  effect  may  be  seen  by  a 
reference  to  Fig.  116.  The  branch  I  of  the  curve  shows  the  cur- 
rent, plotted  against  voltage,  when  the  current  is  in  one  direction; 


DETECTORS  — CRYSTAL  RECTIFIERS 


165 


branch  //  the  corresponding  values  of  the  current  obtained  when 
the  voltage  is  reversed.  The  accompanying  table,  Table  III, 
contains  the  numerical  values  from  which  these  curves  were 
plotted. 

In  the  experiment  whose  result  is  shown  in  Fig.  116  and  Table  III, 
the  specimen  of  carborundum  was  held  in  a  clamp  under  a  pressure 
of  about  500  grams,  and  it  is  seen  from  the  table  that  the  current 
in  one  direction  is  100  times  as  great  as  the  current  in  the  opposite 


2000 


1600 


1200 


800 


400 


20  10  0 


II 


10      20 

Volts 


FIG.  116.     Curve  showing  the  carborundum  contact  to  be  unilaterally 

conductive. 


direction  when  an  electromotive  force  of  10  volts  is  applied  in  the 
two  cases.  With  increase  of  current  through  the  specimen,  the 
ratio  of  the  current  in  the  two  opposite  directions  diminishes.  At 
27.5  volts  Ci  is  only  17  times  Ci. 

In  this  particular  experiment  the  piece  of  carborundum  was  sub- 
merged in  an  oil  bath  designed  to  keep  the  temperature  of  the 
specimen  constant.  The  piece  of  carborundum  was  held  in  a 
clamp,  the  jaws  of  which  served  to  lead  the  current  to  the  speci- 


166 


WIRELESS  TELEGRAPHY 


men.     The  oil,  of  which  the  temperature  was  64°  C.,  came  freely 
into  contact  with  the  crystal. 


TABLE  III 

RELATION  OF  CURRENT  TO   VOLTAGE,   SHOWING    UNILATERAL 
CONDUCTIVITY    OF    A    CARBORUNDUM    CONTACT 


Volts. 

Current  in  Microamperes. 

eye. 

Pi 

Commutator, 
Left. 

C2 
Commutator, 
Right. 

2.2 

1 

2.8 

2 

4.0 

5 

4.7 

10 

5.9 

20 

6.5 

30 

7.3 

40 

8.0 

50 

10.0 

100 

1 

100 

12.1 

150 

12.8 

200 

14.5 

300 

5 

60- 

16.0 

400 

16.8 

500 

10 

50 

17.7 

600 

19.4 

700 

20.0 

800 

20 

40 

21.0 

900 

21.9 

.     1,000 

30 

33 

23.2 

1,200 

50 

24 

25.0 

1,500 

27.5 

2,000 

120 

17 

Similar  effects  were  obtained  at  various  temperatures  between 
—  10°  C.  and  100°  C.,  both  with  and  without  the  use  of  oil  as  a 
bath.  A  like  result  was  had  with  different  specimens  and  under 
different  pressures.  The  relative  values  of  the  positive  and 
negative  currents,  howrever,  varied  from  piece  to  piece,  and 
also  was  different  under  different  conditions  of  temperature 
and  pressure. 

Effects  of  Pressure.  —  Figure  117  shows  a  series  of  current-vol- 
tage measurements  with  a  specimen  of  carborundum  held  in  a 
clamp  under  various  pressures. 

Several  experiments  were  made  with  other  specimens  of 
carborundum  with  considerable  disparity  in  the  results,  and  the 


DETECTORS  —  CRYSTAL  RECTIFIERS 


167 


curves  of  Fig.  117  cannot  be  taken  to  represent  a  general  oc- 
currence. 

For  more  details  on  the  effect  of  pressure  reference  is  made  to 
the  original  publications  in  the  Physical  Review. 


FIG.  117.    Current-voltage  curves  of  carborundum  under  different  pressures. 


Experiments  with  Platinized  Specimens  of  Carborundum.  —  In 

the  effort  to  ascertain  what  part  the  form  of  contact  plays  in  the 
phenomenon  of  unilateral  conductivity  in  crystals,  a  number  of 
specimens  of  carborundum  were  selected  with  opposite  faces  plane 
and  very  approximately  parallel,  and  some  of  the  parallel-faced 
crystals  were  platinized  on  one  or  both  of  their  smooth  surfaces 
by  the  cathode  discharge  so  that  they  could  be  put  into  good 
conducting  contact  with  the  electrodes.  The  metallic  surfaces 
thus  obtained  were  in  many  cases  optically  plane. 

Platinized  on  One  Face  only.  —  Some  of  the  specimens,  plati- 
nized on  one  face  only,  gave  very  remarkable  unilateral  con- 
ductivity. Table  IV  shows  results  obtained  with  one  of  these 
specimens,  designated  116,  when  submitted  to  a  pressure  of  1  kilo- 
gram. This  specimen  was  .6  mm.  thick,  with  area  of  about 
1  sq.  mm.  One  of  the  faces,  which  was  optically  true,  was  heavily 
platinized.  The  other  face  was  somewhat  rough  and  was  without 
platinum.  The  specimen  was  held  in  a  clamp  with  silver  jaws. 


168 


WIRELESS  TELEGRAPHY 


TABLE  IV 

CRYSTAL,    11&.     THICKNESS,  .6  MM;  AREA,  1  SQ.  MM.     PLATINIZED 
ON  ONE  SIDE.     PRESSURE,    1   KG. 


Volts. 

Cj  ,  Current  toward 
Platinum   in 
Microamperes. 

C2,  Current  from 
Platinum   in 
Microamperes. 

C,/C2, 

4.5 

3.92 

g    , 

6 

7.84 

3  <g 

7 

19.6 

0)    +3 

9 

39.2 

£  § 

10 
11 

64.0 
98.0 

5| 

13 

168 

Z3  "M 

15 

282 

£  .2 

16 

350 

M^ 

18 

600 

o  ^ 

21 

1000 

&& 

26 

2000 

p 

30 

3000 

.75 

4,000 

34.5 

4200 

3.92 

1,070 

Careful  examination  showed  that  the  rough,  unplatinized  face  of 
the  crystal  made  contact  at  only  a  few  points  with  the  electrode 
on  that  side.  With  a  given  voltage,  the  current  toward  the 
platinized  face  was  greater  than  the  current  in  the  opposite  direc- 
tion, and  the  conductive  asymmetry  of  the  crystal,  having,  as  it 
did,  one  good  conducting  and  one  high-resistance  contact,  was  very 
great.  At  30  volts  the  current  toward  the  platinized  face  was  4000 
times  the  current  in  the  opposite  direction.  The  results  for  this 
specimen  under  a  pressure  of  1  kg.,  and  also  under  a  pressure  of 
.35  kg.,  are  plotted  in  the  curves  of  Fig.  118.  The  current  toward 
the  platinized  face  is  given  in  the  right-hand  quadrant.  The 
current  in  the  opposite  direction  does  not  appreciably  depart  from 
the  axis. 

When  the  pressure  was  increased  to  2  kg.,  and  then  to  3  kg., 
the  currents  in  both  directions  were  increased  and  the  ratio  of 
Ci/Cz  was  reduced,  so  that  the  current  toward  the  platinum  was 
only  two  or  three  times  as  great  as  the  current  in  the  opposite 
direction  for  a  given  voltage. 

Carborundum  Platinized  on  Both  Sides.  —  When  a  specimen 
of  carborundum  was  platinized  on  two  sides  so  as  to  make  relatively 
good  conducting  contact  with  both  electrodes  of  the  clamp,  the 


DETECTORS  —  CRYSTAL  RECTIFIERS 


169 


30 

l.Kl 
and 
.3E  Kg. 


25  „  .20       15 


84 
fcS 

4.0 

us 

• 

1,8.0 

• 

52.5 

2.0 
1.5 
1.0 

0.5 

10       5        0 


Kg 


15       20 
Volts 


25       90 


0.5 


FIG.  118.     Curve  of  a  carborundum  contact  showing  remarkable  unilateral 

conductivity. 


£ 

£1.2 
gl.O 


r/ 


.04  .03  .12  .16  Jl  JH  JB 

Volts  Alternating 

FIG.  119.     Rectification  of  alternating  current  by  a  crystal-contact  detector. 


170 


WIRELESS  TELEGRAPHY 


ratio  Ci/Cz  of  the  current  in  the  two  opposite  directions  was  only 
1.1-1.6  instead  of  4000,  as  it  had  been  in  the  previous  experiment. 
A  set  of  the  observations  with  the  specimen  platinized  on  both 
faces  is  given  in  Table  V. 

TABLE  V 

SPECIMEN    NO.    19,    PLATINIZED    ON    BOTH    SIDES.       THICKNESS    .82    MM. 

AREA    5    SQ.    MM. 


Volts. 

Current  in  10  4  Amperes. 

eye,. 

Apparent  Resistance  in  Ohms. 

C2. 

C|. 

Et. 

R2. 

1 

1.5 

2.0 

.36 

6660 

5000 

1.5 

6.0 

10.0 

.66 

2500 

1500 

2 

9.5 

15 

.59 

2100 

1330 

3 

20 

30 

.50 

1500 

1000 

4 

37.9 

54.2 

.42 

1060 

740 

5 

68 

95 

.40 

735 

530 

6 

109 

150 

.37 

550 

400 

7 

152 

210 

.38 

455 

332 

8 

217 

288 

1.33 

370 

280 

9 

278 

370 

1.33 

323 

243 

10 

380 

485 

1.28 

263 

207 

11 

460 

620  . 

1.35 

240 

178 

12 

580 

780 

1.35 

207 

154 

13 

760 

970 

.28 

171 

134 

14 

920 

1110 

.22 

152 

125 

15 

1100 

1450 

.32 

136 

103 

16 

1350 

1700 

.26 

119 

94 

17 

1650 

2000 

.21 

102 

85 

18 

2000 

2450 

.23 

90 

73 

19 

2500 

2830 

.13 

76 

57 

20 

2940 

3600 

.23 

68 

55 

21 

4200 

4820 

.13 

50 

43 

Rectification  of  Alternating  Currents  by  the  Crystal  Contact.  - 
In  the  previous  experiments  it  has  been  shown  that-  the  carbo- 
rundum contact  is  unilaterally  conductive;  that  is,  it  gives  a 
greater  current  in  one  direction  than  in  the  opposite  direction 
when  the  same  electromotive  force  is  applied  in  the  two  cases. 
If  this  property  is  manifested  for  rapid  reversals  of  voltage,  an 
alternating  voltage  ought  to  give  more  current  in  one  direction 
than  in  the  other.  The  contact  ought,  therefore,  to  serve  as  a 
rectifier  for  alternating  currents.  Experiment  shows  this  to  be 
true  not  only  for  the  carborundum  detector  but  for  all  the  crystal 
contact  detectors.  For  example,  the  curve  of  Fig.  119  was  ob- 
tained with  the  molybdenite  detector,  by  measuring  with  a  gal- 
vanometer the  direct  current  through  the  detector  when  various 


DETECTORS  —  CRYSTAL  RECTIFIERS 


171 


values  of  60-cycle  alternating  voltage  were  applied  to  the  circuit 
containing  the  detector  and  galvanometer  in  series.  We  shall 
present  in  a  subsequent  chapter  some  oscillograms  obtained  with 
the  crystal  rectifiers.  Let  us,  however,  first  see  how  a  rectifier 
for  small  alternating  currents  may  be  a  detector  for  electric  waves. 


RECTIFIERS     AS     DETECTORS 

Having  seen  in  the  preceding  paragraphs  that  certain  crystal 
contacts  are  rectifiers  of  alternating  current,  let  us  now  reconcile 
this  characteristic  of  the  sensitive  contacts  with  their  action  as  a 
detector  for  electric  waves. 

Two  Characteristics.  —  For  the  purposes  of  this  discussion  l 
we  need  to  fix  our  attention  upon  two  important  characteristics 
of  the  sensitive  contacts  above  investigated. 

First,  the  current  is  not  proportional  to  the  voltage;  and  second, 
the  current  in  the  two  opposite  directions  is  not  the  same  under 
the  same  applied  voltage. 

A  detector  may  possess  one  of  these  characteristics  without  the 


ft 
16 


§12 


L  2  3  4 

Volts 

FIG.  120.     Rising  current-voltage  characteristic  (curve  A)  and 
falling  current- voltage  characteristic  (curve  B). 

other,  or  may  possess  both  together.     A  conductor  or  a  combina- 
tion of  conductors  possessing  the  first  of  these  characteristics  has, 

1  In  this  we  are  following  very  closely  the  arguments  laid  down  by  H. 
Brandes,  Elektrotechnische  Zeitschrift,  Vol.  27,  pp.  1015-1017,  1906,  and 
Science  Abstracts,  No.  2078,  Vol.  9,  1906. 


© 


172  WIRELESS  TELEGRAPHY 

we  shall  say,  a  "  rising  "  or  "  falling  "  characteristic.  (Compare 
respectively  curves  A  and  B,  Fig.  120.)  A  conductor  or  combina- 
tion of  conductors  showing  unequal  currents  in  opposite  direc- 
tions under  the  same  applied  voltage  we  have  called  "unilaterally 
conductive." 

Now  a  unilaterally  conductive  system  is  seen  at  once  to  be  a 
rectifier  for  alternating  currents,  without  any  battery  in  the  cir- 
cuit, because  when  an  alternating  voltage  is  applied,  more  current 
flows  in  one  direction  than  in  the  other. 

A  conductor  or  system  of  conductors  having  a  rising  or  falling 
current-voltage  characteristic,  is  a  rectifier  also,  if  used  with  an 

auxiliary  direct  current  upon 
which  the  alternating  current  is 
superposed.  In  explanation  of 
this  statement,  let  us  suppose 
such  a  detector  D,  Fig.  121,  to 
be  inserted  in  series  with  a  galva- 
nometer G,  a  battery  B,  and  a 
source  of  alternating  voltage  AC. 
Let  us  suppose  that  the  conduc- 

„  ...,.,,     tor  D  has  a  current- volt  age  char- 

FIG.  121.     Detector  in  circuit  with  .     . 

alternating  e.m.f.  actenstic  of  the  form  shown  by 

curve  A,  Fig.  120.    Let  the  e.m.f. 

of  the  battery  be  2  volts.  By  a  reference  to  the  current-voltage 
curve  it  will  be  seen  that  this  will  send  a  direct  current  of  5.3 
microamperes  through  the  circuit.  Now  let  the  impressed  alter- 
nating voltage  have  a  maximum  e.m.f.  of  J  volt.  When  this 
is  in  one  direction  it  will  add  to  the  2  volts  direct,  giving  2.5  volts. 
The  corresponding  current,  from  the  curve,  is  9.2  microamperes. 
When  the  alternating  e.m.f.  is  in  the  opposite  direction,  it  will 
subtract  from  the  local  voltage,  giving  a  total  voltage  of  1.5  volts. 
The  corresponding  current,  from  the  curve,  is  2.7  microamperes. 
Thus,  under  the  action  of  the  impressed  e.m.f.  of  i  volt  (maxi- 
mum) the  current  fluctuates  between  9.2  and  2.7  microamperes. 
All  of  the  intermediate  values  can  also  be  obtained  from  our  knowl- 
edge of  the  impressed  voltage  and  the  current-voltage  curve  A. 
However,  without  such  a  general  investigation  it  is  seen  that  the 
added  voltage  from  the  alternating  source  increases  the  current 
in  one  direction  more  than  the  corresponding  subtracted  voltage 
decreases  it;  and  that  consequently  the  total  effect  of  the  super- 
posed alternating  voltage  is  an  increase  of  current.  In  this  way 


{5D 


DETECTORS  —  CRYSTAL  RECTIFIERS  173 

an  increment  of  direct  current  is  obtained  by  the  superposition  of 
an  alternating  voltage  upon  the  local  direct  voltage;  that  is  to  say, 
the  apparatus  is  a  rectifier. 

In  a  similar  way,  it  may  be  shown  that  if  the  conductor  D  has  a 
falling  characteristic,  it  also  has  a  rectifying  effect,  if  used  with  a 
local  battery;  but  in  this  case  the  effect  of  the  impressed  alternating 
e.m.f  is  to  produce  a  decrease  in  the  lo^al  current. 

Now  a  crystal  contact  which  is  asymmetrically  conductive  and  has 
also  a  rising  characteristic  will  be  a  rectifier  without  a  battery  and 
also  with  a  suitable  battery  in  the  local  circuit.  Whether  it  will 
be  a  better  rectifier  with  or  without  the  battery  depends  on  the 
form  of  the  current-voltage  characteristic. 

WHY  A   RECTIFIER  FOR   SMALL  ALTERNATING   CURRENTS   ACTS  AS   A 
DETECTOR  FOR  ELECTRIC  WAVES 

In  the  preceding  sections  we  have  seen  that  the  detectors  that 
have  certain  characteristics  are  rectifiers  for  alternating  currents. 
In  our  illustration  we  applied  our  alternating  e.m.f.  directly  to  the 
circuit  containing  the  detector  and  the  galvanometer,  or  telephone, 
in  series.  But  when  the  detector  is  used  in  a  wireless  telegraph 
receiving  circuit,  the  alternating 
e.m.f.  is  not  so  applied,  and 
furthermore  has  a  very  high  fre-  ^ 
quency.  How  is  the  action  of  the  \  /  \j  Vy 
detector  to  be  explained  in  that 
case? 

Let  us  take  the  case  of  the 
simple  form  of  receiving  circuit 
shown  in  Fig.  122,  with  or  with- 
out a  battery  in  the  telephone 
circuit. 

A  train  of  incoming  waves  pro- 
duces   an    alternating     e-.m.f.    in       FlG    122     Detector  in  antenna 
the  antenna  circuit.     This  e.m.f.,  circuit, 

when  in  one  direction,  produces  a 

large  current  through  the  detector,  D,  charging  the  antenna. 
When  the  e.m.f.  reverses,  the  current  from  the  antenna  to  the 
ground  through  the  carborundum  is  smaller,  thus'  leaving  the 
antenna  charged  with  a  small  quantity  of  electricity.  The  effect 
of  the  whole  train  of  waves  is  additive,  so  that  this  charge  on  the 


174  WIRELESS  TELEGRAPHY 

antenna  is  cumulative.  The  accumulated  charge  on  the  antenna 
escapes  through  the  telephone  shunted  about  the  carborundum, 
causing  the  diaphragm  to  move.  Each  subsequent  train  of  waves 
causes  a  similar  motion  of  the  diaphragm,  which  is  evidenced  as 
a  note  in  the  telephone  with  the  train  frequency  of  the  waves. 

It  is  immaterial  whether  the  detector  permits  the  larger  current 
to  flow  upward,  charging  the  antenna  positive,  or  permits  the  larger 
current  in  the  downward  direction,  charging  the  antenna  negative. 
The  explanation  is  the  same  in  both  cases. 

With  very  slight  change  this  explanation  can  be  made  to  apply 
also  to  those  cases  in  which  the  detector  is  in  a  condenser  circuit 
coupled  inductively  or  directly  with  the  antenna  circuit, 


CHAPTER  XVIII 
ON    DETECTORS     (Continued) 

FURTHER    EXPERIMENTS    ON    THE    CRYSTAL    RECTIFIERS 

HAVING  seen  in  the  preceding  chapter  that  the  crystal  contacts, 
when  suitable  crystals  are  employed  are  detectors  for  electric  waves 
because  they  are  rectifiers  for  rapid  alternating  currents,  let  us 
experimentally  investigate  the  subject  a  little  further. 

Questions  Arising  in  Connection  with  the  Phenomenon.  — 
Many  interesting  questions  arise  in  connection  with  the  phenome- 
non. Is  the  action  localized  at  the  surface  of  contact  between 
the  crystal  and  the  metallic  electrode  ?  Is  the  action  due  to  elec- 
trolytic polarization?  Is  the  action  thermoelectric,  conditioned 
on  unequal  heating  of  the  two  electrode  contacts  ?  If  the  phenome- 
non is  novel,  how  is  it  related  to  the  hitherto  studied  properties 
of  conductors? 

In  the  experiments  on  carborundum,  performed  by  the  writer 
and  partially  presented  in  the  preceding  chapter,  the  investigation 
of  these  questions  met  with  limitations  on  account  of  the  form  of 
occurrence  of  the  carborundum  in  discrete  masses  to  which  elec- 
trodes could  not  be  rigidly  attached,  so  that  the  conditions  at  the 
electrodes  could  not  be  widely  varied.  However,  by  increasing 
the  pressure  of  the  electrodes  against  the  carborundum  beyond  a 
certain  limit,  and  by  cathodically  platinizing  the  surfaces  of  the 
carborundum  at  both  the  contact  areas,  we  have  seen  that  the 
rectification,  though  not  entirely  eliminated,  was  rendered  very 
imperfect;  that  is  to  say,  the  ratio  of  the  strength  of  the  current  in 
one  direction  to  that  in  the  reverse  direction  approached  unity. 
On  the  other  hand,  platinizing  one  only  of  the  surfaces  of  contact, 
while  the  other  surface  was  left  unplatinized,  generally  rendered 
the  rectification  more  nearly  perfect.  This  fact  indicated  that  the 
seat  of  the  action  was  the  area  of  contact  with  the  electrodes,  and 
that  the  action  at  the  two  contacts  were  usually  in  opposition  to 
each  other,  so  that  when  the  action  at  one  of  the  contacts  was 
reduced  by  platinizing,  the  rectification  at  the  other  contact 
appeared  more  pronounced. 

175 


176  WIRELESS  TELEGRAPHY 

These  characteristics  of  the  phenomenon  are  consistent  with  the 
view  that  the  rectification  is  conditioned  on  the  localization  of  the 
energy  of  the  circuit  at  the  high  resistance  boundary  between  the  two 
different  conductors,  the  crystal  and  the  electrode. 

Now  such  a  localization  of  energy  at  the  boundary  of  the  two 
conductors  is  favorable  to  the  production  of  electrolytic  polariza- 
tion, if  we  may  have  electrolytic  polarization  in  solids,  and  is  also 
favorable  to  the  production  of  a  thermoelectromotive  force,  either 
of  which  might  result  in  rectification. 

Nevertheless,  a  number  of  experiments  have  been  made  which 
indicate  that  neither  electrolysis  nor  thermoelectricity  plays  an 
important  part  in  the  phenomenon. 

On  the  question  of  electrolysis,  the  following  experiment  has  a 
bearing. 

Experiment  Showing  Permanence  of  the  Carborundum  Recti- 
fier. —  In  confirmation  of  the  absence  of  electrolytic  polarization, 
a  durability  test  of  the  carborundum  rectifier  has  been  made  as 
follows:  A  crystal  of  carborundum  inclosed  in  a  glass  tube  with  a 
few  drops  of  oil  and  held  between  brass  electrodes,  one  of  which 
was  pressed  forward  by  a  spiral  spring,  was  kept  under  almost 
daily  observation  *  from  October  23,  1907,  until  March  18,  1908. 
During  these  five  months  more  than  1200  measurements  were  made 
of  the  direct  current  obtained  through  the  crystal  under  different 
direct  and  alternating  voltages.  The  rectifier  was  kept  in  a  tem- 
perature bath  and  was  subjected  to  various  long  periods  of  heating 
and  cooling  ranging  from  0°  to  80°  C.  Notwithstanding  the  long 
continued  exposure  of  the  crystal  to  large  changes  of  temperature, 
and  notwithstanding  the  frequent  loading  of  the  rectifier  with 
current,  it  was  found  at  the  end  of  the  series  that  the  values  of  the 
direct  current  obtained  from  the  crystal  under  a  given  applied 
alternating  voltage  over  a  range  of  current  from  4  to  400  micro- 
amperes (direct)  and  a  range  of  voltage  between  1.5  and  6  volts 
(alternating)  did  not  differ  from  the  corresponding  values  at  the 
beginning  of  the  series  by  an  amount  exceeding  the  limit  of  accu- 
racy of  the  experiment,  which  was  about  ^  of  1  per  cent. 

This  experiment  shows  that  if  there  is  any  kind  of  electrolytic 

1  This  series  of  measurements  was  carried  out  by  Mr.  K.  S.  Johnson,  to 
whom  the  writer  wishes  to  express  his  sincere  thanks.  The  experiment  was 
finally  discontinued  on  account  of  the  accidental  melting  of  the  cement  holding 
in  the  ends  of  the  tube. 


DETECTORS  —  CRYSTAL  RECTIFIERS  177 

action,  it  must  be  of  such  a  character  as  to  change  the  nature  of  the 
electrodes  or  of  the  crystal  only  very  slowly,  if  at  all. 

On  the  Question  of  a  Possible  Thermoelectric  Origin  of  the 
Phenomenon.  —  It  is  apparent  that  the  disposition  of  the  crystal, 
with  a  high-resistance  contact  of  a  metal  against  it  at  one  side  and 
usually  a  comparatively  low-resistance  contact  at  the  other  side, 
is  exactly  the  most  favorable  for  the  development  of  heat  at  the 
high  resistance  junction.  This  heat  being  localized  at  a  very 
small  area,  would  raise  the  temperature  of  that  area  considerably. 
Now  when  the  junction  of  two  dissimilar  conductors  (e.g.,  bismuth 
and  antimony)  is  heated,  an  electromotive  force  is  developed  at 
the  junction.  And  for  all  we  know,  unless  we  try  it,  the  contact 
of  the  crystal  with  the  metal  may  have  an  enormously  higher 
thermoelectromotive  force  developed  than  that  developed  at  pre- 
viously known  thermal  junctions. 

If  this  is  true,  then  when  the  current  is  in  one  direction  the 
thermoelectromotive  force  would  add  to  the  applied  voltage  and 
produce  an  excessive  current,  while  with  the  current  in  the  opposite 
direction  the  thermoelectromotive  force  would  subtract  from  the 
applied  voltage  and  produce  only  a  small  current.  This  explana- 
tion of  the  phenomenon  seems  at  first  alluringly  simple,  and  has 
been  adopted  by  a  number  of  writers  and  inventors,  some  of  whom 
have,  however,  afterwards  changed  their  views.  But  many  per- 
sons still  hold  to  the  idea  that  these  crystal-contact  detectors  are 
thermoelectric  detectors,  and  they  are  so  described  in  many  trade 
catalogues,  especially  in  Europe. 

In  fact,  there  is  so  much  genuine  circumstantial  evidence  hi 
support  of  the  thermoelectric  hypothesis,  that  it  seems  very  im- 
portant to  present  with  some  thoroughness  the  experimental  facts 
that  exclude  this  hypothesis. 

Extension  of  the  Experiments  to  Other  Crystals.  —  In  order  to 
carry  out  such  an  investigation  a  search  was  made  for  other 
crystals  showing  properties  similar  to  carborundum  but  occurring 
in  a  form  more  suitable  for  study.  After  anatase  and  brookite 
and  molybdenite  had  been  discovered  to  be  rectifiers  and  had  been 
tested,  it  was  found  that  the  required  conditions  were  best  full- 
filled  by  molybdenite. 

I  shall  therefore  describe  the  molybdenite  detector.  I  shall 
then  show  and  describe  some  oscillograms  of  alternating  current 
through  several  crystal  detectors,  and  shall  afterwards  return  to 
some  thermoelectric  experiments. 


178 


WIRELESS  TELEGRAPHY 


MOLYBDENITE 

One  of  the  most  sensitive  of  the  rectifiers  thus  far  investigated 
makes  use  of  molybdenite  as  a  member.1  Molybdenite,  with  the 
chemical  formula  MoS2,  is  a  mineral  occurring  in  nature  in  the 
form  of  tabular  hexagonal  prisms  with  eminent  cleavage  parallel 
to  the  base  of  the  prism.  The  cleavage  of  the  crystal  resembles 
that  of  mica,  and  thin  sheets  of  the  mineral  several  square  centi- 
meters in  area  may  be  scaled  off  from  a  large  crystal  of  molyb- 
denite. These  sheets  have  a  metallic  luster  and  look  not  unlike 
sheets  of  lead  foil.  They  can  be  readily  electroplated  with  copper, 
so  that  connecting  wires  may  be  soldered  to  them.  This  property, 
together  with  the  thinness  of  the  sheets  and  the  ease  with  which 
the  thermoelectric  property  of  the  substance  may  be  studied, 
admirably  adapts  it  to  the  present  experiments. 

The  Molybdenite  Rectifier.  —  The  molybdenite  rectifier  also 
acts  as  a  receiver  for  electric  waves  without  a  battery  in  the 

local  circuit. 

A  form  of  mounting  for  the 
molybdenite  is  shown  in  section 
in  Fig.  123.  T  is  a  threaded  brass 
post  on  the  top  of  which  is  placed  a 
disc  of  mica,  N.  On  top  of  the 
mica  is  a  thin  circular  disc  of  the 
molybdenite  My  with  an  area  of 
about  1  square  centimeter,  leaving  a 
projection  of  the  mica  beyond  the 
periphery  of  the  molybdenite.  A 
hollow  cap,  Z),  threaded  inside  and 
having  a  conical  hole  at  the  top,  is 
screwed  down  on  the  post  T  so  as 
to  clamp  the  molybdenite  between 
the  mica  disc 2  and  the  annular 


FIG.  123.     Holder  for 
molybdenite. 


1  See  also  G.  W.  Pierce:  "  A  Simple  Method  of  Measuring  the  Intensity  of 
Sound,"  Proc.  Am.  Acad.  of  Arts  and  Sciences,  Vol.  43,  p.  377  (Feb.,  1908), 
in  which  the  Molybdenite  Rectifier  was  employed.     This  detector  was  also 
independently  discovered  by  Mr.  Greenleaf  Whittier  Pickard. 

2  The  purpose  of  the  mica  disc  under  the  molybdenite  is  to  confine  the 
current  as  much  as  possible  to  the  upper  layer  of  the  molybdenite.     This  was 
done  so  as  not  to  complicate  the  phenomenon  by  conduction  across  the  laminse 
of  the  substance,  and  also  so  that  when  the  detector  is  immersed  in  oil  in  some 
of  the  later  experiments,  the  oil  shall  have  free  play  over  the  conducting 
surface  and  over  the  contacts,  and  serve  the  better  to  avoid  possible  changes 
of  temperature  of  the  essential  parts  of  the  apparatus. 


DETECTORS  —  CRYSTAL  RECTIFIERS 


179 


shoulder  of  the  cap,  with  the  upper  surface  of  the  molybdenite 
exposed  above.  At  the  free  surface  of  the  molybdenite  contact 
is  made  l  with  the  metallic  rod  P. 

The  rod  P  was  either  supported  unadjustably,  as  in  the  author's 
experiments  on  sound,  or  it  was  mounted  in  a  manner  to  permit 
of  ready  adjustment,  as  is  shown  in  Fig.  124.  The  clamp  K 
containing  the  molybdenite  is  metallically  connected  with  the 
binding  post  H  (Fig.  124).  Another  binding  post  is  attached 


FIG.  124.     Mounting  for  molybdenite. 


to  the  metallic  block  A,  on  top  of  which  is  supported  a  stout 
spring  B.  Through  a  hole  in  B  provided  with  a  set-screw,  the 
rod  P  is  allowed  to  drop  down  into  contact  with  the  surface  of 
the  molybdenite  at  K.  The  set-screw  is  then  tightened  against 
P,  and  the  final  adjustment  is  made  by  the  slow-motion  screw  S. 
The  apparatus  is  connected  in  circuit  by  means  of  the  binding 
posts,  so  that  the  current  of  the  circuit  is  made  to  enter  the  molyb- 
denite through  the  contact  area  between  P  and  the  molybdenite 
and  leave  by  way  of  the  contact  between  the  molybdenite  and  the 
cap  C,  or  the  reverse.  It  is  found  that  a  much  larger  current 
flows  in  one  direction  than  in  the  reverse  direction  for  a  given 
applied  electromotive  force. 

The  current-voltage  curves  (see  Figs.  125,  126  and  127)  resemble 
those  of  the  carborundum  detector,  but  large  rectified  currents 

1  In  the  diagrams  of  Fig.  123  and  Fig.  124  the  lower  end  of  the  rod  P  is 
shown  pointed.  It  is  found,  however,  that  the  end  of  the  rod  P  may  be  blunt 
or  even  flat  with  an  area  as  great  as  4  sq.  mm.  without  much  loss  of  sensitive- 
ness of  the  instrument  as  a  receiver  for  electric  waves  or  as  a  rectifier. 


180 


WIRELESS  TELEGRAPHY 


L 


I 


.8       1.0      1.2     1.4      Id      1.8     2.0      2,2 

Volts 


FIG.  125.  Current-voltage  curves  of  the  molybdenite  rectifier.  A,  current 
from  copper  to  molybdenite;  B,  current  in  opposite  direction;  C,  difference 
of  voltage  for  a  given  current. 


Vol 


FIG.  126.     Current-voltage  curves  with  molybdenite  rectifier. 


DETECTORS  —  CRYSTAL  RECTIFIERS 


181 


are  obtained  with  very  small  voltages  in  the  case  of  the  molyb- 
denite, which  characterized  the  molybdenite  rectifier  as  much 
more  sensitive  than  the  carborundum  as  a  detector  for  electric 


waves. 


.3 


123456 

Volts 

FIG.  127.     Current-voltage  curves  with  molybdenite  rectifier. 


OSCILLOGRAPHIC   STUDY   OF  CRYSTAL  RECTIFIERS 

An  oscillogram  is  a  photograph  showing  the  rapidly  changing 
values  of  the  current  in  a  circuit  when  a  rapidly  changing  voltage 
is  applied  to  it.  In  the  case  of  the  crystal  rectifiers  a  current  of 
only  a  few  thousandths  of  an  ampere  could  be  sent  through  the 
crystal  contact  without  destroying  its  rectifying  power.  It  was 
therefore  necessary  to  employ  a  very  sensitive  apparatus,  —  one 
that  would  deflect  with  these  small  values  of  the  current,  and 
would  reverse  when  the  current  reversed,  and  that  at  the  same  time 
would  be  so  rapid  in  its  action  as  not  to  show  any  appreciable  lag 
when  the  current  through  it  was  rapidly  changing.  The  purpose 
of  the  experiment  was  to  see  if  the  current  changes  in  the  detectors 
followed  the  voltage  changes  at  once  or  if  they  lagged  behind,  as 
would  be  the  case  if  the  action  of  the  detector  depended  on  heating 
or  cooling,  because  heating  and  cooling  require  time.  Also,  if 
electrolytic  action  entered  into  the  phenomenon  it  ought  to  show 
in  the  oscillograms. 

After  much  experimenting  the  necessary  sensitiveness  of  appa- 
ratus was  finally  obtained  with  a  Braun's  cathode  tube  oscillograph. 
This  apparatus  makes  use  of  the  fact  that  when  a  high  electro- 
motive force,  say  20,000  volts,  is  applied  to  two  aluminum  elec- 
trodes sealed  into  a  glass  tube,  from  which  the  air  is  pumped  to 


182  WIRELESS  TELEGRAPHY 

a  sufficiently  high  degree  of  exhaustion,  a  stream  of  negatively 
charged  particles  called  the  cathode  stream  is  shot  out  from  the 
negative  electrode,  and  these  particles  of  the  cathode  stream  travel 
away  in  a  straight  line  perpendicular  to  the  negative  electrode. 


110  Volts 


FIG.  128.     Oscillographic  apparatus. 

Reference  is  made  to  Fig.  128.  The  small  flat  disc  in  the  small  end 
of  the  tube  is  the  cathode  The  cathode  particles  are  sent  length- 
wise the  tube,  and  in  Professor  Braun's  apparatus  are  made  to  pass 
through  a  small  diaphragm  so  as  to  limit  the  beam  to  a  small 
cross  section.  Beyond  the  diaphragm  the  beam  passes  through  the 
center  of  the  enlarged  portion  of  the  tube  and  makes  a  bright 
spot  upon  a  fluorescent  screen  at  0.  Now  when  a  magnet  is 
brought  up  near  the  tube  at  MM,  the  cathode  beam  is  deflected 
so  that  the  bright  spot  at  0  moves  perpendicular  to  the  page. 
When  the  magnet  is  reversed,  the  deflection  of  the  spot  is  reversed. 
If  instead  of  using  a  permanent  magnet  at  MM  we  use  electro- 
magnets, as  shown  in  the  figure,  and  if  we  send  an  alternating  cur- 
rent through  the  coils  of  the  electromagnets,  the  deflection  of  the 
spot  is  first  in  one  direction  and  then  in  the  other,  back  and  forth 
across  the  screen  at  0.  A  photographic  camera,  (Fig.  128)  is 
placed  above  the  cathode  tube,  and  an  image  of  the  spot  0  is 
focused  on  the  film  carried  by  a  drum  F.  The  image  plays  back 
and  forth  across  the  film.  If  now  the  film  is  set  in  motion  by  a 
rotation  of  the  drum,  the  to  and  fro  moving  spot  traces  a  wavy 
line  on  the  film.  The  drum  is  driven  at  a  high  speed,  and  in  order 
that  the  wavy  line  on  the  film  may  come  back  on  itself  with  each 
revolution,  the  drum  must  be  driven  synchronously  with  the  alter- 
nating current  which  is  being  oscillographed.  This  was  attained 
by  driving  the  drum  with  a  synchronous  motor  operating  on  the 
same  alternating  current  source  of  60  cycles.  The  synchronism 
of  the  drum  with  the  deflections  of  the  luminescent  spot  was  so  per- 
fect in  the  present  experiments  that  exposures  of  four  minutes 


DETECTORS  —  CRYSTAL  RECTIFIERS  183 

could  be  made,  during  which  time  the  image  of  the  spot  moved 
over  the  sensitive  paper  4800  times,  without  any  failure  of  per- 
fect superposition,  and  without  any  appreciable  fogging  of  the 
paper. 

The  deflecting  electromagnets  MM  had  a  combined  resistance 
of  436  ohms,  and  were  provided  with  soft  iron  cores  about  6  milli- 
meters in  diameter.  With  these  deflecting  coils  a  direct  current 
of  1.5  milliamperes  gave  a  deflection  of  1  cm.  on  a  ground  glass  put 
in  the  place  of  the  sensitive  film  at  the  back  of  the  camera.  A 
calibration  for  different  values  of  direct  current  through  the  coils 
showed  the  deflections  of  the  light  spot  to  be  proportional  to  the 
current,  for  the  small  values  of  the  current  employed,  and  showed 
no  evidence  of  hysteresis  in  the  iron. 

The  Oscillographic  Photographs.  —  Reproductions  (reduced  to 
i)  of  a  characteristic  set  of  the  photographs  obtained  with  a  60- 
cycle  alternating  e.m.f.  are  given  in  Plate  I.  Oscillograph  No.  1 
was  taken  with  the  molybdenite  rectifier  adjusted  to  give  practi- 
cally perfect  rectification.  No.  2  is  with  the  same  rectifier  slightly 
out  of  adjustment  (overloaded),  so  that  the  rectification  is  less 
perfect.  No.  3  is  with  the  same  rectifier  further  out  of  adjustment. 
No.  4  is  an  oscillographic  record  with  the  carborundum  rectifier. 
No.  5  is  with  the  rectifier  of  brookite.  In  taking  No.  2  the  rectifier 
was  submerged  in  oil,  to  test  the  effect  of  cooling. 

Three  Exposures.  —  In  making  these  pictures  the  following 
steps  were  taken:  The  drum  carrying  the  film  was  set  rotating. 
The  high-potential  current  obtained  from  Professor  Trowbridge's 
40,000  volt  storage  battery  was  started  in  the  tube.  The  potential 
V  (Fig.  128)  and  the  contact  of  the  rectifier  were  adjusted  so  that 
the  deflection  of  the  luminescent  spot  on  the  fluorescent  screen 
showed  good  rectification.  Exposure  of  about  2  minutes  was  then 
made.  This  exposure  gave  the  heavy  line  of  the  oscillograms. 

The  switch  at  T  (Fig.  128)  was  then  thrown  open,  so  that  no 
current  was  flowing  in  the  electromagnets  and  the  luminescent  spot 
came  to  its  zero  position.  The  exposure  in  this  position  was  made 
for  a  shorter  time  of  about  40  seconds.  This  traced  a  thin  straight 
line  along  the  centre  of  the  picture  and  gave  the  axis  of  zero 
current. 

The  switch  at  T  was  then  thrown  to  the  position  to  put  the  resist- 
ance R  in  the  circuit  in  place  of  the  crystal.  The  resistance  R 
had  been  previously  adjusted,  so  that  the  amplitude  of  the  deflec- 
tion with  R  in  the  circuit  should  be  equal  to  the  maximum  am- 


184  PLATE  I.    G.  W.  Pierce,  Crystal  Rectifiers. 


DETECTORS  —  CRYSTAL  RECTIFIERS 


185 


plitude  with  the  crystal  in  the  circuit.  With  the  resistance  R  in 
circuit  an  exposure  of  about  1  minute  was  made,  giving  the  light 
sinusoidal  curve  of  the  picture. 

On  each  picture  the  three  exposures  give,  therefore,  (1)  the  form 
of  the  rectified  cycle  as  a  heavy  line,  (2)  the  position  of  the  axis 
of  zero  current,  as  a  straight  line  through  the  figure,  and  (3)  the 
form  and  position  of  the  alternating  current  cycle  when  an 


TABLE  VI 

TABULAR  DESCRIPTION  OF  THE  OSCILLOGRAPHIC  RECORDS  OF  PLATE  I 


Maximum 

No. 

Material  of  Rectifier. 

Condition. 

Rectified 
Current  in 

R.  M.  S. 
Alternat- 

Equiva- 
lent Re- 

Milliam- 

ing  Volts. 

sistance 
in  Ohms. 

peres. 

1 

Molybdenite     j 

Good  adjust- 
ment 

|    4.9 

3.54 

400 

f 

Out  of  best  ad- 

2 

-        i 

justment, 
submerged  in 

„ 

3.54 

400 

I 

oil  and  over- 

I 

loaded 

3 

\ 

Out  of  best  ad- 

}   45 

} 

justment 

1    4-5 

4 

Carborundum  plat- 

Overloaded 

5.4 

22.0 

6000 

inized  on  one  side 

5 

Brookite 

3.0 

2.22 

992 

equivalent  resistance  R  is  substituted  for  the  rectifier.  The  last 
named  cycle  appears  in  the  pictures  as  a  thin-lined  sine  curve. 
This  curve  is  in  phase  with  the  impressed  voltage  immediately 
about  the  crystal,  and  is  referred  to  below  as  the  "  voltage-phase 
curve." 

Coordinates.  —  In  tracing  all  the  curves,  the  motion  of  the  light 
spot  over  the  paper  is  from  left  to  right;  the  time  coordinate  is, 
therefore,  horizontal  and  is  drawn  as  usual  from  left  to  right. 

The  scale  drawn  in  ink  at  the  left-hand  margin  of  each  picture 
gives  the  value  of  the  current,  one  division  being  one  milliampere. 

Conditions.  —  A  tabular  description  of  the  conditions  under 
which  each  of  the  records  was  taken  is  contained  in  Table  VI. 

A  discussion  of  the  records  follows: 

Oscillogram  Nos.  i,  2,  and  3  —  Molybdenite.  —  The  pressure 


186  WIRELESS  TELEGRAPHY 

of  the  copper  rod  against  the  molybdenite  for  good  rectification 
is  slight  and  is  somewhat  difficult  to  attain.  Some  points  of  the 
crystal  are  more  sensitive  than  others,  and  the  crystal  has  to  be 
moved  around  under  the  copper  contact  and  tried  at  several 
different  points  before  the  best  adjustment  can  be  found.  Oscillo- 
gram  No.  1  was  taken  with  a  molybdenite  rectifier  in  good  adjust- 
ment. The  rectification  in  this  case  is  seen  to  be  practically 
perfect;  the  cycle  through  the  specimen  consists  of  a  nearly  sinu- 
soidal curve  for  one  half-period  and  a  practically  straight  line  for 
the  other  half-period.  The  large  current  flows  from  the  copper  to 
the  molybdenite,  and  the  zero  current  from  the  molybdenite  to  the 
copper. 

When  the  pressure  on  the  contact  was  increased  until  a  small 
negative  current  was  permitted  to  pass,  oscillogram  No.  2  was 
obtained.  Increasing  the  pressure  still  more,  so  as  to  get  a  larger 
negative  current,  gave  oscillogram  No.  3. 

One  object  in  taking  these  oscillograms,  together  with  the  vol- 
tage-phase cycle,  was  to  see  if  there  is  any  evidence  of  lag  of  the 
rectified  cycle  with  respect  to  the  voltage-phase  cycle.  No  such 
lag  appears.  On  the  other  hand,  the  rectified  cycles  lead  their 
respective  voltage-phase  cycles  at  three  positions : 

The  first  of  these  positions  of  lead  is  at  the  part  of  the  cycle  in 
which  the  rectified  current  approaches  the  zero  axis  after  having 
traversed  the  upper  half  of  the  curve.  This  advance,  which  is  so 
small  as  to  be  just  perceptible  in  the  oscillograms,  amounts  to 
about  TTjW  of  a  second. 

A  second,  somewhat  larger,  lead  of  the  rectified  cycle  ahead  of 
the  voltage-phase  cycle  is  at  the  point  of  rising  from  the  axis  after 
the  rectified  current  has  followed  for  a  half-period  along  the  zero 
axis.  The  lead  here  is  about  ysVo  second. 

A  third,  very  significant,  lead  of  the  rectified  cycle  is  at  the 
negative  maximum,  as  is  seen  in  the  cases  of  imperfect  rectification, 
oscillograms  Nos.  2  and  3.  Here  the  lead  is  a  considerable  fraction 
of  a  half-period. 

Oscillogram  No.  4  —  Carborundum.  —  Oscillogram  No.  4  was 
obtained  with  a  carborundum  rectifier  consisting  of  a  specimen  of 
carborundum  platinized  on  one  side  and  held  in  a  clamp  under 
a  contact  pressure  of  3  kg.  When  sufficient  current  was  sent 
through  the  carborundum  to  give  deflections  suitable  for  the  oscillo- 
gram, the  carborundum  was  overloaded,  and  permitted  the  current 
to  pass  also  in  the  negative  direction.  The  carborundum  cycle 


DETECTORS  —  CRYSTAL  RECTIFIERS  187 

differs  from  the  molybdenite  cycle  in  the  absence  of  a  lead  at  the 
negative  maximum  and  at  the  point  of  rising  from  the  zero  axis. 
This  anomaly  in  the  case  of  the  carborundum  rectifier  is  seen 
later  to  be  the  effect  of  its  high  resistance. 

Oscillogram  No.  5  —  Brookite.  —  The  form  of  the*  cycle  ob- 
tained in  this  case  is  intermediate  between  the  carborundum  cycle 
and  the  cycle  of  oscillogram  No.  3.  This  is  consistent  with  the 
value  of  its  resistance. 

In  order  to  investigate  the  meaning  of  the  lead  of  the  rectified 
cycles  in  the  several  cases,  the  oscillograms  had  to  be  examined 
mathematically  with  the  aid  of  the  theory  of  alternating  currents. 

Only  the  conclusions  from  this  mathematical  examination  are 
here  given.  The  mathematical  reader  is  referred  to  the  original 
paper.1 

Conclusions  from  an  Examination  of  the  Rectified  Cycle  with 
the  Aid  of  Alternating  Current  Theory.  —  (1)  The  case  of  the 
advance  of  the  rectified  cycle  on  rising  from  the  axis  of  no  current 
is  shown  in  the  mathematical  discussion,  above  referred  to,  to 
be  due  to  the  fact  that  after  a  dormant  half -period  the  current  in 
the  circuit  follows  the  ordinary  exponential  "  building-up  "  curve 
for  a  time  before  coming  into  coincidence  with  the  sine  curve. 
This  building-up  curve  starts  from  the  axis  with  zero  lag,  and  is, 
therefore,  in  advance  of  the  sine  curve.  It  is  chiefly  due  to  the 
self-inductance  in  the  oscillographic  circuits.  To  this  effect  of 
self-inductance  is  to  be  added  the  effect  due  to  the  higher  resist- 
ance of  the  rectifier  for  small  currents  than  for  large  currents. 
This  higher  resistance  brings  the  building-up  curve  a  little  nearer 
to  the  sine  curve. 

(2)  The  slightly  quicker  descent  of  the  rectified  cycle  on  ap- 
proaching the  axis  after  having  traversed  the  upper  half  of  the 
curve  is  also  due  to  this  higher  resistance  of  the  rectifier  when 
traversed  by  smaller  currents. 

(3)  The  very  significant  lead  of  the  negative  maximum  ahead  of 
the  corresponding  voltage-phase  maximum  is  explicable  on  the 
assumption  that  the  rectifier  has  a  much  higher  resistance  in  the 
negative  direction  than  in  the  positive  direction.     We  have  shown 
in  the  mathematical  discussion  that  the  angle  of  lag  of  the  voltage- 
phase  cycle  behind  the  impressed  voltage,  determined  by  the 

1  G.  W.  Pierce:  Physical  Review,  1909,  Vol.  28,  p.  153;  or  Proc.  Am.  Acad. 
of  Arts  and  Sciences,  1909,  Vol.  45,  p.  317. 


188  WIRELESS    TELEGRAPHY 

inductance  and  resistance  of  the  circuit,  is 

--£-  * 

while  in  the  negative  direction,  to  give  proper  amplitude,  the 
substituted  equivalent  resistance  should  be  at  least  6470  +  436 
=  6906  ohms,  whence  the  angle  of  lag  in  this  case  would  be 


Therefore,  the  angle  of  lead  of  the  rectified  cycle  ahead  of  the 
voltage-phase  cycle,  determined  as  the  difference  of  these  two 
angles  of  lag,  is  30.2°.  This  value  agrees  with  the  oscillogram 
No.  2,  for  which  the  calculation  was  made. 

In  this  connection  it  is  interesting  to  notice  that  a  lead  of  this 
negative  maximum  in  the  case  of  the  carborundum  oscillograph 
does  not  appear.  The  explanation  of  this  is  easily  obtained  if  one 
substitutes  for  the  resistance  values  of  the  molybdenite  the  corre- 
sponding values  for  the  circuit  containing  the  carborundum  recti- 
fier. The  equivalent  resistance  of  the  carborundum  in  its  positive 
loop  is  6000  ohms,  so  that  the  angle  of  lag  of  the  voltage-phase 
cycle  with  this  resistance  in  it  is  only  5.6°,  while  in  the  negative 
direction  the  equivalent  resistance  of  the  carborundum  is  about 
20,000  ohms,  giving  an  angle  of  lag  in  the  neighborhood  of  1°. 
The  difference  between  these  two  angles  of  lag,  which  would  give 
the  phase  difference  between  the  carborundum  cycle  and  the 
corresponding  voltage-phase  '  cycle,  would  be  a  quantity  just 
perceptible  on  the  oscillogram,  as  was  verified  in  the  original 
photographs. 

In  conclusion  of  this  discussion  of  the  oscillograms,  I  should  say 
that  we  have  not  been  able  to  detect  in  the  photographs  any 
departure  in  amplitude  or  in  phase  between  the  rectified  cycle 
and  the  voltage-phase  cycle  that  is  not  accounted  for  by  the  in- 
ductance and  resistance  of  the  oscillographic  apparatus  or  by  the 
current-voltage  curves  of  the  rectifier. 

This  means  that  if  there  are  any  terms  contingent  upon  heating 
or  other  effects  which  involve  an  integral  of  a  function  of  the 
current  with  respect  to  the  time,  this  integral  attains  its  final 
value  in  a  time  within  the  limit  of  error  of  measuring  the  oscillo- 
grams, which  is  about  1/6000  second.  This  result  contrasts  with 


DETECTORS  —  CRYSTAL  RECTIFIERS 


189 


the  result  obtained  in  an  oscillographic  study  of  the  electrolytic 
detector,  where  an  integrative  action  was  discovered  (see  next 
chapter). 

THERMOELECTRIC  PROPERTIES  OF  MOLYBDENITE 

In  the  present  section  an  account  is  given  of  the  investigation  of 
the  thermoelectromotive  force  of  molybdenite  against  copper  and 
a  determination  of  the  temperature  coefficient  of  resistance  of 
molybdenite.  Apart  from  their  possible  bearing  on  the  action 
of  the  rectifier,  the  thermoelectric  properties  of  molybdenite  are  of 
interest  in  themselves. 

Thermoelectromotive  Force.  —  Five  specimens  were  mounted 
for  the  study  of  the  thermoelectromotive  force  of  molybdenite 
against  copper.  These  specimens  are  referred  to  as  "  A,"  "  B," 
"  C,"  "  D,"  and  "  E."  The  method  of  mounting  the  specimen  E 
is  shown  in  Fig.  129.  A  thin  sheet  of  molybdenite  .1  or  .2  mm. 


FIG.  129.     Apparatus  for  studying  thermoelectric  properties 
of  molybdenite. 

thick,  2  cm.  wide,  and  8  cm.  long,  was  cemented  between  two  glass 
microscope  slides  G  with  a  cement  made  of  water-glass  and  calcium 
carbonate.  The  molybdenite  was  then  copper-plated  over  a  small 
area  at  each  of  the  exposed  ends  MM,  and  to  these  copper-plated 
areas  were  soldered  copper  wires  .2  mm.  in  diameter,  so  as  to  form 


190 


WIRELESS  TELEGRAPHY 


thermal  junctions  with  the  molybdenite.  The  thermal  junctions 
and  the  ends  of  the  glass  mounting  were  inserted  into  two  brass 
vessels  for  containing  the  temperature  baths  of  oil.  The  joints 
between  the  brass  vessel  and  the  glass  mounting  were  made  tight 
with  the  cement  of  water-glass  and  calcium  carbonate.  The  oil 
baths  were  provided  with  stirrers  driven  by  a  motor.  One  of  the 
baths  was  kept  at  0°  C.,  and  the  other  bath  was  given  various 
temperatures  between  0  and  200°  C.  The  resulting  thermoelec- 
tromotive  force  was  measured  by  means  of  a  potentiometer  to 
which  the  copper  wires  LL  led.  The  results  for  the  specimen 
"E"  are  recorded  in  Table  VII  and  plotted  in  the  curve  of  Fig.  130. 


TABLE  VII 

THERMOELECTROMOTIVE  FORCE  OF  THE  COPPER-MOLYBDENITE 
COUPLE  "E,"  THE  COLD  JUNCTION  BEING  KEPT  AT  ZERO 


Temperature  of 
Hot  Junction. 

E.M.F.  in 
Millivolts. 

Temperature  of 
Hot  Junction. 

E.M.F.  in 

Millivolts. 

10.1 

-    7.5 

99.2 

-    68.4 

14.3 

-10.7 

109.3 

-   75.2 

16.2 

-11.5 

111.6 

-   77.2 

18.7 

-13.8 

116.3 

-   79.2 

21.5 

-16.0 

118.7 

-   83.2 

24.1 

-17.6 

133.2 

-  90.7 

25.6 

-18.5 

141.9 

-   96.9 

33.1 

-24.6 

156.8 

-106.8 

36.2 

-25.9 

166.9 

-113.2 

41.9 

-31.5 

176.8 

-119.0 

51.1 

—  36.7 

179.0 

-120.0 

59.2 

-42.5 

180.9 

-121.5 

67.4 

-48.6 

188.5 

-126.2 

70.8 

-51.2 

192.7 

-128.7 

76.0 

-54.1 

195.0 

-130.0 

80.8 

-57.2 

The  negative  sign  before  the  e.m.f.  in  the  second  and  fourth  columns 
of  Table  VII  indicates  that  this  specimen  of  molybdenite  is  thermoelectri- 
cally  negative  with  respect  to  copper;  that  is  to  say,  the  current  at  the 
hot  junction  flows  from  the  molybdenite  to  copper. 


In  a  similar  way  the  other  four  specimens  "  A,"  "  B,"  "  C,"  and 
"D"  gave  the  values  recorded  in  Table  VIII  and  plotted  in  Fig. 
131.  For  the  purposes  of  comparison  a  part  of  the  curve  obtained 
for  "  E  "  is  also  plotted  in  Fig.  131. 


DETECTORS  —  CRYSTAIy  RECTIFIERS 


191 


Temperature 

"      8      S      £       8 


5 
8 

g 
3 

s 

g 

i 

\ 

X 

s 

\ 

\ 

\ 

\ 

\ 

Si 

\s 

\ 

/ 

\ 

. 

\ 

\ 

\. 

FIG.  130.     Curve  of  thermoelectromotive  force  of  molybdenite  (specimen  E) 
against  copper,  for  various  temperatures  of  the  hot  junction. 


20 


10 


o 
^  0 


10 


40     50      60     70     80 
Temperature 


\ 


NJ 


\ 


FIG.  131.     Thermoelectric  curves  of  various  specimens 
of  molybdenite  against  copper. 


192 


WIRELESS  TELEGRAPHY 


Some  of  the  specimens  (B,  D,  and  E)  are  thermoelectrically 
negative  with  respect  to  copper,  while  the  other  specimens  (A  and 
C)  are  thermoelectrically  positive  with  respect  to  copper.  The 
thermoelectromotive  force  per  degree  differs  largely  with  the 
different  specimens,  as  may  be  seen  by  a  reference  to  Table  IX, 
which  contains  the  thermoelectromotive  force  per  degree  of  the 
different  specimens  of  molybdenite  against  copper  and  against  lead 
(obtained  from  the  known  value  of  the  lead-copper  junction). 
For  comparison  Table  IX  also  gives  the  thermoelectromotive  power 
of  some  other  remarkable  thermoelectric  .elements. 

The  comparison  shows  that  these  specimens  of  molybdenite  have 
very  large  thermoelectromotive  force  against  copper  or  against 
lead.  The  specimens  D  and  E  were  found  to  be  at  the  extreme 
negative  end  of  the  thermoelectric  series. 

The  great  variability  among  the  specimens  studied  may  be  due 
to  an  admixture  of  small  quantities  of  some  other  substance  with 
the  molybdenite,  or  it  may  be  due  to  structural  differences  from 
point  to  point  of  the  crystal.  The  differences  in  the  specimens 
could  not  have  arisen  from  the  copper-plating  or  from  the  heat 
employed  in  soldering  the  junctions,  because  the  specimens  A,  B, 
C,  and  D  were  tested  before  the  copper-plating  and  soldering  was 
done,  and  by  means  of  the  preliminary  test  were  classified  as  posi- 
tive, negative,  positive  and  negative  respectively,  which  agrees 
with  the  determination  after  soldering. 


TABLE  VIII 

MOLYBDENITE-COPPER  JUNCTIONS  A,  B,  C,  D.  THE  COLD  JUNCTION  WAS 
AT  20°  C.  THE  HOT  JUNCTION  WAS  AT  TEMPERATURE  T°  C.  THE 
THERMOELECTROMOTIVE  FORCE  V  IS  IN  MILLIVOLTS 


Junction  A. 

Junction  B. 

Junction  C. 

Junction  D. 

T. 

V. 

T. 

V. 

T. 

V. 

T. 

V. 

31.9 

1.45 

31.6 

-    2.70 

31.7 

2.01 

31.6 

-   4.81 

53.5 

4.63 

54.1 

-    9.21 

55.2 

7.20 

57.5 

-17.9 

76.6 

8.21 

80.0 

-17.1 

87.2 

14.9 

59.8 

-19.4 

89.4 

10.4 

87.4 

-20.0 

94.4 

16.6 

86.7 

-33.7 

97.1 

11.5 

95.3 

-24.2 

The  preliminary  test  was  made  by  touching  the  specimens  with 
two  copper  wires  attached  respectively  to  the  two  terminals  of  a 
galvanometer,  one  of  the  wires  being  slightly  warmer  than  the 


DETECTORS  —  CRYSTAL  RECTIFIERS 


193 


other.  This  preliminary  test  proved  very  interesting  in  that  it  showed 
that  one  may  find  all  over  many  of  the  pieces  cut  from  a  crystal  of 
molybdenite  points  where  the  substance  is  thermoelectrically  positive 
and  other  points  where  it  is  thermoelectrically  negative.  These  posi- 
tive and  negative  points  sometimes  lie  so  near  together  that  with  a 
fine-pointed  exploring  electrode  attached  to  a  galvanometer  and 
warmed  by  heat  conducted  from  the  hand 'one  may  find  the  deflec- 
tions of  the  galvanometer  reversed  from  large  positive  values  to 
large  negative  values  on  making  the  slightest  possible  motion  of 
the  pointer  over  the  crystal. 

Explorations  of  this  kind  failed  to  show  any  definite  orientation 
of  the  thermoelectric  quality  with  respect  to  the  crystallographic 
axes. 

The  existence  of  small  thermoelectrically  positive  and  negative 
patches  in  a  piece  of  the  molybdenite  may  indicate  that  the  ther- 
moelectromotive  force  measured  by  attaching  wires  to  the  speci- 
men is  too  low  on  account  of  the  inclusion  under  the  electrodes  of 
both  positive  and  negative  areas  which  would  partially  neutralize 
the  thermoelectric  action  against  another  electrode. 


TABLE  IX 


Substance. 

Thermoelectromotive  Force  in  Mi- 
crovolts, per  Degree  Centigrade, 
at  20°  C. 

Authority. 

Against  Copper. 

Against  Lead. 

Molybdenite  A  .... 
B.... 
C  .... 
D.... 
E.... 
Silicon  

110 
-230 
175 
-415 
-720 

113 
-227 
178 
-413 
-717 
-400 
-   89 
26 
502 
807 

Present  experiment 

}> 
» 
» 

Frances  G.  Wick1 

Matthiessen2 

» 

}> 
>t 

Bismuth 

Antimony    
Tellurium        

Selenium    

1  Phys.  Rev.,  25,  390.     2  Everett,  Units  and  Physical  Constants. 


It  may  be  said  in  passing  that  the  specimens  D  and  E,  with 
soldered  connections,  still  showed  the  phenomenon  of  rectification 
when  used  with  alternating  currents,  even  when  the  two  junctions 
of  the  copper  with  the  molybdenite  were  in  oil  baths  at  the  same 


194  WIRELESS  TELEGRAPHY 

temperature  as  the  room  and  the  oil  in  the  baths  was  vigorously 
stirred  with  motor-driven  stirrers.  The  rectification  in  this  case 
was,  however,  very  imperfect. 

Temperature  Coefficient  of  Resistance.  —  Another  interesting 
thermal  property  of  the  molybdenite  is  its  temperature  coefficient 
of  resistance.  A  brief  report  of  this  coefficient  is  here  given. 

Two  specimens  of  the  molybdenite  were  made  into  the  form  of 
resistance  thermometers  by  depositing  heavy  copper-plated  areas 
near  the  two  ends  of  thin  pieces  of  the  molybdenite  and  soldering 
thin  copper  strips  to  the  copper  plate.  For  insulation  a  thin  strip 
of  mica  was  placed  over  the  molybdenite,  and  one  of  the  copper 
leads  was  bent  back  over  the  mica  so  that  both  leads  ran  away 
parallel  with  the  mica  insulation  between.  The  whole  conductor 
was  then  placed  between  two  mica  strips  and  inserted  in  a  flat- 
tened brass  tube.  The  tube  was  then  mashed  tight  together  so  as 
to  clamp  securely  the  molybdenite  and  its  leads.  The  end  of  the 
tube  adjacent  to  the  molybdenite  was  soldered  up.  The  leads 
were  brought  out  at  the  other  end  of  the  tube  and  connected  to 
binding  posts  insulated  by  a  hard-rubber  head  from  the  tube. 

The  two  molybdenite  resistances  thus  mounted  are  called  No.  50 
and  No.  51.  The  molybdenite  in  No.  51  was  .65  cm.  wide  by 
.7  cm.  long;  the  thickness  was  about  .3  mm. 

The  resistances  of  these,  two  conductors  were  measured  at  vari- 
ous temperatures  with  the  aid  of  a  Wheatstone  bridge.  They 
showed  no  evidence  of  rectification.  The  values  plotted  in  Fig. 
132  were  obtained.  The  curves  marked  "  50  "  and  "  51  "  give 
the  resistances  of  No.  50  and  No.  51  respectively.  The  ordinates 
for  these  curves  are  at  the  left  margin  of  the  diagram,  and  are  in 
ohms.  The  curves  "  C  50  "  and  "  C  51  "are  for  the  reciprocals 
of  the  resistance  of  No.  50  and  No.  51  respectively.  The  ordinates 
for  these  curves  are  at  the  right-hand  margin  of  the  diagram. 

Each  of  the  specimens  has  a  large  negative  temperature  coeffi- 
cient of  resistance.  With  No.  50,  for  example,  the  resistance  at 
93.1°  C.  is  229  ohms;  at  0°  C.  the  resistance  is  561  ohms;  at  -  76° 
the  resistance  is  3051  ohms;  and  at  the  temperature  of  liquid  air 
the  resistance  of  this  specimen  was  found  to  be  over  6,000,000 
ohms.  This  last  value  is  not  plotted  on  the  curves. 

It  is  interesting  to  note  that  between  —  15°  and  93°  the  temperature- 
conductance  curve  of  each  of  the  specimens  is  a  straight  line. 

At  0°  C.  the  resistance  of  each  of  the  specimens  decreases  about 
1.53  percent  per  degree  centigrade  increase  of  temperature;  at 


DETECTORS  —  CRYSTAL  RECTIFIERS 


195 


20°  the  decrease  of  resistance  per  degree  increase  of  temperature 
is  1.19  percent. 

Plausibility  of  Thermoelectric  Explanation.  —  The  large  thermo- 
electromotive  force  of  the  molybdenite  against  the  common  metals, 
together  with  its  large  negative  temperature  coefficient  of  resist- 
ance, lends  plausibility  to  the  hypothesis  that  the  rectification 
is  due  to  thermoelectricity.  For  if  we  pass  an  electric  current 
through  the  rectifier  and  the  current  begins  to  make  its  way 


-70   -60   -50  -40   -30   -20   -10     0     10     20 
Temperature 


40    50     60     70    80    90 


FIG.  132.     Resistance  and  conductance  of  molybdenite  as  a  function 
of  the  temperature. 


through  a  small  area  at  the  contact,  this  small  area  is  heated  and 
decreases  in  resistance,  so  that  the  greater  part  of  the  current  flows 
through  this  particular  small  area,  heating  it  still  more,  while  the 
portions  of  the  contact  through  which  the  current  has  not  started 
remain  cool  and  continue  to  offer  a  high  resistance.  The  effect 
of  this  action  is  to  confine  the  heating  to  an  extremely  small  area, 
which  is  the  condition  necessary  for  the  extremely  rapid  and 
efficient  action  of  the  rectifier,  on  the  hypothesis  of  a  thermo- 
electric explanation.  That  there  is,  however,  an  insuperable  ob- 


196 


WIRELESS  TELEGRAPHY 


jection  to  this  explanation  of  the  phenomenon  is,  I  think,  made 
clear  in  the  succeeding  experiments,  in  which  it  is  shown  that 
the  thermoelectric  effect  is  often  opposite  to  the  rectification, 
and  that  the  amount  of  heat  associated  with  the  rectification 
accounts  for  less  than  sWoo^  of  the  rectified  current  as  thermo- 
electric. 


EXPERIMENTAL   FACTS  ADVERSE   TO   THE   THERMOELECTRIC   EXPLA- 
NATION  OF   THE   PHENOMENON   OF   RECTIFICATION 

Thermoelectric  Effect  Opposite  to  the  Rectification.  —  A  num- 
ber of  experiments  with  different  specimens  of  molybdenite  were 
made  in  which  the  rectification  and  the  thermoelectric  effect 
could  be  simultaneously  studied.  A  diagram  of  the  arrangement 
of  apparatus  is  given  in  Figure  133.  The  specimen  of  motybdenite 

is  shown  at  M,  and  was  held  down 
upon  a  wooden  base  by  a  spring 
clip.  One  end  of  each  of  the 
specimens,  which  were  easily  in- 
terchangeable in  the  apparatus, 
was  electroplated  with  copper  at 
S.  To  this  copper-plated  area  a 
copper  lead  was  soldered.  A 
copper  rod  C,  supported  as  in 
Figure  124,  was  brought  into  con- 
tact with  the  part  of  the  molyb- 
denite distant  from  the  soldered 
junction.  The  molybdenite  and 
FIG.  133  Apparatus  for  compar-  the  contact  were  put  in  an  elec- 
mg  rectified  current  with  thermo-  ...  .  . 

electric  effect.  trie   circuit   containing  a  galva- 

nometer at  A  and  a  source  of 

variable  alternating  potential  at  V.  The  alternating  potential  V 
could  be  applied  or  omitted  by  closing  or  opening  the  switch  at  T. 
A  small  heating  coil  was  wound  on  the  rod  C,  and  another  similar 
heating  coil  was  wound  on  a  second  copper  rod  E  placed  im- 
mediately below  the  contact  of  C  with  M . 

An  auxiliary  thermal  junction  formed  by  a  small  constantan 

wire  attached  to  the  lower  end  of  the  copper  rod  C  was  connected 

to  a  second  galvanometer  shown  at  G,  for  use  in  a  later  experiment. 

The  copper  rods  C  or  E  could  be  heated  by  the  surrounding 

coils,  and  the  thermal  current  in  the  circuit  through  the  molyb- 


DETECTORS  —  CRYSTAL  RECTIFIERS 


197 


denite  or  the  circuit  through  the  constantan  could  be  read  on  the 
galvanometers  A  or  G.  Also  the  rectified  current  obtained  by 
applying  the  alternating  voltage  V  could  be  read  on  the  galva- 
nometer A.  When  the  thermal  current  or  the  rectified  current 
through  A  is  in  the  direction  of  the  arrow  B,  the  molybdenite, 
following  the  usage  in  thermoelectricity,  is  said  to  be  positive. 
When  the  current  in  A  is  in  the  direction  "opposite  to  the  arrow  B, 
the  molybdenite  is  said  to  be  negative. 

The  results  obtained  with  a  number  of  specimens  of  molybdenite 
when  heat  was  applied  above,  and  when  heat  was  applied  below, 
and  when  the  alternating  voltage  was  applied,  are  contained  in 
Table  X. 

TABLE  X 

SIGN    OF    MOLYBDENITE   WHEN    HEATED    ABOVE    OR    BELOW 
AND    WHEN    SUBJECTED    TO   ALTERNATING    VOLTAGE 


Specimen  No. 

Heated  Above. 

Heated  Below. 

Under  Alternat- 
ing Voltage. 

75 

+ 

81 

_j_ 





Turned   over 

+ 

_ 

_ 

93 

— 

_j_ 

_|_ 

Another  point 

— 

— 

+ 

^ 

— 

— 

•f 

Turned  over 

_ 

_ 

78 

_|_ 

_j_ 

_j_ 

Another  point 

+ 

- 

- 

94 

_ 

1 

+ 

Another  point 

- 

+ 

+ 

From  this  table  it  appears  that  the  thermoelectric  voltage  when 
the  junction  is  heated  by  heat  conducted  from  above,  in  twelve  out 
of  the  thirteen  cases  tried,  is  opposite  to  the  direct  voltage  ob- 
tained when  an  alternating  current  is  passed  through  the  junction. 
When  the  heat  is  conducted  to  the  junction  from  below,  through  the 
molybdenite,  the  thermoelectromotive  force  in  four  cases  is  opposite 
to  the  rectified  voltage,  and  in  nine  cases  is  in  the  same  direction 
as  the  rectified  voltage.  In  only  one  case,  one  point  of  No.  78, 
is  the  rectified  voltage  in  the  same  direction  as  the  thermal  voltage, 
both  when  the  junction  is  heated  from  above  and  when  it  is 
heated  from  below. 

In  all  of  these  cases  the  heat  was  applied  in  the  neighborhood  of 
the  same  junction,  and  there  is  no  opportunity  for  heat  to  get  to 


198  WIRELESS  TELEGRAPHY 

the  other  junction  (copper-plated  and  soldered)  by  conduction, 
on  account  of  the  great  distance  of  the  other  junction  from  the 
source  of  heat.  To  make  this  absolutely  certain  this  distant 
junction  was  in  some  cases  submerged  in  an  oil  bath. 

So  far  as  I  have  been  able  to  learn,  this  phenomenon  of  the 
reversal  of  the  thermoelectro motive  force  at  a  thermal  junction, 
conditioned  on  whether  the  heat  is  conducted  to  the  junction 
through  one  element  of  the  junction  or  the  other  element  of  the 
junction,  is  novel.  It  may  be  explained  by  the  assumption  of 
another  thermal  junction  of  opposite  sign  in  the  molybdenite 
itself  below  and  in  the  immediate  neighborhood  of  the  copper- 
molybdenite  junction.  This  assumption  is  plausible  because  it 
has  been  shown  above  that  the  molybdenite  with  which  these 
experiments  were  performed  is  thermoelectrically  an  extremely 
heterogeneous  substance. 

However,  whatever  the  explanation  of  the  dependence  of  the 
sign  of  the  thermoelectromotive  force  on  the  manner  of  applying 
the  heat,  it  is  seen  that  the  thermoelectric  effect  is  usually  opposite 
in  sign  1  to  the  rectified  effect. 

By  applying  heat  from,  above  and  at  the  same  time  applying  the 
alternating  voltage,  one  can  make  the  thermal  current  and  the 
rectified  current  neutralize  each  other.  This  opposition  of  sign 
of  the  rectified  current  and  the  thermal  current  renders  the  correct- 
ness of  the  thermoelectric  explanation  of  the  phenomenon  of 
rectification  extremely  improbable. 

Insufficient  Heating  of  the  Contact  to  Account  for  Rectification. 
—  The  most  convincing  experiment  on  the  subject  is  the  follow- 
ing: With  the  aid  of  the  auxiliary  thermal  junction  of  copper-con- 
stantan  placed  at  the  contact  of  the  copper  with  the  molybdenite, 
as  shown  in  Fig.  133, it  was  possible  to  look  for  a  rise  of  temperature 
of  the  copper  molybdenite  junction  by  the  alternating  current 
which  was  being  rectified.  If  any  appreciable  heat  were  developed 
at  the  molybdenite  copper  junction,  the  copper-constantan  junc- 
tion ought  to  show  it.  The  following  result  was  obtained: 

When  the  rectified  current  was  118  microamperes,  the  heating 
shown  by  the  copper-constantan  junction  did  not  exceed  .01°  C. 

1  In  the  case  of  silicon-steel,  carbon-steel,  and  tellurium-aluminum,  L.  W. 
Austin  has  found  that  the  rectified  current  generally  flows  in  opposite  direc- 
tion to  that  produced  by  heating  the  junction.  In  his  experiments  (Bulletin 
of  the  Bureau  of  Standards,  5,  No.  1,  August,  1908)  the  heat  was  applied  by 
conduction  from  above  only. 


DETECTORS  —  CRYSTAL  RECTIFIERS  199 

When,  on  the  other  hand,  as  a  control  experiment,  heat  was  applied 
to  the  copper-molybdenite  junction  from  below  so  that  it  had  to  be 
conducted  through  the  molybdenite  and  through  the  copper-molyb- 
denite junction  to  get  to  the  copper-constantan  junction,  the 
heating  shown  by  the  auxiliary  copper-constantan  junction  was 
11.4°C.,  while  the  thermal  current  from  the  copper-molybdenite 
junction  was  only  .2  microamperes.  In  both  the  case  of  the  recti- 
fied current  and  the  case  of  the  application  of  heat  from  below 
the  heat  had  to  be  conducted  from  the  point  of  rectification  to  the 
auxiliary  junction.  Therefore,  with  a  rise  of  temperature  of  the 
auxiliary  junction  1100  times  as  great  as  the  rise  shown  during 
the  rectification,  the  thermal  current  in  the  copper-molybdenite 
circuit  was  5<ro  of  the  rectified  current;  that  is  to  say,  the  rectified 
current,  for  a  rise  of  temperature  of  T<hy  of  a  degree  of  the  auxiliary 
junction  (being  approximately  a  linear  function  of  the  tempera- 
ture) was  less  than  sWoou  of  the  rectified  current  from  an  alter- 
nating current  producing  the  same  rise  of  temperature. 

Summary  of  Conclusions  from  the  Experiments  with  the 
Crystal  Rectifiers.  —  1.  An  examination  of  the  characteristics  of 
contact  detectors  using  carborundum,  anatase,  brookite,  hessite, 
iron  pyrites,  and  silicon  shows  that  we  are  dealing  with  the  same 
kind  of  phenomenon  in  the  case  of  all  these  crystal  substances. 
The  various  other  crystal-contact  detectors  which  I  have  not 
examined  probably  act  in  the  same  way. 

2.  At  the  contact  between  the  crystal  and  a  common  metal, 
or  between  two  different  crystals,  or  between  two  apparently  simi- 
lar crystals,  there  is  asymmetric  conductivity,  permitting  a  much 
greater  current  to  flow  in  one  direction  than  in  the  other  under 
the  same  applied  voltage. 

3.  These  contacts  all   have   a   rising  current-voltage  charac- 
teristic. 

4.  These  crystals  all  have  a  large  thermoelectromotive  force 
against  the  common  metals,  and  the  amount  and  the  direction  of 
this  thermoelectromotive  force  is  different  at  different  points  on 
the  crystalline  bodies. 

5.  The  rectifying  effect  is  also  different  in  amount  and  direction 
at  different  points  of  the  crystalline  body;  the  direction  of  the 
rectifying  effect  is  often  opposite  to  the  effect  that  would  be 
obtained  by  heating  the  contact. 

6.  Thermoelectricity  does  not  explain  the  phenomenon  of  rec- 
tification, but  the  two  effects,  since  both  exist  in  such   marked 


200  WIRELESS  TELEGRAPHY 

degree  in  the  same  bodies,  may  be  related  in  that  both  may  have 
their  seat  in  some  common  property  of  the  materials  employed. 
For  example,  if  we  suppose  that  a  surface  of  separation  between  the 
crystalline  body  and  some  other  body  permits  the  passage  of  electrons 
more  easily  in  one  direction  than  in  the  other,  this  would  account  for 
the  rectifying  effect,  and  would  also  account  for  the  thermoelectric 
effect,  provided  the  velocity  of  the  electrons  is  suitably  different  at 
different  temperatures. 

7.  The  thermoelectric  explanation  of  the  rectifying  effect,  if 
we  had  found  it  to  be  supported  by  the  experiments,  would  have 
correlated  the  phenomenon  of  rectification  at  a  solid  contact  with 
the  body  of  information  that  we  already  have  in  regard  to  thermo- 
electricity, but  we  should  still  have  had  by  no  means  a  complete 
knowledge  of  the  action,   because  our  understanding  of  thermo- 
electricity is  very  incomplete. 

8.  From  experiments  with  thermoelectricity  we  are  familiar 
with  the  fact  that  the  energy  of  an  oscillatory  electric  current 
passing  through  a  high-resistance  contact  is  partially  converted 
into  heat  energy,  and  that  the  heat  energy  so  obtained,  if  produced 
at  a  thermal  junction,  is  again  partially  converted  into  electric 
energy  manifesting  itself  as  a  direct  current.     It  is  perhaps,  after 
all,  more  simple  to  suppose  the  alternating  current  to  be  converted 
into  direct  current  without  the  intermediation  of  heat;  and  this 
seems  to  be  the  case  with  the  crystal-contact  rectifiers.     This 
result  opens  up  a  new  field  for  investigation,  which  may  contribute 
to  a  better  understanding,  not  only  of  thermal  electricity,  but  of 
the  much  larger  question  of  the  mechanism  of  electrical  conduc- 
tivity in  solid  bodies. 


CHAPTER  XIX 

ON  DETECTORS   (Concluded) 
THE    ELECTROLYTIC    DETECTOR,    AND  VACUUM    DETECTORS 

Description  of  the  Electrolytic  Detector.  -  -  The  electrolytic 
detector  for  electric  waves,  as  described  by  Fessenden  1  and  shortly 
after  by  Schloemilch,2  consists  of  a  cell  containing  an  electrolyte 
and  having  one  electrode  of  very  small  area,  usually  in  the  form  of 
an  extremely  fine  wire  of  platinum,  and  as  the  other  electrode  a 
larger  area  of  platinum  or  some  other  metal.  When  used  in  wire- 
less telegraphy  the  two  electrodes  are  connected  in  a  circuit  upon 
which  the  electric  oscillations  are  impressed,  so  that  the  rapidly 
oscillating  electric  currents  in  the  circuit  are  made  to  traverse  the 
cell  of  the  detector.  An  example  of  a  simple  form  of  receiving 
circuit,  with  the  detector  connected  in  the 
antenna,  is  shown  at  MDG  of  Fig.  134.  A 
local  circuit  TED,  through  the  detector,  con- 
tains a  telephone  receiver  T  and  an  adjustable 
source  of  e.m.f.,  which  is  used  to  polarize  the 
detector  by  sending  through  it  and  the  tele- 
phone a  small  direct  current.  Under  the 
action  of  the  electric  oscillations  through  the 
detector  the  current  in  the  telephone  receiver 
is  modified  so  as  to  produce  a  sound  in  the 
telephone  with  a  period  determined  by  the 
train  frequency  of  the  incident  electric  waves.  — 

The  action  is  localized  at  the  contact  of  the  Fl,G-  13f  Circuit  with 

electrolytic  detector, 
rme  wire  with  the  electrolyte. 

Details  of  the  Electrolytic  Detector.  —  The  electrolyte  employed 
in  the  electrolytic  detector  is  usually  20%  nitric  acid,  though 
almost  any  electrolytically  conductive  liquid  (e.g.,  dilute  sulphuric 
acid,  common  salt  solution,  caustic  soda,  etc.)  may  be  used.  For 
a  highly  sensitive  detector  the  fine  platinum  wire  employed  as 

1  Fessenden,  U.  S.  Patent,  No.  727,331,  filed  April  9,  1903;  issued  May  5, 
1903. 

2  Schloemilch,  Elektrotechnische  Zeitschrift,  Vol.  24,  p.  959,  Nov.  19,  1903. 

201 


=€> 

D 


202 


WIRELESS  TELEGRAPHY 


the  sensitive  "  point  "  may  be  as  small  as  one  or  two  ten-thou- 
sandths of  an  inch  in  diameter.  For  a  less  sensitive  detector, 
which  is  not  so  likely  to  be  destroyed  by  strong  signals,  wire  as 
large  as  one-thousandth  of  an  inch  or  even  larger  may  be  used. 

Only  a  very  short  length  of  the  fine  wire  comes  into  contact  with 
the  electrolyte;  for  the  fine  wire  is  either  sealed  into  a  glass  tube 
so  as  to  protect  all  but  the  mere  end  of  the  wire  from  contact  with 
the  electrolyte,  or  the  fine  silver-coated  platinum  wire,  as  at  W, 


FIG.  135.    Professor  Pupin's 
electrolytic  rectifier. 


FIG.  136.     Electrolytic  detector  with  adjust- 
able contact. 


Fig.  136,  is  carried  up  or  down  by  a  micrometer  adjustment  so  as 
to  bring  it  into  contact  with  the  electrolyte.  The  silver  is  removed 
from  about  TV  of  an  inch  of  length  of  the  wire  by  submerging  it  in 
the  electrolyte,  which  is  in  this  case  nitric  acid,  and  sending  a 
current  from  the  local  battery  through  it  for  a  few  minutes,  with 
the  small  wire  as  anode.  This  takes  off  the  silver,  leaving  the  bare 
platinum.  The  point,  so  formed,  is  then  withdrawn  by  a  motion 
of  the  screw  B,  until  only  a  very  minute  area  of  the  bare  platinum 
wire  is  left  in  contact  with  the  electrolyte. 

The  detector  in  this  form  is  used  with  an  adjustable  source  of 
e.m.f.  in  its  local  circuit.  The  fine  platinum  electrode  may  be 
connected  either  to  the  positive  or  the  negative  terminal  of  the 
battery,  but  the  detector  is  usually  more  sensitive  when  this  fine 
platinum  electrode  is  positive  (i.e.,  anode).  The  voltage  in  the 
local  circuit  required  in  this  case  is  about  1.5  volts. 

Variation  in  which  Source  of  Polarizing  Voltage  is  Located  in 
the  Detector  Itself.  —  Instead  of  employing  an  external  voltage 
E  (Fig.  134)  to  polarize  the  detector,  a  similar  effect  can  be  ob- 
tained by  constituting  the  electrodes  in  the  detector  of  different 
metals,  one  of  which  (zinc,  say)  is  attacked  by  the  electrolyte,  and 
the  other  of  which,  the  fine  platinum  wire,  is  inert  to  the  action  of 
the  electrolyte. 


ELECTROLYTIC  AND  VACUUM  DETECTORS 


203 


This  makes  the  detector  itself  a  primary  battery. 

This  arrangement  for  which  a  United  States  patent  has  been 
issued  to  Schloemilch,1  and  also  to  Shoemaker,2  would  seem  to  be 
incapable  of  the  high  sensitiveness  attained  by  the  form  in  which 
the  accurately  adjustable  external  voltage,  as  in  Fig.  134,  is 
employed. 

Regarding  the  Theory  of  the  Electrolytic  Detector.  —  Con- 
siderable diversity  of  opinion  has  been  expressed  by  various  writers 
as  to  the  manner  in  which  the  electrolytic  detector  acts  as  a 
receiver  for  electric  waves.  Professor  Fessenden  in  his  original 
patent  attributes  the  action  to  heat,  and  he  calls  this  form  of 
detector  a  " liquid  barretter."  Pro- 
fessor Armagnat,3  who  has  made 
an  experimental  study  of  the  sub- 
ject, attributes  the  action  to  a 
rectifying  effect  resulting  from 
polarization.  Armagnat  obtained 
a  curve  of  the  form  of  Fig.  137 
for  the  current-voltage  character- 
istic of  the  electrolytic  detector. 
Dr.  L.  W.  Austin 4  also  found  that 
the  electrolytic  detector  acted  as 
a  rectifier  for  small  alternating 
currents,  but  came  to  the  opinion 
that  heat,  chemical  action,  rectifi- 
cation, and  electrostatic  .attraction  FlG< 
across  the  gas  film  might  have  a 
part  in  the  explanation  of  the  phenomenon  when  the  detector  was 
used  with  electric  waves. 

A  doubt  that  arose  in  the  minds  of  some  investigators  of  the 
subject  as  to  a  possible  explanation  of  the  phenomenon  in  terms 
of  rectification  alone  came,  it  seems,  from  the  idea  that  there 
could  not  be  energy  enough  in  the  electric  waves  received  at  great 
distances  to  produce  the  effects  in  any  other  way  than  by  a 
triggering  action,  by  which  the  local  energy  of  the  battery  was 

1  Wilhelm  Schloemilch,  U.  S.  Patent,  No.  936,258,  filed  Oct.  3,  1903,  issued 
Oct.  5,  1909. 

2  Harry  Shoemaker,  U.  S.  Patent,  No.  795,312,  filed  Feb.  13,  1905,  issued 
July  25,  1905. 

3  Armagnat,  Bui.  soc.  franchise,  session  of  April,  1906,  p.  205;  Journal  de 
Physique,  Vol.  5,  p.  748,  1906. 

4  Austin,  Bui.  Bureau  of  Standards,  Vol.  2,  p.  261,  1906. 


204  WIRELESS  TELEGRAPHY 

brought  prominently  into  play.  Now,  however,  since  some  of 
the  crystal  detectors,  that  act  entirely  without  any  local  source 
of  energy,  are  as  sensitive  as  the  electrolytic  detector,  we  see  that 
the  energy  to  produce  the  sounds  in  the  telephone  is  really 
present  in  the  incoming  waves,  and  does  produce  the  sounds  when 
the  incoming  energy  is  applied  to  the  telephone  receiver  with 
the  aid  of  a  suitable  rectifier. 

The  Electrolytic  Detector  as  a  Rectifier.  —  That  an  electrolytic 
cell  with  one  of  the  electrodes  small,  when  suitably  polarized  with 
a  direct  current,  is  a  rectifier  for  alternating  currents  was  first 
shown  by  Professor  M.  I.  Pupin  1  before  such  a  cell  came  into 
commercial  use  as  a  detector  for  electric  waves.  The  following 
account  of  Pupin's  rectifier  is  translated  from  an  article  published 
in  the  "  Jahrbuch  der  Elektrochemie,"  Vol.  6,  p.  35,  1899: 

"  In  Fig.  3  "  (here  reproduced  as  Fig.  135)  "  A  is  a  battery, 
B  an  electrolytic  cell  with  the  platinum  electrodes  a  and  b  and 
acidulated  water.  If  the  polarization  of  the  cell  B  is  as  great 
as  the  e.m.f.  of  A,  no  current  flows  in  the  circuit.  If  one  allows 
an  alternating  current  to  act  upon  the  circuit  ABC,  the  circuit 
contains  resistance,  self-inductance,  and  a  capacity  localized  in 
the  plates  a  and  b.  The  cell  B  acts,  however,  as  a  condenser  only 
so  long  as  the  potential  difference  of  the  plates  a  and  b  is  smaller 
than  the  decomposition  voltage.  If  this  value  is  exceeded,  a 
current  goes  through  the  circuit.  If  the  alternating  current,  for 
example,  has  an  amplitude  that  is  twice  as  great  as  the  e.m.f.  of 
A,  in  case  the  phase  has  the  same  direction  as  A  a  current  flows  in 
the  circuit,  e.g.,  in  the  direction  BC;  when  the  phase  is  oppositely 
directed,  the  condenser  B  sends  a  current  in  the  opposite  direction. 
This  last  can  be  diminished  by  making  the  capacity  of  B  very 
small.  If,  for  example,  the  area  of  one  of  the  electrodes  is  only 
one  square  millimeter,  one  may  easily  rectify  alternating  currents 
with  a  frequency  of  1000  per  second;  with  greater  frequency  the 
electrode  must  naturally  be  made  still  smaller.  It  is  best  to  em- 
ploy a  platinum  wire  sealed  into  glass  —  the  wire  being  cut  off 
immediately  at  the  end  of  the  glass.  The  author  (Pupin)  suc- 
ceeded in  rectifying  electric  oscillations  of  Hertzian  frequency 
and  producing  electrolytic  effects  with  them;  the  wire  for  this 
purpose  was  .025  mm.  in  diameter." 

1  Pupin,  Electrical  World,  Vol.  34,  p.  743,  1899;  Zeitsch.  f.  Elektrochemie, 
Vol.  6,  p.  349,  1899;  Jahrbuch  d.  Elektrochemie,  Vol.  6,  p.  35,  1899;  Bui. 
Am.  Phys.  Soc.,  Vol.  1,  p.  21,  1900. 


ELECTROLYTIC  AND  VACUUM  DETECTORS 


205 


This  quotation  shows  that  Pupin  had  employed  the  electrolytic 
detector  in  1899  as  a  rectifier  for  electric  waves  of  Hertzian  fre- 
quency, and  that  he  had  a  well-defined  explanation  of  the  processes 
occurring  in  the  rectifier.  I  have  made  some  experiments  that 
fall  into  close  agreement  with  Pupin' s  explanation  of  the  phenome- 
non. These  are  described  in  the  succeeding  paragraphs. 

OSCILLOGRAPHIC     STUDY    OF    THE    ELECTROLYTIC    DETECTOR1 

In  these  experiments  the  current  through  the  detector  under  the 
action  of  an  alternating  e.m.f.,  superposed  on  a  polarizing  current, 
is  determined  by  means  of  an  oscillograph.  The  application  of 
the  oscillograph  to  the  problem  gives  the  instantaneous  values  of 
the  current  through  the  detector,  and  permits  an  examination 
of  the  wave  form  of  the  rectified  cycle.  The  oscillographic 
apparatus  was  the  Braun's  tube  described  in  Chapter  XVIII. 

Circuits  Employed  with  the  Detector  in  Taking  the  Oscillo- 
grams.  —  The  electrolytic  detector  used  in  these  experiments  made 


FIG.  138.     Oscillographic  apparatus  and  circuits  for  study  of  electrolytic 

detector. 

use  of  a  platinum  point,  .0002  inch  in  diameter,  dipping  into  20 
per  cent  nitric  acid,  and  was  adjusted  to  high  sensitiveness  as  an 
electric  wave  detector  immediately  before  taking  the  oscillograms. 
A  diagram  of  the  circuits  employed  in  the  experiment,  together 
with  a  sketch  of  the  oscillographic  apparatus,  is  shown  in  Fig.  138. 

1  This  account  is  an  abridgment  of  an  article  by  the  author  on  "The 
Electrolytic  Detector,  Studied  with  the  Aid  of  an  Oscillograph."  Physical 
Review,  1909,  Vol.  28,  p.  56. 


206  WIRELESS  TELEGRAPHY 

The  detector  is  at  Z>,  and  is  connected  in  series  with  the  deflecting 
coils  MM  of  the  oscillograph  and  with  the  variable  sources  of 
voltage  V  and  E.  The  voltage  V  is  taken  from  a  potentiometer 
connected  with  the  60-cycle  alternating  mains  of  the  laboratory. 
E  is  an  adjustable  steady  voltage  taken  from  a  battery.  The 
voltage  at  E  could  be  reversed.  By  opening  the  switch  near  D  the 
electrolytic  detector  could  be  disconnected  from  the  circuit,  and 
by  throwing  this  switch  downward  an  ohmic  resistance  R  could 
be  substituted  for  the  detector. 

In  taking  the  oscillograms  of  Plate  II  the  following  steps  were 
employed:  The  drum  carrying  the  film  was  set  rotating.  The 
high  potential  current  was  started  in  the  tube.  The  chosen  value 
of  the  polarizing  current  was  applied  to  the  circuit  and  was  read 
on  a  direct-current  milliammeter.  The  alternating  current  was 
superposed  on  the  circuit,  and  by  adjustment  of  the  potentiometer 
at  V  the  voltage  of  this  alternating  current  was  given  any  desired 
value. 

The  Exposures.  —  After  the  preliminary  adjustment  of  the 
direct  and  alternating  currents  through  the  detector,  four  expo- 
sures were  made  on  each  picture,  while  the  film  was  being  carried 
around  continuously  by  the  synchronously  driven  drum. 

Axis  of  Zero  Current.  —  This  is  the  lower-  straight  line  across 
the  pictures,  and  was  obtained  by  an  exposure  of  20  seconds  taken 
with  the  circuit  open. 

Axis  of  Polarizing  Current.  —  This  is  the  upper  straight  line 
across  the  picture,  and  was  obtained  with  the  detector  in  circuit 
and  traversed  by  the  polarizing  current.  The  exposure  was  20 
seconds.  In  oscillogram  No.  1  this  axis  is  not  apparent,  because 
on  account  of  the  small  value  of  the  polarized  current  employed  it 
falls  into  coincidence  with  the  axis  of  zero  current. 

The  Rectified  Cycle.  —  This  cycle  may  be  identified  in  the  oscil- 
lograms as  a  positive l  loop  for  a  half-period,  followed  by  a  nearly 
straight  portion  lying  along  the  axis  of  zero  current  for  a  part  of  a 
half-cycle,  and  going  over  into  the  positive  loop  through  an  inter- 
mediate "  building  up  "  segment.  This  cycle  (exposure  of  60  sec.) 
was  taken  with  the  detector  in  circuit,  with  the  alternating  e.m.f. 
applied  to  the  circuit,  and  with  the  polarizing  current  also  flowing. 

The  Voltage-Phase  Cycle.  —  This  is  the  sine  curve  of  the  pic- 
tures, and  was  taken  in  order  to  obtain  the  e.m.f.  immediately 

1  In  describing  the  oscillograms,  values  above  the  axis  of  zero  current  are 
called  positive;  values  below  this  axis  are  called  negative. 


ELECTROLYTIC  AND  VACUUM  DETECTORS     207 

about  the  detector.1  A  similar  curve  was  made  use  of  in  the 
experiments  of  the  preceding  chapter  and  is  there  discussed.  In 
the  present  experiments,  because  of  the  employment  of  the  polar- 
izing current  with  the  rectifier,  a  question  arises  as  to  the  appro- 
priate method  of  taking  this  cycle.  Two  different  methods  were 
tried,  either  of  which,  by  proper  elimination  of  the  constants  of  the 
oscillogr^phic  apparatus,  will  give  the  desired  result.  The  method 
yielding  simplest  results  for  the  voltage-phase  cycle  is  the  following : 
After  the  exposure  for  the  rectified  cycle  had  been  made,  the  alter- 
nating voltage  was  left  unchanged,  and  a  resistance  was  substituted 
for  the  rectifier.  A  double  adjustment  of  the  substituted  resist- 
ance and  the  direct  voltage  was  made  by  successive  approxima- 
tions until  the  result  was  attained  that  (1)  the  direct  voltage  alone 
gave  through  the  substituted  resistance  a  current  equal  to  that 
used  in  polarizing  the  rectifier  and  (2)  the  alternating  voltage 
superposed  on  this  direct  current  gave  a  deflection  of  the  lumi- 
nescent spot  to  a  point  coincident  with  the  maximum  point  attained 
with  the  rectifier  in  the  circuit.  This  means  that  the  voltage- 
phase  cycle  was  taken  with  the  axis  of  polarizing  current  as  axis, 
and  with  amplitude  equal  to  the  maximum  amplitude  of  the  recti- 
fied cycle.  This  method  was  employed  in  oscillograms  1,  2,  and  5. 

The  second  method  of  taking  the  voltage-phase  cycle  was  as 
follows:  The  polarizing  voltage  was  reduced  to  zero,  the  detector 
was  short-circuited,  and  an  alternating  voltage  equal  to  that  used 
with  the  detector  was  applied  to  the  circuit.  This  method  was 
employed  in  oscillograms  3  and  4. 

Coordinates  of  the  Oscillographic  Curves.  —  In  taking  all  of 
the  curves  of  the  oscillograms,  the  motion  of  the  light  spot  over 
the  film  is  from  left  to  right;  the  time  coordinate  is,  therefore,  the 
horizontal  scale  of  the  curves  and  is  drawn  as  usual  from  left  to 
right.  The  current  coordinate  is  given  in  the  scale  drawn  in  ink 
at  the  left-hand  margin  of  each  picture  —  one  division  being  one 
milliampere. 

DISCUSSION  OF    THE    OSCILLOGRAMS    OF    PLATE    II 

The  oscillograms  shown  in  Plate  II  are  reproductions  of  positives 
printed  from  the  films  carried  by  the  rotating  drum.  They  were 
taken  with  a  60-cycle  alternating  current  applied  to  the  circuit 

1  The  ordinary  method,  which  would  be  to  take  the  leads  from  the  two  sides 
of  the  detector  through  a  high  resistance  to  the  oscillograph,  could  not  be  used 
because  the  oscillograph  was  working  at  the  limit  of  its  sensitiveness  on  the 
full  voltage  without  the  added  resistance. 


(208) 


PLATE  II.     G.  W.     Pierce,  The  Electrolytic  Detector. 


ELECTROLYTIC  AND  VACUUM  DETECTORS 


209 


containing  the  electrolytic  detector.  The  reproduction  is  one- 
third  the  size  of  the  original.  The  several  curves  shown  in  the 
plate  were  obtained  with  different  polarizing  currents  superposed 
on  the  circuit.  Table  XI  contains  a  tabulation  of  the  polarizing 
current  and  voltage,  the  applied  alternating  voltage,  the  maximum 
current  through  the  detector,  and  the  substituted  resistance 
employed  in  taking  the  voltage  curve.  e 


TABLE  XI 

TABULAR  DESCRIPTION   OF  THE  OSCILLOGRAPHIC  RECORDS 


Maximum 

No. 

Polarizing  Direct 
Current  in  Milli- 

Polarizing  E.M.F. 
in  Volts 

R.M.S. 
Volts  A.C. 

Positive  Cur- 
rent through 

Equivalent 
Resistance 

araperes. 

Detector   in 

in  Ohms. 

Milliamperes. 

I1 

.1 

1.45 

2.09 

2.37 

4401 

2 

1.0 

5.5 

4.00 

9.6 

70 

32 

1.2 

5.5 

4.00 

9.6 

OO2 

42 

1.4 

Not  measured 

5.00 

10.0 

OO2 

5 

2.2 

» 

5.00 

11.0 

150 

1  It  should  be  noticed  that  the  sensitiveness  of  the  oscillograph  when  No.  1 
was  taken  was  three  times  as  great  as  when  the  other  oscillograms  of  the  plate 
were  taken. 

2  The  voltage-phase  cycle  of  oscillograms  3  and  4  was  taken  with  the  polar- 
izing current  omitted,  so  that  they  have  the  axis  of  no  current  as  axis  of  the 
cycle. 

Point  Anode  or  Cathode  —  the  Large  Loop  in  the  Direction  of 
the  Polarizing  Current.  —  Some  of  the  oscillograms  were  taken 
with  the  polarizing  current  from  the  point  to  the  electrolyte  and 
some  with  the  polarizing  current  in  the  opposite  direction.  Al- 
though the  values  of  the  polarizing  voltage  required  to  produce  a 
given  polarizing  current  were  different  in  the  two  cases  the  general 
characteristics  of  the  cycle  were  the  same.  A  reversal  of  the  polar- 
izing current  reversed  the  rectified  current,  and  whether  the  polar- 
izing current  was  from  the  point  to  electrolyte  or  in  the  opposite 
direction  the  large  loop  of  the  rectified  cycle  (always  oscillographed 
positively)  was  obtained  when  the  alternating  current  was  flowing 
in  the  same  direction  as  the  polarizing  current. 

The  Form  of  the  Rectified  Cycle.  —  The  cycle  obtained  with  the 
rectifier  in  the  circuit  has  the  same  general  form  in  all  the  pictures. 
When  the  current,  having  traversed  the  positive  loop,  comes  to 
the  axis  of  zero  current,  it  follows  along  this  axis  for  a  short  way, 


210 


WIRELESS  TELEGRAPHY 


then  takes  a  small  negative  dip,  becomes  positive  again,  follows 
along  just  above  the  axis  of  zero  current  for  a  short  time,  and  then 
rises  along  a  transition  curve  to  the  positive  loop. 

Calculations  Concerning  the  Form  of  the  Cycle.  —  The  rectified 
cycle,  when  examined  by  comparison  with  the  voltage-phase  cycle, 
makes  a  misleading  impression  unless  one  takes  carefully  into 
account  the  condition  under  which  the  curves  are  obtained.  One 
must  bear  in  mind  that  the  form  of  the  current  through  any  recti- 
fier is  not  determined  by  the  rectifier  alone,  but  is  a  function  also 
of  the  constants  of  the  circuits  employed  with  the  rectifier.  In 
the  present  experiments  the  deflecting  coils  of  the  oscillographic 
apparatus  possessed  appreciable  self -inductance  and  resistance, 
and  these  factors  must  be  taken  into  account. 

Taking  these  factors  into  account,  by  a  mathematical  investi- 
gation not  here  given  I  have  obtained  the  following  results  for 


!' 

I' 

.1 

/     f 

*5 

V 

zs 

x 

/ 

/ 

w 

\> 

\ 

NV 

/ 
1 

1  A 

Y/ 

2 

\ 

& 

1 



\ 

\ 
—-  \ 

\ 

.___ 



4 

--** 

# 

/ 

.___ 

\ 

—  A 

A 

/ 

^     1 
\ 

\ 

Y 

3 

1 

/ 

\ 
\ 

\ 
\ 

\ 

V 

{/ 

\ 
\ 

\ 

/ 

FIG.  139.     Computed  curves. 

oscillogram  No.  2.  The  applied  alternating  e.m.f.  is  represented 
by  the  dotted  curve  of  Fig.  139,  with  volts  at  right-hand  margin. 
The  resulting  voltage-phase  curve  calculated  from  this  e.m.f. 
and  the  constants  of  the  circuit  is  displaced  38°  to  the  right 
from  the  e.m.f.  curve  and  is  given  by  the  continuous-line  sine 
curve.  The  rectified  cycle,  calculated  approximately  from  the 
current-voltage  characteristic  of  the  detector  and  the  applied  vol- 
tage gives  a  curve  of  the  form  of  the  heavy  line  in  Fig.  4.  This 
calculation  accounts  for  the  general  form  of  the  rectified  cycle  and 
its  relation  to  the  voltage-phase  cycle.  The  electrolytic  detector 
has  an  important  peculiarity ;  which  is  shown  by  the  oscillogram, 
and  to  which  attention  is  now  directed. 

Evidence  of  Polarization  Capacity.  —  On  oscillograms  1,  2  and 
3  there  is  a  small  positive  rise  of  the  photographic  curves  in  the 


ELECTROLYTIC  AND  VACUUM  DETECTORS  211 

region  to  the  immediate  right  of  the  negative  maximum.  This 
rise  is  more  striking  in  the  original  photographs  than  in  the  repro- 
ductions; and,  though  small,  it  deserves  attention,  because  the 
occurrence  of  this  small  positive  maximum  is  evidence  of  the 
existence  for  about  15V^  of  a  second  of  a  positive  e.m.f. 
greater  than  the  e.m.f.  immediately  following.  Now  in  this 
part  of  the  cycle  the  externally  applied  e.m.f.  is  greater  follow- 
ing the  rise  than  during  the  rise;  therefore  the  rise  indicates  the 
existence  of  a  positive  e.m.f.  in  the  circuit  itself.  This  is  capable 
of  the  following  explanation  in  terms  of  the  theory  of  polarization. 
After  the  prevalent  external  e.m.f.  has  been  in  a  negative  direc- 
tion and  has  returned  to  zero,  the  polarization  tension  which  has 
been  opposing  the  negative  current  at  the  electrode  continues  to 
exist  for  a  short  time  and  produces  a  positive  current.  This 
action,  resembling  that  of  a  capacity,  is  familiarly  known  as  the 
polarization  capacity  of  the  electrode.  By  the  existence  of  the 
small  positive  maximum  near  the  axis  of  the  cycle,  the  oscillogram 
shows  that  the  polarization  capacity  of  the  electrode  is  not  entirely 
negligible.  Evidence  of  the  existence  of  this  polarization  capacity 
is  clearly  given  by  the  oscillograms  1,  2,  and  3.  The  oscillograms 
4  and  5,  while  not  having  a  positive  maximum  near  the  axis,  show 
also  a  striking  tendency  toward  a  maximum  at  this  point,  which  is, 
however,  masked  by  the  rapid  rise  of  the  building-up  curve  in  this 
part  of  the  cycle. 

CONCLUSIONS  IN  REGARD  TO  THE  ELECTROLYTIC  DETECTOR 

L  The  whole  phenomenon  of  the  rectification  of  small  alter- 
nating currents  by  the  electrolytic  detector  seems  to  be  explicable 
in  terms  of  the  theory  of  electrolytic  polarization. 

2.  The  polarization  capacity  of  the  small  platinum  electrode  is 
not  entirely  negligible,  even  with  currents  making  only  60  cycles 
per  second.     The  polarization  capacity  may,  however,  aid  in  pro- 
ducing a  rectified  current  as  well  as  in  opposing  this  effect,  and 
apart  from  the  effect  of  this  capacity  on  the  tuning  of  the  circuit, 
does  not  detract  from  the  utility  of  the  rectifier  as  a  detector  for 
electric  waves. 

3.  The  present  conclusions  in  regard  to  the  action  of  the  detector 
are  entirely  in  accord  with  Pupin's  original  brief  description  of  the 
phenomenon  as  quoted  above. 


212  WIRELESS  TELEGRAPHY 

COMPARISON    OF    THE    ELECTROLYTIC    DETECTOR    WITH    THE 
CRYSTAL   RECTIFIERS 

The  resemblance  of  the  oscillograms  with  the  electrolytic  de- 
tector to  those  with  the  crystal  rectifiers  l  is  close,  in  so  far  as  de- 
pends on  the  fact  that  both  classes  of  rectifiers  are  nearly  perfect 2 
rectifiers  when  employed  under  their  best  conditions.  The  electro- 
lytic rectifier,  in  order  to  approximate  perfection 3  as  a  rectifier, 
must  be  polarized  by  the  superposition  of  a  direct  current;  while 
the  use  of  the  direct  current  with  the  crystal  rectifier,  does  not 
always  materially  improve  the  rectification.  Also  the  two  rectifiers 
are  different,  in  that  the  electrolytic  rectifier  shows  evidence  of  elec- 
trolytic polarization  capacity,  which,  so  far  as  may  be  judged  from 
the  oscillograms,  is  absent  with  the  crystal  rectifier.  The  experiment 
with  the  electrolytic  detector,  since  it  shows  in  the  matter  of  polar- 
ization capacity  the  integrative  action  of  this  detector,  which  was 
sought  for  and  not  found  with  the  crystal  rectifier,  is  thus  an 
interesting  "control  "  experiment. 

In  the  matter  of  sensitiveness  the  best  crystal  rectifiers  are 
about  equal  to  the  electrolytic  detector. 

VACUUM    DETECTOR 

Another  highly  sensitive  detector,  by  which  the  electrical  oscilla- 
tions at  a  wireless  telegraph  receiving  station  are  rectified  and 
detected,  makes  use  of  the  unilateral  conductivity  of  a  vacuous 
space  containing  electrons  produced  by  an  incandescent  body.  A 
rectifier  for  electric  oscillations  making  use  of  this  principle,  in- 
vented by  Professor  J.  A.  Fleming,4  and  called  by  him  an  "oscilla- 
tion valve  "  is  represented  in  Fig.  140.  In  this  figure  a  is  a  glass 
bulb,  a  little  smaller  than  an  ordinary  incandescent  lamp  bulb; 
6  is  a  carbon  filament,  like  that  of  an  incandescent  lamp,  which 
is  heated  to  incandescence  by  connection  through  the  leads  /  and 
e  with  a  battery  h.  Surrounding  the  filament  but  not  touching 
it  is  a  metallic  cylinder  c.  The  bulb  is  pumped  to  a  high  degree  of 
exhaustion.  When  the  filament  is  raised  to  incandescence  by  a 

1  Pierce,  Part  II.,  L  c. 

2  A  rectifier  is  called  "nearly  perfect "  when  the  ratio  of  the  current  in  one 
direction  to  that  in  the  opposite  direction  is  large. 

3  The  current  through  the  electrolytic  rectifier  is  slightly  asymmetric  when 
no  polarizing  current  is  employed. 

4  Proc.  Roy.  Soc.  London,  1905,  Vol.  74,  p.  476;  also  U.  S.  Patent,  No. 
803,684,  filed  April  19,  1905,  issued  Nov.  7,  1905. 


ELECTROLYTIC  AND  VACUUM  DETECTORS 


213 


current  through  it,  negative  electrons  are  sent  off  from  it  and  render 
the  space  between  the  filament  and  the  cylinder  conductive  for 
an  electric  current,  provided  the  e.m.f.  producing  this  current  is 
directed  from  the  cylinder  to  the  hot  filament.  In  case  the 
e.m.f.  is  applied  in  the  opposite  direction,  no  current,  or  a  much 
smaller  current,  flows.  An  oscillating  e.m.f.  applied  to  the  cylin- 
der and  filament  produces  more  current  in  one  direction  than 
in  the  opposite  direction. 

One  method  of  connecting  the  valve  into  a  wireless  telegraph 
receiving  circuit  is  shown  in  the  diagram,  which  is  taken  from 


FIG.  140.     Professor  Fleming's 
vacuum  tube  rectifier. 


FIG.  141.  Circuit  employed 
by  Dr.  DeForest  with  vacuum 
detector. 


Professor  Fleming's  U.  S.  Patent  Specifications.  Here  the  valve 
is  in  a  circuit  connected  inductively  with  a  wireless  telegraph 
antenna.  Electrical  oscillations  in  the  antenna  induce  an  oscil- 
lating electromotive  force  in  the  coil  k,  and  this  oscillating  e.m.f. 
sends  more  current  in  one  direction  than  in  the  opposite  direction 
through  the  valve  and  through  the  current-indicating  instrument  I. 
A  modification  of  the  method  of  connecting  the  indicating 
instrument  to  the  oscillation  valve  has  been  made  by  DeForest 
so  as  to  permit  the  use  of  a  telephone  as  indicator.  A  diagram  of 


214  WIRELESS  TELEGRAPHY 

a  circuit  of  this  form  taken  from  DeForest's  U.  S.  Patent  Specifi- 
cations 1  is  shown  in  Fig.  141,  in  which  F  represents  a  telephone 
receiver  and  H  a  battery  connected  in  the  local  circuit  through 
the  vacuum  rectifier  B  (which  Dr.  DeForest  calls  an  audion). 

1  See  U.  S.  Patent,  No.  836,070,  filed  Jan.  18,  1906,  divided   May  19, 
1906,  issued  Nov.  13,  1905. 


CHAPTER  XX 
ELECTRICAL   RESONANCE 

r 

WAVE  METERS.     RESONANCE  IN  SIMPLE  CONDENSER  CIRCUITS 

ON  account  of  the  multiplicity  of  facts  requiring  presentation 
in  an  elementary  discussion  of  electric  wave  phenomena,  it  is  often 
difficult  to  decide  what  is  the  most  direct  course  to  follow.  For 
a  part  of  the  way,  in  the  earlier  chapters,  we  were  able  to  proceed 
almost  in  the  historic  order.  Up  to  about  the  year  1900,  the 
growth  of  knowledge  of  electric  waves,  so  far  as  pertains  to  wireless 
telegraphy,  occurred  as  a  fairly  direct  sequence  of  important 
events,  which  have  been  sketched  in  Chapters  I  to  XIII.  About 
the  year  1900  the  literature  of  the  subject  began  to  multiply 
enormously  and  practical  progress  began  to  develop  in  many 
directions.  Two  main  branches  of  this  development  we  have 
already  pursued,  in  a  discussion  of  the  propagation  of  the  electric 
waves  to  long  distances  over  the  surface  of  the  earth  and  in  a 
discussion  of  some  of  the  detectors  used  in  receiving  the  signals. 
We  shall  now  begin  the  study  of  a  third  main  branch  of  the  sub- 
ject; namely,  Electrical  Resonance. 

Introduction  to  a  Study  of  Electrical  Resonance.  —  In  previous 
chapters  attention  has  been  called  to  the  importance  of  bringing 
different  parts  of  the  sending  and  receiving  circuits  into  resonance 
with  one  another.  By  this  means  the  strength  of  the  signals  is 
increased,  and  the  interference  arising  when  several  stations  are 
operated  simultaneously  is  partially  eliminated. 

The  main  elements  of  variation  in  attuning  circuits  one  to 
another  are  inductance  and  capacity.  Preparatory  to  the  study 
of  more  complex  cases  of  resonance,  let  us  recall  the  experiments 
of  Sir  Oliver  Lodge,  described  in  Chapter  VIII,  in  which  two  Ley- 
den-jar  circuits  were  attuned  to  each  other.  One  of  the  Ley  den- 
jar  circuits,  which  I  shall  call  the  oscillating  circuit,  was  provided 
with  a  spark  gap,  and  was  charged  by  an  electric  machine  and 
allowed  to  discharge.  The  other  Leyden-jar  circuit  (compare  Fig. 
142)  was  at  a  distance  of  perhaps  a  meter  or  two  from  the  oscil- 
lating circuit,  and  could  be  adjusted  as  to  period  of  vibration  by  a 

215 


216 


WIRELESS    TELEGRAPHY 


variation  of  its  self -inductance  by  means  of  the  slider  C'D'.  When 
brought  into  resonance  by  adjusting  its  period  to  that  of  the  oscil- 
lation circuit,  this  second  circuit  was  thrown  into  a  violent  state 
of  electrical  oscillation  which  might  even  break  through  the  glass 
of  the  receiving  Ley  den  jar  unless  provision  for  preventing  this 
were  made  by  providing  it  with  a  spark  gap  in  shunt  with  the  jar. 


FIG.  142.    .Lodge's  resonant  circuits. 

The  presence  of  a  maximum  sparking  across  this  gap  served  to 
indicate  that  the  jars  were  in  resonance. 

Drude 's  Use  of  Lodge's  Resonant  Receiving  Circuit  for  Deter- 
mining the  Period  of  the  Oscillating  Circuit.  —  In  1902  Professor 
Paul  Drude  L  published  a  description  of  a  resonant  method  for 
determining  the  period  of  an  oscillatory  condenser  discharge. 
Drude  used  an  apparatus  in  every  way  similar  to  Lodge's  receiving 
circuit,  with,  however,  capacity  and  inductance  of  such  shape  as 
to  be  easily  calculable,  and  with  a  scale  attached  to  the  inductance, 
so  that  the  period  of  the  receiving  circuit  was  known  for  any  par- 
ticular adjustment  of  the  variable  inductance.  Such  a  calibrated 
receiving  circuit  is  a  frequency  meter  or  a  wave  meter.  Reference 
is  made  to  Fig.  143.  Suppose  that  it  is  required  to  determine  the 
period  of  the  oscillating  circuit,  shown  as  Circuit  I.  The  fre- 
quency meter,  shown  as  Circuit  II,  is  brought  up  near  the  oscilla- 

1  Annalen  der  Physik,  Vol.  9,  p.  611,  1902;  see  also  Vol.  60,  p.  17,  1897. 


ELECTRICAL    RESONANCE  217 

tory  circuit,  and  by  adjusting  the  slider  S  of  Circuit  II  this  circuit 
may  be  brought  into  resonance  with  the  Circuit  I  of  unknown 
period.  The  condition  of  resonance  is  indicated  by  the  maximum 
glow  in  a  sensitive  vacuum  tube  in  contact  with  one  of  the  plates 
of  the  condenser  of  the  frequency  meter.  When  this  resonant 


()sciHation 
Circuit 


Scale 


^   Drude's 
NX  Frequency 
Meter 


Vacuum  Tube 
Indicator 


FIG.  143.     Drude's  resonant  method  of  measuring  wave-length 
and  frequency. 

adjustment  has  been  made,  the  position  of  the  pointer  P  on  the 
scale  is  read,  and  from  this  reading  the  period  of  the  frequency 
meter  is  known,  for  by  calculation  Drude  has  calibrated  the  fre- 
quency meter  in  terms  of  the  period  corresponding  to  any  par- 
ticular adjustment  of  the  pointer  on  the  scale. 

The  period  of  the  frequency  meter  at  resonance  is  the  same  as 
that  of  the  oscillating  circuit;  which  is,  therefore,  also  known. 

Likewise,  the  wave  length  in  air  that  Circuit  I  emits  is  known, 
for  this  wave  length  is  the  velocity  of  light  times  the  period.1 

In  terms  of  units, 

Wave  length  in  meters  =  3  X  108  X  period  in  seconds. 

By  means  of  this  apparatus  Drude  was  able  to  determine  wave 
lengths  between  2  and  445  meters. 

Doenitz's  Wave  Meter.  —  Dr.  Johann  Doenitz  2  of  Berlin,  Ger- 
many, has  constructed  a  wave  meter  that  is  in  a  very  compact  and 
convenient  form  for  measuring  the  wave  lengths  of  wireless  teleg- 
raphy. Instead  of  a  gradually  variable  inductance,  as  in  Drude's 
apparatus,  Doenitz's  instrument  has  a  gradually  variable  condenser 

1  See  Chapter  X. 

2  Elektrotechnische  Zeitschrift,  Vol.  24,  pp.  920-925,  1903.     German  Patent, 
No.  149,350,  from  April  4,  1903.    U.  S.  Patent,  No.  763,164,  filed  Sept.  15, 
1903,  issued  June  21,  1904. 


218 


WIRELESS  TELEGRAPHY 


fb  (Fig.  144).  This  variable  condenser  consists  of  two  sets  of  semi- 
circular plates,  one  fixed  and  the  other  movable  by  rotation  on  a 
vertical  axis.  The  capacity  of  the  condenser  is  thus  changed  by 
bringing  a  larger  or  smaller  area  of  the  two  sets  of  plates  into 
interlapping  position.  This  is  the  condenser  of  Korda  x  described 


FIG.  144.     Doenitz  wave  meter. 

in  Chapter  XIV.  The  condenser  is  provided  with  a  pointer 
passing  over  a  scale  t.  This  scale  on  the  wave  meter  is  calibrated 
directly  in  wave  lengths. 

In  series  with  the  variable  condenser  is  a  loop  of  wire  s,  and 
another  smaller  loop  i.  When  the  instrument  is  brought  up  near 
an  oscillating  circuit  so  that  the  oscillations  act  inductively  on  the 
loop  s,  currents  are  induced  in  the  wave-meter  circuit,  and  these 
currents  are  the  larger  the  nearer  the  period  of  the  wave  meter 

1  Korda,  German  Patent,  No.  72,447,  issued  Dec.  13,  1893. 


ELECTRICAL  RESONANCE  219 

approaches  resonance  with  the  oscillating  circuit  whose  wave 
length  is  to  be  measured.  Resonance  is  determined  by  noting 
the  amount  of  current  in  the  wave-meter  circuit.  This  is  done 
by  means  of  a  Harris  or  Riess  hot-wire  air  thermometer  h,  which 
is,  however,  not  connected  directly  into  the  wave  meter  circuit, 
but  is  coupled  with  it  by  means  of  the  oscillation  transformer  Hi. 
The  action  of  this  transformer  and  thermometer  is  as  follows: 
The  primary  i  of  the  transformer  is  in  the  wave-meter  circuit;  the 
secondary  ii  of  the  transformer  is  in  series  with  a  resistance  w, 
designed  to  be  heated  by  the  current  through  it.  This  heating  of 
the  resistance  heats  a  quantity  of  air  in  a  glass  bulb  surrounding 
the  resistance,  causing  this  air  to  expand,  and  to  push  up  a  column 
of  mercury  in  the  bent  tube  h.  As  the  wave  meter  approaches 
resonance  with  the  oscillation  circuit,  the  rise  of  the  column  of 
mercury  in  the  bent  tube  increases. 

By  reading  this  indicator,  not  only  can  one  determine  the  reso- 
nant adjustment  of  the  wave-meter  circuit,  but  one  can  also  form 
some  idea  of  the  sharpness  of  resonance  by  noting  whether  small 
or  large  variations  of  the  condenser  are  required  for  a  given  rise 
of  the  indicator. 

The  range  of  wave  lengths  measurable  by  Doenitz 's  wave  meter 
is  changed  by  substituting  various  coils  of  different  numbers  of 
turns  for  the  receiving  loop  s.  For  each  of  the  coils  there  is  a 
corresponding  calibration  of  the  scale. 

Sample  of  Observations  Made  with  a  Doenitz  Wave  Meter.  - 
The  curves  of  Fig.  145  were  obtained  l  by  a  Doenitz  wave  meter. 
The  curves  show  the  scale  reading  of  the  air  thermometer  for  vari- 
ous settings  of  the  wave  meter.  Curve  I  was  obtained  by  tuning 
the  wave  meter  to  an  oscillating  antenna  circuit;  Curve  II  was 
obtained  by  tuning  the  wave  meter  to  an  oscillating  condenser 
circuit.  The  condenser  circuit  and  the  antenna  circuit  are  seen 
to  have  the  same  wave  length,  320  meters,  indicated  by  the  fact 
that  this  value,  320  meters,  is  the  reading  of  the  wave-meter  scale 
when  the  thermometer  scale  reading  is  a  maximum.  Now  when 
the  two  circuits  of  curves  I  and  II  were  coupled  together,  and  the 
wave  meter  applied  to  a  study  of  the  oscillations  occurring  in  the 
coupled  system,  the  results  plotted  in  Curve  III  were  obtained. 
The  resonance  curve  in  this  case  has  two  maxima.  To  this  subject 
we  shall  return. 

1  Figure  145  is  copied  with  some  slight  modifications  from  Lieutenant- 
Commander  S.  S.  Robison's  Manual  of  Wireless  Telegraphy,  1906. 


THE 

UNIVERSITY 

OF 


220 


WIRELESS  TELEGRAPHY 


Fleming's  Wave  Meter.  —  A  wave  meter  devised  by  Professor 
Fleming,1  and  called  by  him  a  cymometer,  is  adjustable  to  reso- 
nance by  gradual  variation  together  of  both  the  capacity  and  the 
inductance.  A  photograph  of  the  instrument  is  shown  in  Fig. 
146.  The  condenser  consists  of  two  concentric  brass  tubes  0 
and  I,  separated  by  a  vulcanite  dielectric  V.  The  variable  in- 
ductance is  a  coil  of  wire  LM  wound  on  a  tube  of  vulcanite,  and 
is  varied  by  a  clip  K,  sliding  over  the  bared  wire  of  the  coil.  The 


250  300  350 

Wave-length  in  Meters 

FIG.  145.     Curves  obtained  with  the  Doenitz  wave  meter. 

electric  oscillations  to  be  measured  act  inductively  on  the  receiv- 
ing loop,  consisting  of  the  condenser  01,  the  inductance  LM,  and 
the  wire  PQ,  which  are  in  series.  The  clip  K  of  the  inductance, 
the  tube  0  of  the  condenser,  and  the  pointer  passing  over  the  scale 
S  are  moved  together  by  the  handle  H,  so  that  the  capacity  and 
the  inductance  of  the  instrument  are  increased  or  decreased  to- 
gether and  almost  uniformly  along  with  the  motion  of  the  pointer. 
The  condition  of  resonance  is  indicated  by  a  maximum  glow  of  a 
Geissler  tube  G  attached  to  the  terminals  of  the  tubular  condenser. 

1  Fleming:   The  Principles  of  Electric  Wave  Telegraphy,  p.  404.     Long- 
mans, 1906. 


ELECTRICAL  RESONANCE 


221 


An  advantage  of  Fleming's  cymometer  over  other  forms  of 
wave  meter  arises  in  the  fact  that  the  scale  readings  are  nearly 
proportional  to  the  wave  length  (giving  a  nearly  uniform  scale 
when  calibrated  in  wave  lengths),  whereas  with  instruments  of 
the  Doenitz  type  the  wave  length  is  nearly  proportional  to  the 
square  root x  of  the  capacity  of  the  adjustable  condenser,  so  that 
the  divisions  on  the  scale  become  wider  apart  as  the  wave  lengtH 
increases. 

Fleming's  instrument  has,  however,  the  disadvantage  of  lack 
of  compactness,  for  the  inductance  and  condenser  of  this  instru- 
ment are  from  one  to  two  meters  long. 

Pierce  Wave  Meter.  —  I  have  designed  a  wave  meter  that  has 
met  with  some  use  in  practical  application  to  wireless  telegraphy. 
It  consists  of  a  Korda  semicircular  plate  condenser  C  (Fig.  147),  in 
series  with  a  loop  L  for  receiving  the  inductive  action,  and  in 


FIG.  146.     Fleming  cymometer. 

series  with  a  specially  constructed  high-frequency  telephone 
receiver  T.  A  pointer  carried  by  the  axle  of  the  movable  plates 
of  the  condenser  passes  over  a  scale,  which  is  calibrated  directly 
in  wave  lengths. 

At  resonance,  a  maximum  sound  is  produced  in  the  high-fre- 
quency telephone  receiver.  On  account  of  the  high  sensitiveness 
of  the  telephone  receiver  the  wave  length  of  currents  in  which 
the  oscillations  are  extremely  feeble  may  be  determined,  and  also, 
on  account  of  this  high  sensitiveness,  the  condenser  can  be  made 
very  compact  and  light,  so  that  the  whole  instrument  in  the 
standard  form  weighs  only  14  pounds. 

1  This  the  reader  may  verify  by  examining  the  formula  X  =  2  TTU  x/LC. 


222 


WIRELESS  TELEGRAPHY 


For  extending  the  range  of  the  instrument  to  long  wave  lengths, 
an  inductance  (to  right  of  condenser)  is  included  in  the  instrument 
and  can  be  thrown  in  or  out  of  circuit  by  the  switch  S. 

Also  the  receptor  loop  L  has  a  double  rotation.  One  rotation 
is  about  an  axis  running  from  right  to  left  in  the  picture,  so  that 
the  loop  can  be  placed  parallel  to  any  oscillating  circuit;  and  the 
other  rotation  is  about  an  axis  perpendicular  to  the  figure,  so  that 
the  loop  may  be  folded  back  over  the  pointer  and  inclosed  in 
the  cover  of  the  instrument,  as  is  shown  in  the  lower  cut  of 
Fig.  147. 


FIG.  147.    Pierce  wave  meter. 

Calibration  of  Wave  Meters.  —  One  method  of  calibrating  a 
wave  meter  is  by  tuning  it  to  resonance  with  various  lengths  of 
two  parallel  wires.  This  method,  which  we  have  already  de- 
scribed, is  applicable  up  to  about  200  meters.  For  greater  wave 
lengths  the  parallel-wire  method  is  cumbersome,  and,  according 
to  Diesselhorst,  shows  a  systematic  error,  increasing  with  increase 
of  wave  length.  With  wave  lengths  above  200  meters  I  have 
employed  the  device  of  photographing  the  spark  of  a  discharge 
circuit  with  the  aid  of  a  revolving  mirror.  The  revolving  mirror 
apparatus  was  like  that  of  Fig.  4,  with,  however,  the  addition  of 


ELECTRICAL  RESONANCE  223 

an  accurate  device,  called  a  "  stroboscope,"  for  determining  the 
period  of  revolution  of  the  mirror. 

Having  the  period  of  revolution  of  the  mirror  and  the  distance 
between  spark-terminal  images  on  photographs  like  those  of  Fig.  3, 
one  has  a  direct  measurement  of  the  period  T  of  the  discharge  of 
a  given  oscillating  circuit.  By  constructing  a  large  number  of  such 
oscillatory  discharge  circuits  giving  various  periods  of  discharge, 
or,  better,  by  using  a  discharge  circuit  whose  period  could  be  varied 
at  will,  one  may  obtain  accurate  values  of  various  periods  by  the 
use  of  the  revolving  mirror;  and  from  the  various  periods  T  one 
can  obtain  the  wave  length  X  in  air  of  the  emitted  wave  by  the 

f°rmula  X  =  v  X  T, 

where  v  =  3  X  108  meters  per  second,  T  is  the  time  in  seconds  of 
one  complete  oscillation  of  the  circuit,  and  X  is  wave  length  in 
meters. 

The  wave  meter  to  be  calibrated  is  now  set  to  resonance  with 
each  of  these  known  wave  lengths  and  the  wave  length  is  written 
at  its  appropriate  position  on  the  scale  of  the  instrument. 

Another  method  of  calibrating  a  wave  meter  is  by  tuning  it 
to  resonance  with  circuits  of  which  the  period  is  known  by  calcu- 
lation from  a  knowledge  of  capacity  and  inductance. 

Method  of  Using  a  Wave  Meter.  —  Let  it  be  required  to  deter- 
mine the  wave  length  in  air  emitted  by  the  oscillation  circuit  S, 


FIG.  148.     Position  of  wave  meter  for  determining  the  wave  length 
or  frequency  of  the  circuit  S. 

Fig.  148.  The  wave  meter  must  be  placed  in  such  a  position  that 
the  magnetic  force  from  S  links  with  the  loop  L  of  the  wave  meter; 
the  oscillations  in  S  then  act  inductively  on  the  wave  meter. 
This  action  is  a  maximum  when  the  loop  L  is  close  up  to  S  and  in 
a  plane  parallel  with  it.  It  is,  however,  not  advisable  to  have  the 
two  circuits  too  close  together,  because  in  this  case  the  oscillations 


224 


WIRELESS  TELEGRAPHY 


induced  in  the  wave-meter  circuit  react  on  the  oscillating  circuit 
and  change  its  period. 

With  the  wave  meter  in  inductive  relation  to  the  discharge 
circuit,  by  adjusting  the  condenser  of  the  wave  meter,  a  maximum 
deflection  is  obtained  in  the  hot-wire  air  thermometer  in  the  case 
of  the  Doenitz  wave  meter.  This  deflection  is  a  maximum  when 
the  wave  meter  is  adjusted  to  resonance  with  the  discharge  circuit; 
and  when  this  adjustment  has  been  made  the  required  wave  length 
is  read  off  directly  on  the  calibrated  scale. 

With  the  use  of  the  Fleming  wave  meter  a  maximum  glow  is 
obtained  in  the  Geissler  tube,  at  resonance,  and  the  corresponding 
wave  length  is  directly  read. 

With  the  Pierce  wave  meter  a  maximum  sound  is  obtained  in 
the  high-frequency  telephone,  and  the  corresponding  wave  length 
is  directly  read. 

Use  of  the  Wave  Meter  in  the  Determination  of  the  Capacity 
of  the  Discharge  Condenser.  —  Professor  Fleming  has  pointed  out 
the  utility  of  the  wave  meter  in  the  determination  of  the  capacity 
of  a  condenser.  His  method  consists  of  discharging  the  condenser 
across  a  spark  gap  through  a  known  inductance  and  measuring 
the  wave  length  produced.  He  then  calculates  the  capacity  C 
by  use  of  the  formula 

X  =  v  .  2  TT  VZC7 

where  X  =  wave  length  in  meters  measured  by  the  wave  meter, 
v  =  3  X  108  (the  velocity  of  light  in  meters  per  second),  and  L  =  the 
known  value  of  the  inductance  through  which  the  discharge  occurs. 

A  sample  set  of  observations  that  I  have  taken  in  this  way  is 
shown  in  Table  XII.  The  values  of  the  inductance  in  the  discharge 
circuit  (see  first  column)  were  obtained  by  a  bridge  method. 

TABLE  XII 

DETERMINATION  OF  CAPACITY  BY  THE  WAVE  METER.     LEYDEN  JAR  NO.  45 


Inductance  in  Discharge 
Circuit  in  Henrys. 

Wave  Length  in  Meters. 

Capacity  in  Farads  Computed 
by  Thomson's  Formula. 

3.10X  It)"5 

690 

.  00432  X  10~6 

4.90 

865 

.00432 

6.61 

1005 

.00430 

8.35 

1130 

.00432 

10.0 

1235 

.00430 

12.0 

1345 

.00427 

14.05 

1450 

.00418 

16.1 

1560 

.00423 

Mean,  .  00428  X  10~6±1  per  cent. 

ELECTRICAL  RESONANCE  225 

The  last  column  contains  eight  independent  determinations  of 
the  capacity  with  an  average  error  of  only  1%. 

This  is  one  of  the  best  methods  of  determining  the  capacity  of  a 
condenser  under  conditions  of  actual  use. 

Effect  of  Resistance  on  the  Sharpness  of  Resonance.  —  In  tun- 
ing a  condenser  circuit  with  adjustable  capacity  or  inductance  to 
resonance  with  an  oscillating  circuit,  as  .was  done  in  the  wave 
metrical  experiments  above  described,  we  have  a  simple  case  of 
the  kind  of  tuning  that  is  made  use  of  at  a  receiving  station  when 
it  is  desired  to  receive  signals  of  one  wave  length  and  exclude  signals 
of  a  different  wave  length. 

One  of  the  main  difficulties  in  completely  excluding  undesired 
signals  arises  from  the  fact  that  the  detectors  used  in  receiving 
the  signals  have  a  high  resistance. 

Let  us  see  how  the  sharpness  of  resonance  is  affected  by  resist- 
ance of  the  receiving  circuit,  in  the  simple  case  in  which  a  con- 
denser circuit  (e.g.,  the  wave-meter  circuit)  is  attuned  to  a  given 
wave  length. 

As  an  example,  I  shall  take  a  case  in  which  the  constants  of  the 
receiving  circuit  are  within  the  range  employed  in  wireless  teleg- 
raphy. In  Fig.  149  suppose  that  L  is  an  inductance  of  .0001 
henry,  /  an  instrument  for  measuring  the 
oscillatory  current  (root  of  mean  square  cur- 
rent) produced  by  an  incoming  electric 
wave,  which  is  supposed  to  have  a  wave 
length  \i  =  300  meters;  C  is  a  variable  ca- 
pacity, and  this  capacity  is  supposed  to  be 
calibrated  directly  in  wave  lengths,  \2.  Let 
the  receiving  circuit  be  set  at  various  wave 
lengths  and  let  the  corresponding  current 
be  read  on  the  instrument  7.  FIG.  149.  Simple  oscil- 

By  a  calculation  that  is  not  here  repro- 
duced, it  can  be  shown  that  the  results  plotted  in  Fig.  150  will  be 
obtained.     The  relative  current   is  plotted  vertically,  while  the 
settings  of  the  wave  length  of  the  receiving  circuit  divided  by  the 
wave  length  of  the  incident  wave  (\2/Xi)  are  plotted  horizontally. 

The  different  curves  in  the  diagram  show  the  effects  of  putting 
different  values  of  the  resistance  R  into  the  receiving  circuit.  A 
maximum  current  is  received  in  each  case  when  X2  =  Xi,  but  the 
sharpness  or  flatness  of  the  curves  depends  on  the  value  of  R.  When 
R  =  628  ohms  the  top  curve  is  obtained.  This  curve  is  nearly 


226 


WIRELESS  TELEGRAPHY 


flat,  so  that  large  changes  in, the  period  of  the  receiving  circuit  pro- 
duce only  small  diminutions  of  the  received  current.  Going  suc- 
cessively to  the  values  R  =  314,  207,  125,  63,  and  6.3  ohms,  we 
get  sharper  and  sharper  resonance,  shown  by  the  curves  corre- 
sponding to  these  values  of  R. 

By  a  further  examination  of  this  set  of  curves  we  can  see  how 
well  the  receiving  circuit  with  various  values  of  resistance  can 
discriminate  between  signals  of  different  wave  lengths  coming  at 
the  same  time.  Suppose,  for  example,  with  the  300  meter  wave 
coming,  we  try  to  get  a  message  of  wave  length  1.20  X  300  =  360 


628  Ohms 


6.3  Ohms 


.70      .80       .90     1.00     1.10    1.20     1.30     1.40     1.50    1.60     1.70    1.80     1-90 
Wave-length  Relative  to  Resonant  Wave-length 


FIG.  150.     Effect  of  resistance  on  sharpness  of  resonance,  assuming  a  constant 
inductance  of  .0001  henry.     Wave  length  varied  by  varying  capacity. 

meters.  If  we  have  only  6.3  ohms  in  the  receiving  circuit,  and 
set  for  360  meters  (1.2  in  horizontal  scale),  we  should  get  about  3% 
of  current  from  the  300  meter  wave  along  with  the  full  value  of  the 
360  meter  wave.  This  would  usually  not  cause  any  difficulty. 
If;  on  the  other  hand,  our  receiving  circuit  has  a  resistance  of  63 
ohms  we  should  receive  33%  of  the  300  meter  wave  along  with  the 
360  meter  wave ;  and  with  a  resistance  of  628  ohms  in  the  receiving 
circuit,  we  should  receive  95%  of  the  full  current  of  the  undesired 
300  meter  wave  when  we  were  in  tune  for  the  360  meter  wave. 


ELECTRICAL  RESONANCE  227 

In  this  problem  I  have  supposed  ;that  the  waves  which  are 
arriving  are  themselves  undamped.  If  they  also  have  strong 
damping,  the  interference  would  be  a  little  greater  than  that 
described,  but  the  main  imperfections  of  tuning  are  due  to  the 
resistance  of  the  receiving  station  and  not  to  the  lack  of  purity  of 
the  wave  from  the  sending  station.  The  illustration  shows  that 
we  cannot  get  very  sharp  resonance  so  long  as  we  have  to  use  a 
high  resistance  (the  detectors)  in  the  particular  receiving  circuit 
here  employed.  This  difficulty  is,  however,  considerably  reduced 
by  the  use  of  coupled  circuits  at  the  sending  and  receiving  stations, 
in  the  place  of  the  simple  condenser  circuit  of  this  computation. 

In  the  next  chapter  some  facts  in  regard  to  resonance  with 
coupled  circuits  will  be  presented. 


CHAPTER  XXI 
ON  RESONANCE  (Continued) 

ON  THE   ELECTRICAL   OSCILLATIONS  OF   CONNECTED   SYSTEMS   OF 
CONDENSER    CIRCUITS 

.HAVING  briefly  examined  the  conditions  of  resonance  in  simple 
condenser  circuits,  let  us  next  consider  the  case  of  the  coupled 
circuits  involving  principles  that  are  now  generally  employed  in 
sending  and  receiving  the  signals  of  wireless  telegraphy  and  wireless 
telephony. 

Reason  for  Using  Coupled  Sending  Circuits.  —  The  chief 
reason  for  the  use  of  coupled  circuits  at  the  sending  station  is  as 
follows :  A  closed  condenser  circuit  is  not  a  good  radiator  of  electric 
energy,  hence  an  antenna  is  employed  for  the  purpose  of  radiating 
the  energy.  But  on  account  of  the  comparatively  small  capacity 
of  the  antenna  we  cannot  easily  apply  large  amounts  of  power  l 
directly  to  the  antenna  without  using  a  very  long  spark  gap  in  the 
antenna,  so  as  to  get  the  necessary  high  potential.  Now  the  use 
of  a  long  spark  gap  carries  with  it  disadvantages;  it  does  not  pro- 
duce good  oscillations. 

To  avoid  this  disadvantage,  the  high  potential  in  the  antenna  is 
obtained,  not  by  the  use  of  a  long  spark  gap,  but  by  the  induc- 
tive action  of  a  discharge  occurring  in  a  condenser  circuit  con- 
nected with  the  antenna  and  put  into  resonant  relation  with  it, 
as  shown  in  Figs.  151  and  152.  The  large  amount  of  power  in 
the  condenser  circuit  is  attained  by  the  largeness  of  the  capacity 
instead  of  by  the  length  of  the  spark  gap.  By  the  use  of  a  suitably 
large  capacity  in  the  condenser  circuit  we  can  obtain  tremendous 
current  in  this  circuit,  which  will  induce  very  large  potential  in  the 
antenna,  if  the  antenna  is  in  resonance  with  the  condenser  circuit. 
We  thus  get  a  large  amount  of  radiation. 

It  is  proposed  to  describe  some  experiments  on  the  oscillations  of 
connected  systems  of  circuits.     Attention  is  here  chiefly  confined 
to  the  sending  station.     The  receiving  station  will  be  examined 
later, 
j  p          _  No.  of  charges  per  second  X  Capacity  X  (Maximum  Potential)2 

228 


RESONANCE  —  OSCILLATIONS  OF  COUPLED  SYSTEMS     229 

Simplified  Form  of  Circuits.  —  In  order  to  simplify  the  con- 
ditions somewhat,  in  the  present  experiments,  instead  of  employ- 
ing the  wireless  telegraph  circuits  with  the  antenna  constituting 


U>-1^ 


Key 


FIG.  151.     Inductively  coupled 
transmitting  station. 


FIG.  152.     Direct  coupled  transmit- 
ting station. 


FIG.  153.     Inductively  coupled  condenser     FIG.  154.     Direct  coupled  con- 
circuits,  with  the  antenna  and  ground  denser  circuits, 
of  Fig.  151  replaced  by  a  condenser. 

the  capacity  of  the  secondary  circuit  (such  an  antenna  being  in 
the  form  of  a  capacity  distributed  along  a  wire  also  possessing 
inductance),  this  antenna,  for  the  purposes  of  these  experiments, 
is  replaced  by  a  condenser,  so  as  to  have  a  localized  capacity  in 


230 


WIRELESS  TELEGRAPHY 


each  of  the  circuits.     While  this  change  does  not  simplify  the 
experiments,  it  enables  us  to  apply  certain  fairly  simple  theoretical 


FIG.  155.     Sketch  of  inductively  connected  condenser  circuits. 

formulas  to  the  examination  of  the  result.  Without  these  for- 
mulas, we  should  have  difficulty  in  seeing  any  interrelation  among 
the  results  and  the  constants  of  the  circuits  used  in  the  experiments. 


FIG.  156.     Sketch  of  direct  coupled  condenser  circuits. 

Our  simplified  circuits  are  of  the  forms  shown  in  diagram  in 
Figs.  153  and  154  and  in  sketch  in  Figs.  155  and  156. 


RESONANCE  —  OSCILLATIONS  OF  COUPLED  SYSTEMS     231 

In  Fig.  155,  which  represents  the  inductively  connected  system, 
two  condensers  C\  and  C2  are  connected  to  two  coils  L\  and  L2, 
which  are  inductively  related  but  insulated  from  each  other.  The 
number  of  active  turns  of  wire  on  each  of  the  coils  may  be  varied ; 
Z/2  is  varied  by  the  clip  contacts,  and  L\  is  varied  by  a  wheel 
contact  that  may  be  moved  along  the  inner  spiral  by  a  rotation 
of  the  drum  on  which  the  inner  spiral  is  "»vound. 

Each  of  the  condenser  circuits  is  provided  with  a  spark  gap,  so 
that  either  circuit,  when  connected  to  a  step-up  transformer,  may 
be  used  as  the  discharge  circuit.  The  other  circuit  may  then  be 
looked  upon  as  a  secondary  circuit.  When  the  spark  gap  of  the 
secondary  is  opened  too  wide  to  permit  the  passage  of  a  spark, 
or,  what  is  the  same  thing,  when  the  secondary  is  removed,  the 
period  of  oscillation  is  the  period  of  the  primary  alone.  When, 
on  the  other  hand,  the  secondary  is  left  in  place  and  the  spark  gap 
of  the  secondary  is  closed  (compare  Fig.  153),  the  oscillations  of 
the  discharge  circuit  C\  L\  induce  oscillations  in  the  secondary 
circuit  C2  L2,  and  we  have  a  periodic  flow  of  current  in  both 
circuits.  It  is  proposed  to  give  an  account  of  some  measurements 
of  the  wave  length  produced  in  the  circuits  when  uncoupled 
and  then  when  coupled  with  each  other, .  and  to  compare  the 
measured  values  with  values  computed  from  certain  useful 
formulas. 

In  the  Direct  Coupled  System,  represented  in  Fig.  156,  which  was 
also  studied,  the  transformer  of  the  inductive  coupling  is  replaced 
by  an  auto-transformer;  that  is,  the  two  condensers  C\  and  C2  are 
made  to  discharge  through  parts  of  the  same  coil.  In  this  case, 
also,  both  the  inductances  LI  and  L2  can  be  varied  independently 
by  the  motion  of  the  contacts  W  and  S.  Also,  both  the  condenser 
circuits  are  provided  with  spark  gaps,  so  that  either  circuit  may  be 
caused  to  oscillate  alone  or  to  constitute  the  discharge  circuit  in  a 
connected  system  with  closed  secondary. 

These  two  forms  of  circuits,  Figs.  155  and  156,  are  derived  from 
the  ordinary  wireless  telegraph  circuits  by  replacing  the  antenna 
and  ground  of  the  wireless  telegraph  station  by  the  two  coatings 
of  a  condenser  respectively.  The  circuits  in  these  simplified 
forms  will  yield  results  that  will  aid  in  understanding  the  actual 
wireless  telegraph  circuits,  which  are  to  be  examined  in  subsequent 
chapters. 

Dimensions  of  the  Inductances.  —  The  coils  employed  in  the 
apparatus  shown  in  Figs.  155  and  156  had  the  following  dimensions: 


232 


WIRELESS  TELEGRAPHY 


Coil. 

No.  Turns. 

Diam.  Wire. 

Coil  Diam. 

Pitch. 

Outer  of  Fig.  155 

24 

.208  cm. 

18  cm. 

.81  cm. 

Inner  of  Fig.  155 

51.5 

.208 

13 

.42 

Coil  of  Fig.  156 

51.5 

.208 

13 

.42 

The  inductances  of  various  numbers  of  turns  of  these  several 
coils  were  measured  on  a  Rayleigh's  bridge,  and  these  values  are 
recorded  in  the  subsequent  tables  which  contain  the  wave-length 
measurements. 

General  Statement  of  the  Results.  —  Because  of  the  difficulty 
of  following  the  details  of  the  experiments,  I  shall  make  at  the 
outset  a  general  statement  as  to  the  results. 

When  two  circuits  are  coupled  together,  either  inductively  or 
directly,  the  primary  circuit  will  have  two  periods  of  oscillation, 
and  the  secondary  will  have  the  same  two  periods  of  oscillation; 
and  this  is  true,  with  a  few  exceptions,  even  when  both  of  the  cir- 
cuits have  been  attuned  to  the  same  period  before  being  coupled 
together.  This  double  periodicity  of  the  oscillation  of  the  coupled 
circuits  produces  two  distinct  wave  lengths,  so  that  a  coupled 
system  emits  two  waves. 

Also,  the  energy  of  the  t oscillation  is  at  first  all  in  the  primary 
circuit,  and  gradually  passes  over  into  the  secondary  circuit, 
during  which  process  the  current  in  the  primary  becomes  less 
and  less  with  each  vibration,  while  the  current  in  the  secondary 
becomes  more  and  more  with  each  vibration.  After  the  energy 
has  all  gone  into  the  secondary,  the  current  in  the  primary  becomes 

zero.  Then  the  energy  gradually 
comes  back  into  the  primary  and 
the  current  in  the  secondary  be- 
comes zero.  This  process  may 
repeat  itself  many  times. 

Experiment  with  Sympathetic 
Pendulums.  —  As  a  digression, 
in  order  to  see  just  how  this 
takes  place,  the  reader  is  asked  to 
set  up  a  simple  apparatus  like  that 
shown  in  Fig.  157,  which  consists 
of  two  pendulums  hung  from  a  loosely  suspended  transverse  cord 
about  3  feet  long.  This  supporting  cord  may  be  tied  to  any  two 


FIG.  157.     Coupled  pendulum. 


RESONANCE  — OSCILLATIONS  OF  COUPLED  SYSTEMS     233 

convenient  objects;  for  example,  the  backs  of  two  chairs.  The 
pendulum  bobs  may  be  any  two  small  bodies  of  about  the  same 
weight — two  heavy  nails  will  do.  At  first  make  the  lengths  of  the 
threads  supporting  the  two  pendulum  bobs  the  same.  Now  leave 
one  of  the  bobs  at  rest,  pull  back  the  other  in  a  direction  at  right 
angles  to  the  plane  of  the  strings,  and  then  release  it.  Note  what 
happens.  Try  the  effect  of  making  the  cross  cord  tighter  or  looser, 
and  also  the  effect  of  making  the  two  pendulums  of  unequal  length. 

The  vibratory  motion  of  the  pendulums  represents  very  well 
the  electrical  vibratory  motion  that  takes  place  with  the  coupled 
condenser  circuits. 

Oscillograms  of  the  Pendulum  Motion.  —  In  order  to  show 
graphically  the  nature  of  the  pendulum  motion,  I  have  elaborated 
the  pendulum  apparatus  a  little,  and  taken  a  moving  picture 


FIG.  158.     Coupled  pendulum  with  arrangement  for  photo- 
graphing the  motion. 

(oscillogram)  of  the  motion  of  each  of  the  pendulum  bobs.  To  do 
this,  a  camera  was  placed  in  the  position  shown  at  C  in  Fig.  158. 
At  the  back  of  the  camera  is  a  small  horizontal  slit  A,  and  back  of 
this  slit  is  a  sheet  of  bromide  paper  F  carried  by  a  rotating  drum  D. 
In  order  to  have  a  bright  object  upon  which  to  make  the  exposure, 
a  small  Nernst  glower  G  was  hung  just  above  one  of  the  pendulum 
bobs.  This  Nernst  filament  was  put  into  an  electric  circuit  by 
means  of  the  small  wife  W,  which  also  served  as  the  suspension 
for  the  pendulum,  and  by  means  of  the  return  wire  R,  which  was 
carried  up  in  such  a  manner  as  not  to  interfere  with  the  freedom 
of  motion  of  the  pendulum.  The  current  was  started  in  the  glower 
by  heating  it  with  a  match  while  the  current  was  on.  As  the 
pendulum  swung,  the  image  of  the  Nernst  glower  moved  back  and 
forth  along  the  slit  A .  A  small  horizontally  moving  point  of  light 
thus  entered  the  slit  and  fell  upon  the  film.  If  now  the  sensitive 


234 


WIRELESS  TELEGRAPHY 


paper  is  given  a  uniform  motion  by  the  rotation  of  the  drum,  the 
paper  moves  vertically  past  the  slit,  while  the  image  of  the  swing- 


K   A 


f\   A 


\  r\   i 


FIG.  159.     Photographs  of  motion  of  the  coupled  pendulum. 

ing  light  moves  horizontally  along  the  slit.  The  combined  effect 
of  these  two  motions  is  a  wavy  line  on  the  photographic  paper. 
Four  curves  thus  obtained  are  shown  in  Fig.  159.  These  curves 


RESONANCE  — OSCILLATIONS  OF  COUPLED  SYSTEMS     235 

show  the  displacement  of  the  bob  plotted  vertically,  against  time 
plotted  horizontally. 

The  first  curve  P,  of  Fig.  159,  was  obtained  by  leaving  the 
ball  M  initially  at  rest,  and  pulling  aside  and  releasing  ball  L 
(Fig.  158).  The  motion  here  corresponds  to  the  primary  current 
in  the  coupled  condenser  circuits.  The  second  curve  S  was 
obtained  by  leaving  the  ball  L  initially  at  rest  and  releasing  M. 
This  curve  corresponds  to  the  secondary  current  of  the  coupled 
condenser  circuit.  The  two  cords  supporting  L  and  M  were  of 
the  same  length  in  the  case  of  these  two  experiments. 

As  another  experiment,  the  two  cords  were  both  equally  short- 
ened, and  the  transverse  supporting  cord  was  loosened;  the  curves 
P'  and  S'  were  obtained  for  the  motion  of  the  ball  L  initially  dis- 
placed (primary)  and  initially  at  rest  (secondary)  respectively. 

The  curves  P  and  S  or  P'  and  S'  represent  very  well  the  electrical 
vibratory  motion  of  the  coupled  condenser  circuits,  if  we  think  of 
the  displacement  of  the  bob  in  the  two  curves  as  representing 
the  current  in  the  primary  and  secondary  circuits  of  the  coupled- 
condenser  oscillation. 

How  the  Curves  Show  the  Existence  of  Two  Periods.  —  Each 
of  the  curves  of  Fig.  159  shows  the  existence  of  two  periods,  in  the 
motion  of  the  pendulum,  by  the  presence  of  "beats."  If  two  vibra- 
tions of  different  periods  coexist  in  the  same  system,  the  slower  of 
these  vibrations  will  fall  more  and  more  behind  the  other  in  phase 
until  the  two  vibrations  become  just  opposite  to  each  other  and 
neutralize  each  other;  then  the  slower  vibration  will  again  fall 
more  and  more  behind  till  it  is  a  whole  vibration  behind  the  faster, 
and  the  two  vibrations  will  then  add  and  intensify  each  other. 
This  is  what  has  happened  in  the  experiment  with  the  pendulums. 
The  same  thing  happens  with  the  electrical  vibrations  of  the  con- 
denser circuits  that  are  coupled  together. 

Theoretical  Values  of  Wave  Lengths  in  the  Coupled  Circuits.  - 
Let  us  now  return  to  the  experiments  with  the  condenser  circuits. 
By  the  use  of  the  wave  meter  we  can  pick  out  and  measure  each 
of  the  periods  or  the  corresponding  wave  lengths  of  the  connected 
system  of  condenser  circuits.  When  this  has  been  done,  we  shall 
find  that  the  wave  lengths  obtained  satisfy  the  following  theoretical 
relations  l : 

1  Lord  Rayleigh,  Theory  of  Sound;  J.  v.  Geitler,  Sitz.  d.  k.  Akad.  d.  Wiss.  z. 
Wien,  February  and  October,  1905;  B.  Galitzine,  Petersb.  Ber.,  May  and  June, 
1895;  V.  Bjerknes,  Ann.  der  Physik,  Vol.  55,  p.  120,  1895;  Oberbeck,  Ann.  der 


236 


WIRELESS  TELEGRAPHY 


x/= 


X2'  = 


X22+  V(X12-X22)2H-4r2X12X22 


-  X22)«+ 


(1) 


(2) 


In  these  equations 

Xi=  the  natural  wave  length  of  the  primary  alone, 
X2  =  the  natural  wave  length  of  the  secondary  alone, 
r  =  the  coefficient  of  coupling,  defined  by  the  equation, 


— ,  (3) 

where 

M  =  mutual  inductance  between  the  two  circuits, 

LI  =  self-inductance  of  the  primary, 

L2  =  self-inductance  of  the  secondary,  and  X/  and  X/  are  the 

resultant  wave  lengths  in 
both  primary  and  second- 
ary when  the  circuits  are 
coupled  together. 

I  shall  give  1  a  few  mea- 
surements of  the  wave 
lengths  obtained  with  the 
coupled  condenser  circuits, 
together  with  a  compari- 
son with  values  computed 
from  formulas  (1)  and  (2). 
Experiment  with  Induc- 
tively Connected  Circuits. 
Lz  =  24  Turns  of  Outer 
Coil.  (Fig.  155)  X2  =  1060 
Meters. —  The  results  ob- 

FIG.  160.    Curves  showing  observed  and  cal-  tained  in  this  experiment 
culated  values  of  the  two  wave  lengths  ,          ,     .       ™  _ 

produced  by  two  condenser  circuits  indue-  are    plotted    in    rig.    loU, 

tively  coupled.  an(j  a  complete  record  of 

the  observations  is  given  in  Table  XIII. 

Physik,  Vol.  55,  p.  623,  1895;  Domalip  and  Kolacek,  Ann.  der  Physik,  Vol.  57, 
p.  731,  1896;  M.  Wien,  Ann.  der  Physik,  Vol.  61,  p.  151,  1897,  and  Ann.  der 
Physik,  Vol.  8,  p.  686,  1902;  compare  also  Webster,  Theory  of  Electricity  and 
Magnetism,  p.  499,  1897,  and  Fleming,  The  Principles  of  Electric  Wave  Teleg- 
raphy, p.  209,  1906;  also  Cohen,  Bui.  Bu.  of  Standards,  Vol.  5,  p.  511,  1909. 

1  For  a  fuller  description  of  these  experiments  and  several  others  of  a  similar 
character  see  Pierce:  Physical  Review,  Vol.  24,  p.  152,  1907. 


1600 
1400 
1200 
10CO 
800 
600 
400 
200 

^ 

E.^I.C. 

Outer  2' 

T 

> 

s 

/ 

c 

Oj- 

C2  = 

—  'X-2-2 

—  R— 

-.0043. 
=.0048 
-—.  1C50 

_—  *— 

2m.f. 
2 
M 

s 

/ 

z 

^^ 

>? 

/ 

/ 

z 

t 

~&r* 

^' 

«  I 

"£" 

Xo 

^ 

s* 

/ 

2CO     400     600     800    1000    1200    1400   16( 

Xi    Meters            "  "  *  x  Observed 

RESONANCE  —  OSCILLATIONS  OF  COUPLED  SYSTEMS      237 


TABLE  XIII 

INDUCTIVELY    CONNECTED    SYSTEM 

Primary  capacity  .00432  microfarad. 

Primary  inductance  varied. 

Secondary  capacity  .00482. 

Secondary  inductance  24  turns  outer  coil,!/.,  =  6.60  X  10  ~5    henry. 

Wave  length  of  secondary  X2  =  1060  meters." 


Turns  Primary. 

LI 
Primary  Inductance. 
Henry. 

M 

Henry. 

T2 

50 

15.85X  10~5 

6.52X  10~5 

.412 

45 

13.9 

6.14 

.421 

40 

11.8 

5.80 

.430 

35 

10.0 

5.12 

.397 

30 

8.20 

4.45 

.360 

25 

6.50 

3.56 

.295 

20 

4.82 

2.70 

.228 

15 

3.15 

1.95 

.183 

10 

1.72 

1.20 

.128 

5 

.69 

.47 

.048 

3 

.32 

.23 

.0277 

Calculated. 

Observed. 

Turnc 

\ 

1  Urllb 

Primary. 

Al 
Meters. 

V 

V 

V 

X2' 

Meters. 

Meters. 

Meters. 

Meters. 

50 

1560 

1740 

727 

1750 

710 

45 

1460 

1670 

712 

1650 

685 

40 

1350 

1567 

686 

1570 

465 

35 

1230 

1462 

680 

1480 

660 

30 

1130 

1390 

660 

1370 

660 

25 

1000 

1273 

685 

1280 

660 

20 

870 

1185 

680 

1185 

630 

15 

700 

1127 

595 

1125 

565 

10 

510 

1080 

467 

1090 

460 

5 

300 

1060 

292 

1040 

285 

3 

210 

1062 

193 

1075 

210 

The  method  of  taking  the  observations  is  as  follows :  First,  the 
condenser  Cz  (  =  .00482  mf.)  was  connected  in  series  with  24  turns 
of  the  outer  coil  (Fig.  155)  and  was  provided  with  a  spark  gap. 
In  this  position,  with  the  inner  coil  thrown  out  of  circuit  by  dis- 
connecting both  plates  of  its  condenser,  the  wave  length  X2  was 
found  to  be  1060  meters.  Next,  with  the  secondary  condenser 
disconnected,  the  wave  length  of  the  primary  (inner)  circuit  was 
determined  with  its  condenser  Ci  (  =  .00432  mf.)  connected  in 
series  with  50  turns  of  the  inner  coil.  This  wave  length  \i  was 
1560  meters.  Next,  with  the  primary  left  unaltered,  the  second- 
ary was  closed  by  attaching  its  condensers  without  spark  gap  to 
the  24  turns  of  the  outer  coil.  This  is  the  case  of  the  closed  second- 


238  WIRELESS  TELEGRAPHY 

ary,  and  when  the  discharge  was  established  in  the  primary,  the 
wave  lengths  were  found  to  be  X2'  =  710  meters  and  X/  1750 
meters.  The  value  of  X2'  and  X'2  were  plotted  against  Xi  =  1560, 
Fig.  160.  Now  decreasing  the  primary  inductance  to  45  turns,  the 
values  Xi  =  1460,  X/  =  1650  and  X2'  =  685  were  obtained,  and 
the  last  two  values  were  plotted  against  the  value  of  Xi,  and  so 
forth.  The  complete  curves  of  Fig.  160  were  obtained  in  this  way. 
In  the  curves  of  Fig.  160  the  crosses  are  the  observed  values  and 
the  circles  are  the  corresponding  calculated  values.  The  45°  line 
between  the  two  curves  may  be  looked  upon  as  Xi  plotted  against 
itself,  while  a  horizontal  line  across  the  figure  at  1060  meters  (not 
shown)  would  represent  the  graph  of  X2.  With  this  in  mind  it  will 
be  seen  that  the  two  derived  wave  lengths  X/  and  X2'  approach  X2 
and  Xi  respectively  toward  the  origin.  The  observed  and  the 
calculated  values  are  in  satisfactory  agreement. 

The  formulas  for  the  calculation  of  X/  and  X2'  are  the  formulas 

(1)  and  (2)  of  page  236,  which  involve  merely  the  independent 
periods  of  the  two  circuits  and  their  coefficient  of  coupling.     The 
latter  quantity  was  obtained  by  the  measurement  on  a  Rayleigh's 
bridge  of  Lj,  L2  and  M  for  each  setting  of  the  oscillation  circuit. 
The  values  of  these  inductances  and  the  values  of  r  calculated  from 
them  is  also  included  in  Table  I. 

The  intensity  of  the  various  periods  of  the  circuits  under  the 
different  conditions  of  the  experiment  varies  greatly.  No  attempt 
was  made  to  determine  these  intensities  and  the  experiments  are 
designed  merely  to  show  the  wave-length  relations. 

Experiments  with  the  Inductively  Connected  System  in  the 
Special  Case  Where  X2  =  Xj.  —  A  case  of  especial  interest  is  the 
case  in  which  the  primary  and  secondary  have  the  same  independ- 
ent periods.  This  is  the  case  of  so-called  "  resonance  "  between 
the  two  circuits.  In  this  case  the  wave-length  formulas  (1)  and 

(2)  become  greatly  simplified,  as  may  be  seen  by  substituting  X2  = 
Xi  in  these  equations,  which  under  this  condition  become 

(X/)2  =  Xl2(l+r),  (4) 

(X2')2  =  X!2(1  -r).  (5) 

In  the  present  experiment  the  two  independent  wave  lengths  Xi  and 
X2  were  made  equal,  and  the  wave  lengths  produced  by  the  com- 
pound system  were  then  measured  and  compared  with  calculations 
from  the  formulas  (4)  and  (5).  Two  wave  lengths  X/  and  X2'  were 


RESONANCE  —  OSCILLATIONS  OF  COUPLED  SYSTEMS     239 

obtained  both  by  measurement  and  by  calculation.  The  observed 
and  calculated  results  are  plotted  in  the  curves  of  Fig.  161.  In 
this  case  also  the  agreement  is  fairly  satisfactory. 

These  two  experiments  with  the  inductively  connected  system 
of  circuits  give  an  experimental  verification  of  the  formulas 
(1),  (2),  (3),  (4)  and  (5),  and  serve  to  show  how  the  wave  lengths 
obtained  with  the  connected  system  depend  on  the  constants  of 


1200 


10CO 


800 


600 


400 


200 


E.^I.C. 
C,  =  00432  m 


C2  =00482 

X2 


200        400         600         800        1000      12(K 
\a  Meters  4 +++ Observed 

oooo  Calculated 


FIG.  161.     Curves  of  wave  lengths  obtained  with  inductively  coupled 
condenser  circuits  having  individually  the  same  period. 


the  two  circuits.  We  shall  return  to  this  subject  after  giving 
briefly  the  results  of  an  experiment  with  the  direct  coupled  system 
of  circuits. 

Experiment  with  the  Direct  Coupled  Circuit.  —  C2  =  .00178 
Microfarads,  L2  =  25.5  Turns  =  6.7  X  io~5  Henrys,  X2  =  645 
Meters.  —  The  apparatus  for  this  experiment  with  the  direct 
circuit  is  shown  in  Fig.  156.  The  steps  of  the  experiment  are 
similar  to  those  with  the  other  system  of  circuits.  The  observed 
and  calculated  values  of  the  wave  lengths  in  the  compound  oscil- 
lating system  are  plotted  in  Fig.  162.  The  formulas  of  calculation 
are  the  formulas  (1)  and  (2),  and  the  agreement  between  the 
observed  and  calculated  results  (crosses  and  circles)  is  seen  to  be 
satisfactory. 


240 


WIRELESS  TELEGRAPHY 


Xi 


D.C. 

SEC  at  25.5 


One  interesting  result  shown  by  this  experiment  is  the  fact  that 
the  curve  X2'  comes  down  to  the  horizontal  axis  in  the  neighbor- 
hood of  Xi  =  1010  me- 
ters. This  point  is  the 
point  of  so-called  per- 
fect coupling,  and  was 
obtained  when  the  pri- 
mary and  secondary 
condensers  were  both 
connected  through  the 
same  inductance,  25.5 
turns  of  the  coil,  as 
shown  in  Fig.  163. 


1600 


1400 


1200 


1000 


800 


600 


400 


200 


.00432 
00178 
645-: 


m.f. 


200      400      600      800     1000    1200     1400    1600 

\,   Meters  -H-+  +  Observed 

Xl  K  cooo  calculated 

FIG.  162.  Curves  showing  observed  and  calcu- 
lated values  of  the  two  wave  lengths  produced 
by  two  condenser  circuits  directly  coupled. 


r 

lid 


FIG.  163.  Diagram  of  a 
case  of  coupling  that 
gives  but  a  single 
wave  length. 


In  this  case  we  may  explain  the  result  in  two  ways: 

(I.)  The  two  condensers  may  be  looked  upon  as  discharging  in 

parallel  through  the  same  inductance,  and  producing,  therefore, 

only  one  wave  of  wave  length 

Xi'  =  2  TT  •  v  •  VLi(Ci+  C2)  =  vV+  \22.  (6) 

II.   This  result  is  also  obtainable  from  the  theoretical  equations 
(1)  andf(2) 


=  4 

T 


-  X22)2+ 


X2' 


v- 


X22-      (Xi2-  X22)2+  4r2X12X22 


(2) 


For  when  the  primary  and  secondary  condensers  are  connected 
about  the  same  inductance, 

Ll  =  L2  =  M, 


therefore 


M2 


=  1. 


RESONANCE  —  OSCILLATIONS  OF  COUPLED  SYSTEMS     241 

When  r  is  equal  to  unity  the  coupling  is  said  to  be  perfect  and  the 
equations  (1)  and  (2)  become 

Xi'  =  vV  +  X,2; 
and  X2'  =  0. 

That  is  to  say,  the  oscillation,  as  shown  also  by  method  I, 
becomes  single-  valued. 

The  case  of  perfect  coupling  was  not  observed  in  the  experiments 
with  the  inductively  coupled  system,  because  for  perfect  coupling 
the  primary  and  secondary  coils  must  have  the  same  number  of 
windings  and  the  two  coils  must  be  so  close  together  as  to  be 
practically  coincident,  —  conditions  that  could  not  be  realized  with 
the  inductive  coupling. 

Close  Coupling  and  Loose  Coupling.  —  One  of  the  most  interest- 
ing facts  derivable  from  an  examination  of  the  equations  (1)  and 
(2),  which  are  verified  by  the  experiments,  is  the  influence  of  the 
coefficient  of  coupling  (T)  on  the  wave  lengths  produced  by  the 
coupled  circuits.  In  general,  two  wave  lengths  are  obtained  when 
a  coupled  system  of  circuits  is  set  into  oscillation.  This  duplicity 
of  the  wave  length  is  often  an  inconvenience  in  wireless  telegraphy, 
because,  to  avoid  interference  when  a  neighbor  is  sending  a  mes- 
sage we  do  not  wish  to  hear,  it  is  necessary  to  tune  to  avoid,  not 
one  undesired  wave,  but  two. 

The  influence  of  the  coefficient  of  coupling  on  the  wave  length 
is  very  easy  to  investigate  in  case  the  primary  and  secondary  of 
the  coupled  system  are  attuned  to  the  same  wave  length  X,  as  they 
generally  are  in  practice.  In  this  case,  the  formulas  for  the  com- 
pound wave  lengths  X/  and  X2'  become  the  simple  forms  of  equation 
(4)  and  (5);  namely, 

(Xi')2  =  X2  (1  +  r),  (4) 

and  (X2')2  -  X2  (1  -  r).  (5) 

Dividing  each  of  these  equations  by  X2,  and  extracting  the  square 
root,  we  have, 


-=  VT+~r  CO 

A 

*L  =  VT=~r  (8) 

A 

Now,  putting  in  various  values  of  r  =  (.1,  .2,  .3,  etc.,  up  to  1.0), 
we  obtain  the  relative  values  of  X/  and  X2',  shown  in  the  curves  of 
Fig.  164. 


242 


WIRELESS  TELEGRAPHY 


These  are  all  of  the  possible  values  of  the  derived  wave  lengths 
X/  and  X2'  for  the  given  wave  length  X,  because  r  cannot  be  greater 
than  unity. 

Circuits  coupled  together  with  a  large  value  of  r  are  called 
dose-coupled  circuits;  those  coupled  with  a  small  value  of  r  are 
called  loose-coupled.  The  looser  the  coupling,  the  nearer  the  two 
derived  wave  lengths  approach  the  wave  length  of  each  condenser 

1.0 


0       .2       .4'      .6        ,8       1.0      1.2      1.4      1.6 

Wa^e-lengths  Cbupled -=- Wave-lengths  Uncoupled 

FIG.  164.     Effect  of  coefficient  of  coupling  on  the  resultant 
wave  lengths  of  a  coupled  circuit. 

circuit  alone.  For  sharp  resonance,  then,  the  coupling  ought  to  be 
loose;  while  for  strength  of  signals  the  coupling  ought  not  to  be 
too  loose.  i 

Similar  considerations  apply  more  or  less  directly  to  the  receiv- 
ing circuits  also.  This  subject,  of  the  closeness  or  looseness  of  the 
coupling,  will  come  up  again  in  connection  with  the  actual  wireless 
telegraph  sending  and  receiving  circuits,  comprising  an  antenna 
circuit  coupled  with  a  condenser  circuit,  which  are  discussed  in  the 
next  chapter. 


CHAPTER  XXII 


TUNING  THE   SENDING   STATION 

HAVING  investigated,  in  the  preceding  chapter,  the  condi- 
tions of  resonance  and  the  manner  of  vibration  of  two  condenser 
circuits  connected  together,  it  is  proposed  now  to  consider  the 
actual  wireless  telegraph  sending  circuits.  For  this  purpose  let 

us  examine  the  method  of 
adjusting  the  direct  coupled 
or  the  inductively  coupled 
sending  station  to  resonance. 

0A  diagram  of  a  direct  coupled 
sending  station  is  shown  in 
Fig.  165.  The  condenser  C, 
repeatedly  and  periodically 
charged  from  a  transformer 
Tr,  discharges  through  a  spark 
gap  G  and  a  few  turns  P  of 
a  "  helix."  The  oscillations  in 
this  circuit  act  inductively  and 
produce  oscillations  in  the  an- 
tenna circuit  consisting  of  the 
antenna,  the  coils  S  of  the 
helix,  and  the  ground  E.  A 
maximum  effect  is  produced 
when  these  two  circuits  are 
properly  adjusted  to  each 

other.      A    photograph,    Fig. 
FIG.  165.     Direct^coupled  transmitting    166>  is  gjven  to  ghow  the  CQn_ 

struction  of  the  sending  helix 

(right)  and  a  method  of  inclosing  the  spark  gap  for  reducing  the 
noise  of  the  spark. 

A  diagram  of  the  inductively  coupled  sending  circuit  is  shown 
in  Fig.  167.  Here  the  primary  and  secondary  inductances  are 
parts  P  and  S  of  two  separate  helices.  These  two  helices  may  be 
one  above  the  other,  as  represented  in  the  diagram,  or  may  be  one 

243 


244 


WIRELESS  TELEGRAPHY 


FIG.  166.     Showing  construction  of  helix  and  spark  gap  for  a 
direct  coupled  transmitter. 

inside  of  the  other,  as  shown  in  the  photograph  of  Fig.  168.     They 

must,  however,  be  separated  by 
sufficient  distance  to  prevent 
sparking  between  them  when 
the  station  is  in  operation.  In 
this  system  of  circuits  also,  the 
condenser  circuit  and  the  antenna 
circuit  must  be  adjusted  to  reso- 
nance, in  order  to  get  strong  oscil- 
lations in  the  antenna. 

We  shall  show  the  details 
of  the  process  of  attuning  the 
primary  and  secondary  of  the 
coupled  circuits  to  resonance  (1) 
by  the  use  of  a  wave  meter, 
and  (2)  by  the  use  of  a  hot-wire 
ammeter. 

Wave-Metrical  Method  of  At- 
tuning a  Direct  Coupled  Sending 

FIG.  167.    Inductively  coupled       Station.  —  To  adjust   the   station 
transmitting  station.  ^o  resonance  one  first  disconnects 

the  condenser  circuit,  as  shown  in  Fig.  169,  and  places  the  wave 


TUNING  THE  SENDING  STATION 


245 


meter  WM  up  near  the  helix.  The  lower  end  of  this  helix  is  con- 
nected through  a  spark  gap  to  the  ground.  The  secondary  of  the 
station's  transformer  is  connected  about  the  spark  gap.  The 


FIG.  168.     Showing  construction  of  the  helices  of  an  inductively  coupled 

transmitting  station. 


antenna  is  connected  by  means  of  a  clip  contact  K  to  some  particu- 
lar number  of  turns  of  the  helix.  The  transformer  is  set  into  opera- 
tion so  as  to  produce  a  spark  at  the  gap.1  This  sets  up  oscillations 
in  the  antenna  circuit,  and  the  wave  meter  is  adjusted  to  resonance 
with  these  oscillations.  The  wave  length  is  read,  and  this  reading 

1  In  this  case,  where  the  spark  gap  is  in  the  antenna  circuit,  there  is  a  tend- 
ency for  the  spark  to  go  over  into  an  arc  and  not  produce  good  oscillations. 
This  may  be  obviated  by  playing  a  small  blast  of  air  on  the  spark. 


246 


WIRELESS  TELEGRAPHY 


of  wave  length,  together  with  the  number  of  turns  of  helix,  is 
entered  in  a  table.  The  clip  contact  is  now  moved  to  another 
point  on  the  helix,  thus  putting  in  more  or  less  inductance  in  the 
circuit,  and  the  wave  length  is  again  determined  and  entered  with 
the  number  of  turns  in  the  table.  A  table  is  thus  formed  for  the 
wave  length  corresponding  to  various  numbers  of  turns  of  the  helix. 


Condenser 
Disconnected 


FIG.  169.     Method  of  determining  wave  length  of  antenna  circuit. 

These  results  are  then  plotted,  and  give  a  curve  like  that  marked 
"  Antenna  Circuit  "  in  Fig.  170. 

A  similar  operation  is  performed  with  the  condenser  circuit.  In 
this  case  the  antenna  and  ground,  see  Fig.  171,  are  disconnected; 
and  the  condenser  circuit,  with  the  spark  gap  in  series,  is  con- 
nected with  various  numbers  of  turns  of  the  helix;  and  the  wave 
length  for  each  case  is  determined,  and  a  curve  of  wave  lengths 
against  turns  is  plotted.  The  curve  for  this  case  is  put  on 
the  same  chart  with  the  antenna  observations,  and  marked 
"  Condenser  Circuit,"  Fig.  170.  By  a  reference  to  the  curves 
we  can  now  obtain  the  number  of  turns  required  either  in 
the  condenser  circuit  or  in  the  antenna  circuit  to  produce 


TUNING  THE  SENDING  STATION 


247 


1000 


Turnsof  Helix 


FIG.  170.     Curves  showing  wave  lengths  of  antenna  circuit  and 
condenser  circuit  with  different  numbers  of  turns  of  the  helix. 


FIG.  171.     Method  of  measuring  wave  length  of  the  condenser  circuit. 


248 


WIRELESS  TELEGRAPHY 


a  given  wave  length.  For  example,  let  it  be  required  to  have 
both  the  condenser  circuit  and  the  antenna  circuit  produce  a  wave 
length  of  420  meters.  One  sees  that  to  get  this  wave  length  in 
the  condenser  circuit  one  must  use  1.4  turns  of  the  helix,  and  to 
have  the  same  wave  length  in  the  antenna  circuit  when  alone, 
one  must  use  in  this  circuit  5.1  turns. 

Hence,  if  we  connect  the  condenser  about  1.4  turns  of  the  helix, 
and  the  antenna  and  ground  about  5.1  turns  of  the  helix,  we  shall 
have  the  two  circuits  in  resonance,1  and  shall  get  powerful  oscilla- 
tions induced  in  the  antenna  circuit  under  the  action  of  the  dis- 
charge in  the  condenser  circuit.  The  method  of  making  the 
required  connections  is  shown  in  Fig.  165. 

Although  the  primary  and  secondary  are  now  connected  in 
resonance,  the  electrical  vibration  of  the  system  is  not  a  simple 
vibration  giving  420  meters  wave  length.  If  we  bring  the  wave 
meter  up  near  the  coupled  system  in  operation,  two  positions  of 
resonance  are  found  on  the  wave  meter  corresponding  to  two  wave 
lengths.  In  the  actual  case,  from  which 
the  above  numerical  values  were  taken, 
these  two  wave  lengths  obtained  were 
X2'  =  358  meters  and  A/  =  462  meters. 
This  duplicity  of  resultant  wave  length 
exists  in  the  antenna  circuit  and  also  in 
the  condenser  circuit  and  therefore  gives 
rise  to  a  series  of  beats  like  those  obtained 
with  the  coupled  pendulum  experiments, 
described  in  the  preceding  chapter. 

Photograph  of  the  Double  Oscillation  in 
the  Antenna  Circuit.  —  In  a  wireless  tele- 
graph station  attuned  to  resonance,  as  just 
described,  I  inserted  a  small  spark  gap  in 
the  lead  to  ground  just  below  the  helix, 
and  took  a  revolving-mirror  photograph,  a 
negative  of  which  is  shown  in  Fig.  172. 
mirror  spark  of  the  Although  this  photograph  had  to  be  made 
with  a  veiT  brief  exposure  and  is  therefore 
faint,  the  beats  are  clearly  visible,  and 
at  about  every  fourth  oscillation  the  beats 
reduce  the  antenna  current  to  zero. 


FIG.  172.       Rotating 


inductively  coupled 
on' 


Compare  Paragraph  on  "Detuning"  on  p.  251. 


TUNING  THE  SENDING  STATION  249 

Adjustment  of  Direct  Coupled  Sending  Station  to  Resonance 
with'  the  Aid  of  a  Hot-wire  Ammeter.  —  Another  method  of 
adjusting  the  condenser  circuit  and  the  antenna  circuit  to  resonance 
makes  use  of  a  hot-wire  ammeter,  inserted  in  the  antenna  circuit  as 
represented  at  A,  Fig.  165.  This  instrument  contains  a  fine  wire 
through  which  the  oscillations  pass,  producing  heat.  The  heated 
wire  expands,  and  by  means  of  a  delicate  gearing  attachment,  the 
sagging  of  the  expanding  wire  acts  upon  a  hand  passing  over  a 
dial.  The  movement  of  the  hand  over  the  dial  is  thus  an  indica- 
tion of  the  amount  of  current  passing  through  the  sensitive  wire. 
The  instrument  may  be  calibrated  directly  in  amperes,  but  this 
calibration  (chiefly  on  account  of  the  shunts  that  have  to  be 
employed)  is  without  much  absolute  value,  when  the  hot-wire 
ammeter  is  used  with  the  very  rapid  oscillations  of  wireless  teleg- 
raphy. Nevertheless,  a  maximum  deflection  of  the  instrument 
indicates  a  maximum  of  current  in  the  antenna,  and  this  is  all 
that  is  required  of  the  hot-wire  ammeter  in  order  to  decide  when 
the  antenna  and  condenser  circuits  are  in  resonance. 

Instead  of  inserting  the  hot-wire  ammeter  in  the  antenna  above 
the  helix,  it  may  just  as  well  be  placed  in  the  lead  from  the  helix 
to  the  ground.  In  either  case  oscillations  in  the  antenna  circuit 
pass  through  the  instrument. 

To  tune  up  a  station  with  a  hot-wire  ammeter,  let  the  station  be 
coupled  up  as  shown  in  Fig.  165.  Set  the  transformer  in  action, 
and  read  the  hot-wire  ammeter.  Now  keeping  the  spark  gap  con- 
stant, and  leaving  the  antenna  clip  K  unchanged,  move  the  clip 
K'  of  the  condenser  circuit  to  a  different  number  of  turns  of  the 
helix,  and  again  read  the  current.  Make  a  table  containing  the 
number  of  turns  of  helix  in  primary  circuit  and  corresponding  hot- 
wire ammeter  readings.  Then  plot  a  curve  of  readings  against 
turns  in  the  form  shown  in  Fig.  173.  From  this  figure  it  is  seen 
that  the  maximum  reading  of  the  ammeter  was  obtained  when  the 
primary  was  discharging  through  1.3  turns  of  the  helix.  This  is, 
therefore,  the  adjustment  that  must  be  given  to  the  primary  in- 
ductance in  order  to  bring  the  condenser  circuit  into  resonance  with 
the  antenna  circuit,  for  the  fixed  value  of  the  secondary  induc- 
tance employed  throughout  the  adjustment. 

Since  the  readings  of  the  hot-wire  ammeter  depend  on  the  values 
of  the  mean  square  current  through  it,  one  can,  by  a  process  like 
that  described,  find  out  just  what  conditions  of  the  two  circuits 
give  the  greatest  mean  square  current  in  the  antenna,  and  if 


250 


WIRELESS  TELEGRAPHY 


everything  is  kept  constant  in  the  experiment  except  the  induct- 
ance variation,  one  can  determine  the  resonance  adjustment  by  the 
maximum  reading  of  the  ammeter.  But  in  extending  the  use  of 
this  instrument  to  other  conditions  it  is  necessary  to  keep  in  mind 
that  the  readings  of  the  ammeter  do  not  give  any  information 
of  the  current  amplitude  of  an  individual  oscillation;  it  always  gives 


s4 

<u 

a 


1 


2345  6 

Turns  of  Helix 

FIG.  173.     Hot-wire-ammeter  resonance  curve  of  direct  coupled 
,  sending  station. 

the  average  integral  effect  of  a  large  number  of  oscillations,  and 
this  is  not  by  any  means  the  determining  factor  in  the  transmission 
of  messages. 

The  wave  meter  method  of  attuning  the  circuits  is  to  be  pre- 
ferred, because  it  gives  the  actual  wave  lengths  finally  attained, 
and  this  is  necessary  when  it  is  required  to  set  several  stations 
so  that  they  will  emit  particular  predetermined  wave  lengths. 

Tuning  the  Inductively  Coupled  Transmitter.  —  From  what  has 
been  said  in  regard  to  the  tuning  of  the  direct  coupled  transmitter, 
no  difficulty  will  be  encountered  in  making  the  small  modifications 
that  are  necessary  to  adapt  the  directions  to  inductively  coupled 
apparatus.  So  the  discussion  will  not  be  repeated. 

Coefficient  of  Coupling.  —  In  some  apparatus  of  the  inductively 
coupled  type  the  distance  between  the  primary  helix  and  the 
secondary  helix  can  be  varied;  this  varies  the  mutual  inductance 
of  the  circuits,  and  consequently  the  coefficient  of  coupling.  The 
diminution  of  this  coefficient  by  increasing  the  distance  between 


TUNING  THE  SENDING  STATION  251 

the  primary  and  secondary  helices  brings  the  two  resultant  wave 
lengths  produced  by  the  station  closer  together,  and  gives  a  sharper 
wave  system  than  that  obtained  with  a  large  coefficient  of  cou- 
pling. The  coefficient  of  coupling  of  the  direct  coupled  system 
also  may  be  varied,  for  example,  by  introducing  more  or  less 
inductance  (not  mutual)  in  one  of  the  circuits. 

The  question  as  to  the  best  coefficient  of  coupling  to  employ 
at  the  transmitting  station  is  difficult  to  decide.  The  question 
is  complicated  by  the  conditions  that  exist  at  the  receiving  station 
as  well  as  at  the  sending  station.  I  shall  therefore  defer  a  con- 
sideration of  this  question  until  after  a  discussion  of  the  resonant 
relations  at  the  receiving  station. 

The  Detuning  of  Coupled  Circuits.  —  We  have  shown  in  the 
preceding  paragraphs  how  the  condenser  circuit  and  the  antenna 
circuit  may  be  adjusted  to  resonance.  This  gives  in  the  coupled 
system  a  maximum  flow  of  current  and  a  maximum  radiation  of 
energy  from  the  antenna.  The  energy  radiated  is,  however,  in 
the  form  of  two  waves  of  different  wave  lengths.  Suppose  this 
doubly  periodic  wave  to  be  received  by  a  receiving  circuit.  Can  we 
not  tune  the  receiving  circuit  either  to  the  one  or  to  the  other  of 
the  received  wave  lengths?  And  would  it  not  be  preferable  to 
adjust  the  transmitting  condenser  circuit  to  a  little  longer  or  a 
little  shorter  wave  than  the  transmitting  antenna  circuit  in  order 
to  strengthen  the  longer  or  the  shorter  wave  of  the  coupled  system 
at  the  expense  of  the  other  wave  which  is  not  to  be  used  at  the 
receiving  circuit?  Professor  M.  Wien  1  shows  that  a  small  ad- 
vantage (in  some  cases  as  great  as  30%)  may  be  derived  from  a 
process  of  this  kind  provided  the  condenser  circuit  and  the  antenna 
circuit  are  differently  damped.  In  his  experiments  Wien  used 
a  simple,  low-resistance  receiving  circuit,  and  I  am  unable  to  say 
how  great  would  be  the  advantage  in  a  similar  detuning  operation, 
when  the  coupled  receiving  circuits  and  the  high-resistance  de- 
tectors of  actual  practice  are  used  at  the  receiving  apparatus. 
In  my  own  experiments  I  have  never  detected  any  appreciable 
advantage  in  detuning  an  actual  sending  station. 

Possible  Existence  of  Three  Wave  Lengths  in  a  Coupled  Sys- 
tem. —  With  the  condenser  circuit  and  the  antenna  circuit  attuned 
to  the  same  independent  wave  length,  as  in  the  case  of  our  wave 
metrical  illustration  on  page  248,  there  is  the  possibility  of  the 

1  Annalen  der  Physik,  Vol.  25,  p.  1,  1908. 


252  WIRELESS  TELEGRAPHY 

wave  meter  giving  indications  of  three  wave  lengths  instead  of 
two.  In  the  case  described,  two  of  the  wave  lengths  would  be 
358  meters  and  462  meters,  and  there  would  also  be  a  third  wave 
length  which  would  be  the  wave  length  of  the  uncoupled  antenna 
circuit;  namely,  420  meters.  The  reason  is  this:  After  a  certain 
number  of  oscillations  the  current  in  the  condenser  circuit  becomes 
so  small  that  the  spark  in  this  circuit  extinguishes.  This  opens 
the  primary  circuit,  and  we  no  longer  have  a  coupled  system;  so 
that  the  secondary  goes  on  oscillating  with  its  own  natural  period. 
This  is  shown  in  my  spark  photograph  on  page  248.  After  about 
four  beats  shown  by  the  minima  in  the  picture,  the  beats  cease 
and  the  secondary  circuit  goes  on  oscillating.  In  the  picture  it 
can  be  seen  that  up  above  the  point  where  the  beats  have  ceased 
the  oscillation  is  a  simple  oscillation,  and  these  in  the  original 
photograph  can  be  followed  for  more  than  twenty  oscillations. 
The  result  is  like  that  which  would  be  obtained  with  the  two 
coupled  pendulums  if  we  should  cut  loose  the  primary  pendulum 
at  one  of  its  positions  of  rest,  leaving  the  secondary  to  vibrate 
alone. 

With  the  electric  circuits  this  effect  of  stopping  the  primary 
current  and  allowing  the  secondary  to  go  on  vibrating  has  been 
employed  with  considerable  success  in  the  quenched-spark  method 
of  producing  oscillations,  which  is  treated  in  the  next  chapter. 


CHAPTER  XXIII 

SOME   RECENT   METHODS    OF    EXCITING    ELECTRIC    WAVES 
THE  SINGING  ARC,  THE  SINGINQ  SPARK,  AND  THE 
QUENCHED  SPARK 

THUS  far  in  this  account,  practically  only  one  method  of  pro- 
ducing oscillations  at  the  sending  station  has  been  described; 
namely,  the  method  making  use  of  the  spark  discharge  of  a  con- 
denser which  has  been  charged  from  an  alternating  current 
transformer  or  an  induction  coil.  Electric  waves  produced  in 
this  way  occur  in  discrete  trains. 

Recently  several  new  methods  of  exciting  the  oscillations  have 
come  into  use.  We  shall  begin  the  discussion  of  these  newer 
methods  by  describing  the  "  singing  arc,"  which  is  a  wide  departure 
from  the  ordinary  spark  discharge.  The  singing  arc  operates  on  a 
direct  current  source,  produces  a  practically  continuous  sequence 
of  waves,  and  has  met  with  application,  not  only  to  wireless  teleg- 
raphy, but  also  to  wireless  telephony.  The  history  of  the  sing- 
ing arc  may  be  traced  back  more  or  less  connectedly  to  an  early 
experiment  by  Elihu  Thomson. 

Elihu  Thomson's  Continuous  Current  Spark.  —  In  1892  Pro- 
fessor Elihu  Thomson  l  found  that  electric  oscillations  could  be 
produced  from  a  500-volt  direct  current  source  by  connecting  the 
source  through  a  resistance  with  a  spark  gap  which  was  shunted 
by  a  condenser  and  inductance.  This  form  of  circuit  is  repre- 
sented in  Fig.  174.  A  source  of  direct  electromotive  force  of  500 
volts  is  shown  at  E.  This  is  connected  in  series  with  a  resistance 
R  and  a  spark  gap.  In  parallel  with  the  gap  a  condenser  C  and 
a  self-inductance  L  are  shunted.  Under  these  conditions  electric 
oscillations  were  found  to  be  present  in  the  condenser  circuit.  In 
the  effort  to  intensify  and  steady  the  effects  Professor  Thomson 
used  a  blast  of  air  or  a  magnet  to  blow  out  the  spark.  This  appa- 
ratus of  Professor  Thomson  with  some  modifications  and  improve- 
ments has  been  reverted  to  in  some  of  the  recent  developments  of 
wireless  telegraphy  and  telephony. 

1  U.  S.  Patent,  No.  500,630,  July  4,  1892. 
253 


254 


WIRELESS  TELEGRAPHY 


Simon's  Talking  Arc.  —  Let  us  also  recall  beginnings  made  in 
another  direction.  In  1898,  Professor  H.  Th.  Simon,1  of  Gottingen 
in  Germany,  found  that  the  vapor  path  of  an  ordinary  electric 
arc  lamp  could  be  set  into  mechanical  vibration  by  variation  of  the 
current  through  the  arc,  and  that  the  vibrating  vapor  path  would 
communicate  its  disturbances  to  the  air  in  the  form  of  sounds. 
In  this  way,  if  a  microphone  transmitter  is  employed  to  vary 


Spark  Gap  o 


FIG.  174.     Diagram  of  Elihu  Thomson's 
direct  current  spark. 


A  re 


FIG.  175.     Professor  Simon's 
talking  arc. 


the  current  through  the  arc,  as  shown  in  Fig.  175,  the  arc  can  be 
made  to  reproduce  speech  with  sufficient  intensity  to  be  heard 
throughout  a  large  auditorium.  The  experiment  is  very  striking 
and  interesting. 

Duddell's  Singing  Arc.  —  In  1900  Duddall2  published  an  account 
of.  some  similar  experiments  with  the  arc,  in  which  the  arc  was 
made  to  produce  electric  oscillations  and  to  give  out  a  musical 
note.     This  was  brought  about  by  shunting  the  arc  with  a  con- 
denser and   inductance,   in   a 
manner  resembling  that  em- 
ployed   by    Elihu    Thomson. 
The  Duddell  circuit  is  shown 
in  Fig.  176,  and   differs  from 
the    corresponding    circuit   of 
Elihu    Thomson   by  the  sub- 
stitution  of    an   arc  between 
carbon  electrodes  for  the  me- 
tallic arc  or  spark  of  Thomson. 
In  Duddell's  apparatus  (Fig. 

176)  the  arc  A,  consisting  of  two  solid  carbon  electrodes,  is  con- 
nected in  series  with  a  direct-current  generator  E,  a  resistance  R  and 
a  self-inductance  L.  About  the  arc  are  shunted  a  condenser  C  and 

1  Wied.  Ann.,  Vol.  64,  p.  233,  1898;  Physikalische  Zeitschrift,  Vol.  2,  p.  253, 
1901. 

2  Journ.  lust,  of  Elec.  Eng.,  Vol.  30,  p.  232,  1900. 


FIG.  176.     Duddell's  singing  arc. 


RECENT  METHODS  OF  EXCITING  ELECTRIC  WAVES       255 


an  inductance  S.  With  proper  adjustments  of  the  various  parts 
of  the  circuit  the  arc  emits  a  musical  sound  which  in  Duddell's 
experiments  could  be  plainly  heard  to  a  distance  of  several  meters. 
The  pitch  of  the  note  can  be  varied  by  varying  the  capacity  C  or 
the  inductance  S.  The  experiment  is  highly  interesting  when  one 
varies  the  capacity  C  by  means  of  a  set  of  keys  and  thereby  pro- 
duces a  succession  of  notes  of  different  pitches. 

In  addition  to  the  evidence  afforded  by  the  emission  of  musical 
sounds,  the  shunt  circuit  comprising  the  condenser  C,  the  induc- 
tance L  and  the  arc  A,  may  be  shown  also  by  its  inductive  action 
on  a  neighboring  circuit  to  be  traversed  by  a  pulsating  or  oscillat- 
ing current.  We  have  thus  a  pulsating  or  oscillating  current  pro- 
duced from  a  direct-current  source. 

Why  the  Arc  Gives  Rise  to  Pulsating  Currents.  —  The  expla- 
nation of  the  production  of  oscillatory  currents  and  audible  sounds 
by  the  arc  shunted  with  a  condenser  has  been  the  subject  of  a 

considerable  amount  of  theo- 
retical and  experimental  inves- 
tigation.1 Duddell's  original 
account  of  the  phenomenon  con- 
tains a  simple  explanation,  which 
is  substantially  as  follows: 

The  electric  arc  between  car- 
bon terminals  has  a  falling  volt- 
ampere  characteristic  like  that 
shown  in  Fig.  177.  With  an  in- 
crease of  current  through  the 
arc  the  voltage  between  the  arc 
terminals  decreases.  For  this 
reason,  when  the  arc  is  connected 
16  in  series  with  a  source  of  voltage 

FIG.  177.    Vo7t"ampere  character-    and  is  "struck"  by  bringing  the 
istic  of  carbon  arc.  terminals  together  and  then  sep- 

arating them,  the  current  through  the  arc  tends  to  increase  to  a 
very  large  value,  and  must  be  restrained  by  a  suitable  resistance 
R  in  circuit  with  the  arc  (see  Fig.  176). 

Suppose,  now,  that  when  the  arc  is  quietly  burning,  a  condenser 
C  and  inductance  S  are  together  connected  about  the  arc.  The 

1  For  a  theoretical  treatment  of  this  subject  the  mathematical  reader  is 
referred  to  an  article  by  H.  Th.  Simon,  Physikalische  Zeitschrift,  Vol.  7,  p.  433, 
1906. 


40 


30 


8 
Amperes 


12 


256  WIRELESS  TELEGRAPHY 

condenser  begins  to  charge.  This  takes  current  from  the  arc  and 
in  consequence  the  voltage  between  the  arc  terminals  increases; 
this  causes  more  current  to  flow  into  the  condenser.  Finally,  the 
condenser  is  charged  to  the  same  voltage  as  that  between  the  ter- 
minals of  the  arc,  but  on  account  of  the  inductance  in  series  with 
the  condenser  the  current  into  the  condenser  continues  for  a  time 
after  this  condition  is  reached.  This  results  in  a  potential  dif- 
ference at  the  condenser  higher  than  that  at  the  arc,  which  finally 
results  in  a  cessation  of  the  current  into  the  condenser.  The 
condenser  then  begins  to  discharge  through  the  arc,  causing  a  drop 
in  the  arc  voltage  and  a  further  discharge  of  the  condenser. 
While  the  condenser  is  discharging,  the  inductance  in  series  with 
the  condenser  tends  to  preserve  the  discharging  current,  so  that 
the  condenser  potential  falls  below  that  of  the  arc.  After  the 
discharge  has  gone  on  to  a  sufficient  extent,  a  minimum  of  con- 
denser potential  is  reached,  and  the  process  again  reverses. 

The  arc  and  the  condenser  circuit  are  thus  in  an  unstable  con- 
dition and  the  condenser  continues  to  charge  and  discharge,  thus 
repeatedly  impoverishing  and  replenishing  the  arc  as  to  current. 
Whatever  energy  is  expended  in  this  oscillation  circuit  is  drawn 
from  the  direct-current  source. 

The  fluctuating  current  through  the  arc,  which  is  a  path  of  con- 
ducting vapor,  causes  the  vapor  path  to  contract  and  expand  peri- 
odically, and  thus  gives  a  continuous  train  of  periodic  disturbances 
to  the  air,  which  are  heard  as  a  musical  note  provided  their  period 
of  vibration  is  within  the  range  of  audibility. 

It  is,  however,  not  the  musical  note,  but  the  oscillating  current 
in  the  condenser  circuit,  that  is  of  interest  in  connection  with  wire- 
less telegraphy  and  telephony.  For  the  purposes  of  wireless  telegra- 
phy and  telephony  it  is  important  that  the  frequency  of  oscillation 
should  be  high;  namely,  between  one  hundred  thousand  and  one 
million  per  second.  With  the  ordinary  Duddell  arrangement  of 
a  carbon  arc  in  air  this  high  frequency  of  oscillation  does  not 
seem  to  be  easily  obtainable,  at  least  not  with  a  large  amount  of 
energy  in  the  oscillating  circuit. 

Poulsen's  Improvement  of  the  Arc  Method  of  Producing  Oscil- 
lations.—  In  1903  Valdemar  Poulsen  1  of  Copenhagen  made  an 
important  improvement  in  the  arc  method  of  producing  high- 
frequency  oscillations.  This  improvement  by  Poulsen  consisted 

1  British  Patent,  No.  15,  599,  July  14,  1903.  See  also  Science  Abstracts, 
Vol.  8,  p.  521,  Abstract  No.  1620,  1905. 


RECENT  METHODS  OF  EXCITING  ELECTRIC  WAVES      257 

primarily  in  placing  the  arc  in  an  atmosphere  of  coal  gas  or  hydro- 
gen, and  in  employing  for  the  arc  one  terminal  of  carbon  ( — )  and 
the  other  terminal  of  a  water-cooled  cylinder  of  copper  (+)  (cf. 
Fig.  178).  For  the  purpose  of  effecting  the  cooling  of  the  copper 
electrode,  it  was  made  hollow,  and  through  it  a  stream  of  water  was 
circulated.  Water  was  also  circulated  through  a  worm  within 
the  jacket  inclosing  the  coal  gas  or  hydrogen  about  the  arc,  so  as 
to  prevent  undue  heating  of  this  jacket.  To  enhance  the  strength 


Gas  Inlet 


FIG.  178.     Mr.  Poulsen's  singing-arc  generator  of  electric  waves. 

and  the  frequency  of  the  oscillations,  the  poles  of  a  powerful  electro- 
magnet NS  are  inserted,  gas-tight,  into  the  chamber,  and  placed  so 
as  to  give  a  magnetic  field  transverse  to  the  arc.  The  carbon 
terminal  of  the  arc  is  slowly  rotated  by  a  clockwork  or  electric 
motor.  This  is  to  prevent  the  formation  by  the  arc  of  inequalities 
in  the  surface  of  the  carbon  electrode.  When  all  of  these  pre- 
cautions indicated  by  Poulsen  are  taken,  the  oscillations  may  be 
given  a  frequency  as  high  as  a  million  or  more  per  second,1  which 
brings  them  well  within  the  range  useful  for  wireless  telegraphy  and 
telephony. 

The  source  of  current  is  a  direct  current  generator  D,  giving 

1  By  the  use  of  an  arc  having  a  water-cooled  copper  cathode  and  a  silver- 
point  anode,  N.  Stschodro  (Ann.  d.  Phys.  Vol.  27,  p.  225,  1908),  has  obtained 
more  than  300,000,000  oscillations  per  second,  and  has  performed  Hertz's 
mirror  experiment  with  the  electric  waves  so  produced. 


258 


WIRELESS  TELEGRAPHY 


about  500  volts.  Leads  from  the  generator  pass  in  series  through 
the  arc  and  around  the  electromagnet  NS.  About  the  arc  is 
shunted  the  condenser  C  and  the  inductance  P.  The  high- 
frequency  oscillation  takes  place  in  the  circuit  ACP,  and  these 
oscillations  are  impressed  upon  the  antenna  by  means  of  the 
oscillation  transformer  PS. 

Comparison  of  Arc  in  Coal  Gas  or  Hydrogen  with  Arc  in  Air.  - 
A  characteristic  difference  between  the  electric  arc  in  an  atmos- 
phere of  coal  gas  or  hydrogen 
and  an  arc  of  equal  length  in 
air  is  shown  in  the  volt-ampere 
curves  of  Fig.  179,  taken  from 
an  investigation  by  Mr.  W.  L. 
Upson.1  It  is  seen  that  the 
arc  in  hydrogen  shows  a 
greater  fall  of  voltage  with  a 
given  increase  of  current  than 
does  the  arc  in  air.  For  this 
reason  a  more  energetic  oscil- 
lation of  high  frequency  can 
be  obtained  from  the  arc  in 
hydrogen  than  from  the  arc 
in  air,  as  may  be  seen  from  the 
following  reasoning: 

To  obtain  the  high-frequency 
oscillation  a  condenser  of  small 


BC 


Amperes 

FIG.  179.  Volt-ampere  characteristic  of 
carbon  arc  in  air  and  copper-carbon 
arc  in  hydrogen  (Mr.  Upson). 


capacity  must  be  used  in  the  shunt  circuit;  whereas  for  a  slow 
frequency  of  oscillation  a  large  capacity  may  be  used.  Now  a  small 
condenser,  as  is  required  for  the  high  frequency,  takes  only  a  small 
amount  of  current  to  charge  it,  and  for  this  charge  to  be  energetic  it 
is  essential  that  it  should  rise  to  a  high  voltage.  It  is  therefore 
essential  that  the  shunting  of  a  small  amount  of  current  from  the 
arc  should  cause  a  large  rise  of  potential  at  the  arc  in  order  to  get 
energetic  oscillations  in  the  shunt  circuit.  From  the  volt-ampere 
curve  of  hydrogen  this  is  seen  to  be  what  happens  in  case  the  arc 
is  in  an  atmosphere  of  hydrogen.  In  order  to  get  equivalent 
steepness  of  the  volt-ampere  curve  in  air,  it  is  seen  to  be  necessary 
to  work  with  very  small  currents  in  the  arc;  whence  it  follows 
that  with  a  small  current  through  the  arc,  oscillations  of  high 


1  Phil.  Mag.,  July,  1907. 


RECENT   METHODS  OF  EXCITING  ELECTRIC  WAVES      259 

frequency  can  be  obtained,  even  with  the  arc  in  air.  The  sur- 
rounding of  the  arc  with  an  atmosphere  of  hydrogen  permits  these 
high-frequency  oscillations  to  be  obtained  also  with  a  large  current 
(10  to  12  amperes)  through  the  arc,  which  is  a  valuable  asset  for 
the  sustenance  of  energetic  oscillations. 

Instead  of  employing  an  atmosphere  of  hydrogen  about  the  arc, 
ordinary  coal  gas,  such  as  is  used  in  illumination,  produces  also 
very  good  results. 

One  method  of  feeding  the  gas  into  the  chamber  is  to  lead  it  in 
continuously  by  a  rubber  tube  connected  with  the  gas  jet  of  the 
illuminating  system.  The  gas,  after  passing  through  the  chamber 
about  the  arc,  is  conducted  away  by  a  rubber  tube  leading  to  the 
outside  of  the  building,  or  else  it  is  led  to  a  gas  burner  and 
ignited  to  prevent  it  from  escaping  unconsumed  into  the  room. 

The  Use  of  Other  Hydrocarbon  Gases  and  the  Use  of  Steam 
About   the  Arc.  —  Instead  of  coal  gas  or  hydrogen,  almost  any 
other  gaseous  hydrocarbon  may  also  be  employed  with  the  arc  to 
enhance  the  energy  and  improve  the  con- 
stancy of  the  high-frequency  oscillations. 
For  example,  the  combustion  products  of 
an  alcohol  flame  will  produce  effects  in  a 
degree  similar  to  effects  with  the  coal  gas. 
These  combustion   products  may  be  sup- 
plied to  the  arc  by  means  of  a  small  alcohol 
lamp  placed  beneath  the  arc,  as  is  shown      IG<   alcohol          al 


""1^" 

A 


in  Fig.  180. 

Similar  beneficial  effects  upon  the  oscillations  are  produced  by 
the  gases  formed  by  the  volatilization  of  a  liquid  hydrocarbon, 
such  as  turpentine,  pentane,  amyl  alcohol,  etc.  In  this  case  the 
liquid  hydrocarbon  is  allowed  to  fall  drop  by  drop  into  a  cup- 
shaped  depression  in  one  electrode,  where  it  is  volatilized  and 
surrounds  the  arc  with  an  atmosphere  of  gas. 

Dr.  Lee  DeForest  l  has  suggested  steam  as  an  atmosphere  for 
the  arc,  and  has  shown  several  methods  of  supplying  steam  to  the 
arc.  One  of  these  methods  is  depicted  in  Fig.  181  taken  from 
DeForest's  United  States  patent  specifications. 

Use  of  Several  Arcs  in  Series.  —  To  obviate  the  necessity  of  the 

magnetic  field  and  the  coal-gas  atmosphere,  as  used  with  the 

Poulsen  arc,  the  Telefunken  Company  of  Germany  employs  several 

arcs  in  series,  thus  obtaining  a  high  effective  voltage.     Only  a 

1  U.  S.  Patent,  No.  850,917,  issued  April  23,  1907. 


260  WIRELESS  TELEGRAPHY 

small  current  is  sent  through  the  arcs.  The  use  of  a  small  current 
through  the  arcs,  as  has  been  pointed  out  above,  utilizes  the  steep 
part  of  the  volt-ampere  curve  of  Fig.  179,  so  as  to  obtain  large 
fluctuations  of  current  even  with  the  arc  in  air.  The  arcs  of  the 
Telefunken  apparatus  have  carbon  cathodes  and  water-cooled 
copper  anodes,  arranged  as  in  Fig.  182,  which  shows  six  of  these 
arcs  in  series.  The  tubes  T,  T,  .  .  .  are  of  copper,  and  are  filled 
with  water  for  cooling.  The  bottom  of  each  of  the  tubes,  which 
are  the  positive  electrodes  of  the  arcs,  is  recessed  and  in  this 
recess  the  arc  is  maintained.  Provisions  are  made  for  striking 
all  of  the  arcs  at  once,  and  for  separately  adjusting  their  arc 
lengths.  The  arcs  have  a  combined  terminal  voltage  of  220  volts 
and  require  about  5  amperes.  An  oscillation  circuit  comprising 
the  condenser  C  and  the  inductance  P  is  shunted  about  the  arcs. 
The  oscillations  are  communicated  to  the  antenna  by  means  of  the 
oscillation  transformer  PS. 

On  the  Period  of  Oscillations  Produced  by  the  Duddell  and 
Poulsen  Arcs.  —  The  period  of  the  oscillations  of  the  condenser 
circuit  shunted  about  the  Duddell,  Poulsen  or  Telefunken  arc  is 
not  determined  completely  by  the  value  of  the  capacity  and 
the  inductance  in  the  oscillating  circuit,  but  is  a  function  also  of 
the  length  of  the  arc,  the  current  through  it,  the  material  of  the 
terminals,  and  the  nature  and  pressure  of  the  surrounding  gas. 
Mr.  G.  W.  Nasmyth,1  by  a  quasi-theoretical  discussion  of  the 
problem,  has  derived  the  following  expression  for  the  time  of  one 
complete  oscillation: 


i  _  R_ 

ic  r 

V  ~ 


in  which  L,  C,  and  R  are  the  self-inductance,  capacity  and  ohmic 
resistance  of  the  oscillating  circuit;  I  is  the  length  of  the  arc,  A 
the  current  through  the  arc,  and  c  and  d  are  constants  depending 
on  the  nature  of  the  terminals  of  the  arc  and  the  gas  surrounding  it. 

Mr.  Nasmyth  finds  experimental  confirmation  of  this  formula 
for  a  large  range  of  frequencies. 

On  the  Continuity  of  the  Oscillations  Produced  by  the  Arc.  - 
Instead  of  being  broken  up  into  separate  discrete  trains,  as  are  the 
electric  waves  produced  by  the  spark  discharge  of  a  condenser, 

1  Physical  Review,  Vol.  27,  p.  117,  1908. 


RECENT  METHODS   OF  EXCITING  ELECTRIC  WAVES      261 


FIG.  181.     DeForest's  arc  in  steam. 


T  T  T  T  T  T 


FIG.  182.     Telefunken  arcs  in  series. 


262  WIRELESS  TELEGRAPHY 

the  waves  emitted  from  a  circuit  connected  with  a  fluctuating  arc 
follow  one  another  in  a  continuous  sequence.  Such  a  sequence  is 
called  a  "  persistent  train  of  waves."  The  waves  are  sometimes 
called  "  undamped."  This  is  true  in  the  sense  that  all  the  waves 
have  equal  amplitude.  It  is,  however,  not  true  in  the  sense  that 
each  oscillation  is  exactly  sinusoidal  in  form.  Under  favorable 
conditions,  however,  the  current  may  be  very  nearly  sinusoidal,  as 
has  been  shown  by  oscillograms  taken  by  Professor  Simon.1 

Use  of  Persistent  Oscillations  in  Wireless  Telegraphy.  —  Al- 
though the  individual  impulses  of  a  persistent  train  of  waves  are 
not  by  any  means  so  intense  as  the  maxima  obtained  with  the 
spark-discharge  method  of  excitation,  yet  these  impulses,  arriving 
continuously  at  the  receiving  station,  may  produce  an  integral 
effect  that  compares  with  that  produced  by  the  waves  originating 
at  a  station  actuated  by  the  spark  discharge.  Up  to  the  present 
this  result  does  not  seem  to  have  been  achieved,  so  that  up  to  the 
present  time  the  greatest  distances  of  telegraphic  transmission 
have  not  been  attained  with  the  singing-arc  excitation. 

For  telegraphic  signaling,  it  is  evident  that  the  telephone 
receiver  cannot  be  employed  to  respond  to  an  unmodified  contin- 
uously arriving  train  of  waves,  because  the  frequency  of  these 
waves  is  beyond  the  limit  of  audibility  and  beyond  the  range  of  the 
telephone  receiver.  In  order  to  make  these  signals  audible  in  the 
telephone  receiver,  used  with  the  detector  at  the  receiving  station, 
the  train  of  waves  emitted  by  the  sending  station  must  be  modified 
so  as  to  give  them  a  train  frequency  of  audible  pitch.  This  is 
done  by  inserting  an  interrupter,  or  "  chopper,"  in  the  oscillating 
circuit  or  in  the  sending  antenna  circuit.  The  interrupter  breaks 
up  the  persistent  series  of  oscillations  into  discontinuous  groups 
separated  by  dormant  periods,  and  these  groups,  arriving  one  after 
the  other  at  the  receiving  station,  will,  after  being  suitably  rectified 
by  the  detector,  give  the  required  periodic  current  in  the  telephone 
receiver. 

Instead  of  actually  interrupting  the  oscillating  circuit  or  the 
antenna,  the  interrupter  may  be  used  to  throw  an  inductance  in  or 
out  at  the  sending  station  and  thereby  periodically  change  the 
resonance  relations  at  the  sending  station.  Such  a  throwing  of 
the  circuits  in  or  out  of  resonance  would  produce  a  periodic 
strengthening  and  weakening  -of  the  effects  received,  which  would 
therefore  be  audible. 

1  Physikalische  Zeitschrift,  Vol.  7,  p.  433,  1906. 


RECENT  METHODS  OF  EXCITING  ELECTRIC  WAVES      263 


Instead  of  having  the  interrupter  or  detuning  vibrator  at  the 
sending  station,  it  may  be  used  at  the  receiving  station,  as  has 
been  proposed  by  Poulsen.1  A  diagram  of  a  circuit  in  which  this  is 
done,  taken  from  Mr.  Poulsen's  United  States  patent  specification, 
is  shown  in  Fig.  183.  The  receiving  antenna  circuit  a  is  induc- 
tively connected  with  the  condenser  circuit  6,  c,  d.  In  shunt  about 
the  condenser  d  is  a  detector  s  with  its  accessories.  A  vibrating  in- 
terrupter at /is  adapted  to  connect  another  condenser  k  periodically 
in  parallel  with  the  condenser  c. 
When  the  contact  is  interrup- 
ted at  /,  assuming  that  the  os- 
cillation circuit  is  tuned  to 
resonance  under  these  circum- 
stances, intense  oscillations 
will  appear  in  this  circuit,  and 
by  means  of  the  detector  at 
s,  which  rectifies  the  oscilla- 
tions, an  integral  current  will 
pass  through  the  telephone. 
If  now  the  contact  at  /  is 
closed,  the  circuit  is  thrown 
out  of  resonance,  oscillations 
in  the  circuit  b,  c,  d  cease, 
and  the  current  in  the  tele- 


FIG.  183.  Receiving  circuit  for  persist- 
ent waves  (Poulsen). 


phone  ceases.  When  the  con- 
tact at  /  is  again  opened, 
another  integral  current  passes  through  the  telephone,  which  in 
this  way  is  made  to  respond  with  a  sound  of  pitch  determined  by 
the  frequency  of  the  interrupter.2 

In  order  to  obviate  the  necessity  of  these  interrupter  devices 
at  the  sending  or  receiving  circuit,  an  Einthoven  galvanometer 
at  the  receiving  station  may  be  used  instead  of  the  receiving  tele- 
phone. Einthovenfs  instrument  will  respond  to  the  uninterrupted 
train  of  waves  and  possesses  a  sensitiveness  even  greater  than  the 
telephone  receiver.  The  deflections  of  this  galvanometer  may  be 
photographically  recorded.  Although  this  instrument,  with  the 
necessary  moving  film  for  taking  the  photographic  record  of  the 
message,  is  not  quite  so  simple  to  install  or  to  operate  as  the  circuit 

1  U.  S.  Patent,  No.  897,  779,  applied  for  March  6,  1907,  issued  Sept.  1,  1908. 

2  The  explanation  given  by  Mr.  Poulsen  in  his  patent  specification  is  incon- 
sistent with  the  explanation  here  given. 


264  WIRELESS  TELEGRAPHY 

with  telephone  receiver,  it  is,  however,  sometimes  an  advantage  to 
have  a  photographed  record  of  the  dots  and  dashes,  rather  than  to 
depend  upon  a  correct  reading  of  the  message  by  ear. 

The  advantage  of  a  written  record  in  the  case  of  wireless  teleg- 
raphy is  perhaps  not  so  great  as  in  the  case  of  the  land-line  mes- 
sages, where  the  operator  may  be  dispensed  with  at  small  stations 
for  a  good  part  of  the  day  and  the  recording  apparatus  be  de- 
pended upon  completely.  This  cannot  at  present  be  done  so 
well  with  the  wireless  messages,  especially  in  the  case  of  transmis- 
sion of  messages  to  ships  at  sea,  because  it  is  important  to  have 
the  receipt  of  the  message  acknowledged  as  soon  as  it  is  finished; 
for  otherwise,  on  account  of  the  uncertainty  of  the  position  of  the 
ship  to  which  the  message  is  sent  and  the  uncertainty  as  to  whether 
the  message  has  been  received  unless  acknowledged,  considerable 
misgiving  might  arise  in  the  mind  of  the  sender  of  the  message. 
The  receiving  operator  cannot,  therefore,  well  be  dispensed  with. 
The  presence  of  the  receiving  operator  is  also  constantly  required 
in  order  to  effect  the  tuning  of  the  apparatus  so  as  to  adjust  it  to 
the  different  wave  lengths  employed  by  different  sending  stations, 
and  to  eliminate  signals  of  undesired  wave  lengths.  Although  the 
recording  apparatus  at  the  receiving  station  is  now  used  to  some 
extent,  the  detector  and  telephone  receiver  are  the  main  depend- 
ence for  translating  the  electric  waves  into  intelligible  signals. 

This,  however,  is  a  digression  from  the  subject  under  considera- 
tion, which  is  the  persistent  train  of  waves  produced  by  the  singing- 
arc  method  of  excitation  at  the  sending  station. 

Advantages  of  the  Singing-Arc  Excitation.  —  The  singing-arc 
method  of  excitation  has  the  advantage  for  wireless  telegraphy 
that  it  permits  sharper  tuning  at  the  receiving  station,  and  conse- 
quently better  discrimination  between  signals  of  different  wave 
lengths.  This  advantage  arises  chiefly  from  the  fact  that  the  per- 
sistent train  of  oscillations  gives  opportunity  for  the  current  at 
the  receiving  station  to  build  up  to  what  is  called  a  steady  state, 
and  on  this  account  the  high  resistance  of  the  receiving  detectors 
produces  a  less  deleterious  effect  on  the  sharpness  of  tuning.  How- 
ever, the  effect  of  the  high  resistance  of  the  detector  cannot  be 
completely  eliminated,  and  the  gain  in  sharpness  of  resonance  due 
to  having  a  persistent  train  of  oscillations  does  not  completely 
remove  the  difficulties  that  arise  from  interference.  Some  numeri- 
cal calculations  described  near  the  end  of  Chapter  XXIV  and  made 
on  the  assumption  that  the  incoming  waves  are  undamped,  show 


RECENT  METHODS  OF  EXCITING  ELECTRIC  WAVES      265 

that  the  interference  difficulties,  even  with  undamped  waves,  are 
still  considerable. 

The  main  advantage  in  the  singing-arc  method  of  excitation 
arises  in  the  applicability  to  wireless  telephony  of  this  method  of 
producing  electric  oscillations.  Wireless  telephony  is  briefly  con- 
sidered in  a  subsequent  chapter. 

As  a  continuation  of  the  discussion  of  novel  methods  of  pro- 
ducing oscillations,  we  shall  next  describe  the  Lepel  arc  and  the 
quenched  spark. 

The  Lepel  Arc.  —  In  a  German  Patent,  No.  24,757,  filed  Aug.  20, 
1907,  Baron  von  Lepel  has  described  a  very  simple  and  efficient 
form  of  discharge  gap  which  is  capable  of  operating  on  either  a 
direct  or  an  alternating-current  source.  It  consists  simply  of 
two  circular  discs  of  copper  with  a  thin  sheet  of  paper  between 
them.  The  discharge  occurs  between  the  discs  and  through  the 
paper.  A  small  perforation  made  near  the  center  of  the  paper 
affords  a  suitable  starting  place  for  the  discharge.  As  the  dis- 
charge continues,  the  paper  is  gradually  burned  away  from  the 
center  outwards.  This  burning  away  takes  place  in  an  atmos- 
phere deficient  in  oxygen,  and  consequently  requires  several  hours 
to  use  up  all  the  paper.  A  circular  groove  cut  near  the  outside 
edge  of  the  adjacent  faces  of  the  copper  plates  prevents  the  arc 
from  getting  to  the  outer  edge  of  the  discs  and  there  being  exposed 
to  the  air.  The  essential  feature  of  the  Lepel  gap  is  that  the  spark 
or  arc  shall  be  very  short  and  shall  occur  in  the  space  which  is 
deficient  in  oxygen.  The  presence  of  the  products  of  combustion 
of  the  paper  enhances  the  efficiency  of  the  arc.  The  arc  will 
operate  on  a  direct  current  source,  and  gives  discrete  trains  of 
oscillations  of  which  the  pitch  may  be  made  very  high  and  may 
be  regulated  by  regulating  the  condenser  about  the  gap  and  the 
rheostat  placed  in  the  leads  to  the  current  supply. 

The  series  of  discharges  obtained  from  the  direct-current  supply 
occurs  in  a  manner  resembling  the  occurrence  of  the  series  of  dis- 
charges obtained  by  Elihu  Thomson  with  his  singing  spark,  as 
described  on  page  253.  In  addition  each  discharge  is  rapidly 
quenched  and  gives  the  quenched-spark  effect  described  under 
the  next  heading. 

The  discs  of  the  Lepel  arc  are  3  to  5  inches  in  diameter,  and,  for 
rapid  conduction  away  of  the  heat  generated,  are  made  of  copper 
or  silver,  which  have  high  conductivity  for  heat.  Tl?e  discs  may 
also  be  made  hollow,  and  are  then  cooled  by  the  admission  of 


266  WIRELESS  TELEGRAPHY 

circulating  water.  The  space  between  the  two  discs  is  about  .01 
inch.  The  arc  operates  on  300  to  500  volts  direct  and  employs 
a  current  of  from  1  to  2  amperes.  The  inventor  1  claims  to  have 
transmitted  messages  to  a  distance  of  300  miles  with  less  than 
\  kilowatt  of  power.  On  account  of  the  low  voltages  employed, 
the  sending  condenser  is  made  of  mica  or  paraffined  paper  and 
occupies  a  space  of  only  4  cubic  inches.  The  apparatus  is  thus 
seen  to  be  very  efficient  and  easily  portable.  Baron  Lepel's 
discharger  combines  the  principle  of  the  singing  arc  with  that  of 
the  quenched  spark. 

The  Quenched  Spark.  —  In  discussing  this  subject,  let  us  recall 
the  facts  established  in  Chapters  XXI  and  XXII  that  a  system 
of  two  circuits  inductively  or  directly  coupled  together  possesses 
two  separate  and  distinct  wave  lengths,  even  when  the  two  circuits 
are  individually  attuned  to  the  same  period  before  coupling 
together.  The  existence  of  this  double  periodicity  in  the  oscilla- 
tion of  the  coupled  system  is  a  distinct  disadvantage,  both  because 
of  the  difficulty  of  establishing  sharp  resonance  with  such  a  doubly 
periodic  wave,  and  also  because  of  its  wastefulness  of  transmitting 
energy. 

A  remedy  for  this  defect,  as  was  first  pointed  out  by  Professor 
Max  Wien,2  consists  in  the  use  in  the  primary  circuit  of  a  spark 
that  quenches  itself  out  after  it  has  made  a  few  oscillations.  This 
opens  the  primary  circuit  so  that  it  is  no  longer  a  circuit  in  active 
relation  to  the  secondary,  and  allows  the  secondary  (i.e.,  the 
antenna  circuit)  to  go  on  oscillating  with  its  free  natural  period. 
As  an  illustration  of  the  manner  in  which  this  works  let  us  recall 
our  sympathetic  pendulum  experiment  of  Fig.  159.  It  will  be 
remembered  that  the  secondary  pendulum  is  undergoing  a  maxi- 
mum of  displacement  when  the  primary  is  at  rest.  Now,  if  the 
primary  pendulum  is  disconnected  or  stopped  while  it  is  at  its 
point  of  rest,  the  secondary,  which  is  describing  its  maximum 
excursion,  will  go  on  vibrating  with  its  large  amplitude,  and  will 
not  have  to  expend  a  part  of  its  -energy  in  setting  up  vibrations 
again  in  the  primary.  The  secondary  will,  therefore,  decrease  in 
amplitude  only  because  of  its  own  damping.  This  is  represented 
in  the  curves  of  Fig.  184,  which  also  represent  the  action  in  the 

1  For  further  discussion  of  Lepel's  invention,  together  with  the  inventor's 
claim  of  priority  against  Count  Arco,  see  London  Electrician,  Vol.  63,  pp.  142, 
174,  374,  1909. 

2  Physikalische  Zeitschrift,  Vol.  7,  p.  871,  1906. 


RECENT   METHODS  OF  EXCITING  ELECTRIC  WAVES      267 

corresponding  electrical  case.  P  and  S  represent  the  current  in 
the  primary  and  secondary  oscillating  circuit  having  hi  the  primary 
an  ordinary  spark  gap.  P'  and  S'  represent  the  current  in  the 
primary  and  secondary  of  a  system  having  a  quenched  spark  in 
the  primary.  The  spark  is  quenched  when  the  energy  in  the 
primary  attains  its  first  minimum.  If  this  spark  does  not  recover 
its  conductivity  again,  the  secondary  oscillation  continues  with 
its  own  free  period  and  damping  as  represented  in  S'. 

Now  it  has  been  shown  that  a  very  short  spark  kept  well  cooled 
has  exactly  this  characteristic  of  rapidly  extinguishing  after  a 

Primary  and  Secondary,   Ordinary  Spark 


Primary  and  Secondary,  Quenched  Spark 


FIG.  184.     Curves  showing  oscillations  with  ordinary  spark  and  with  quenched 

spark. 

few  oscillations,  as  is  represented  by  the  curve  P'.  A  method  of 
attaining  a  similar  result  with  a  comparatively  large  amount  of 
power  consists  in  using  several  gaps  of  the  Lepel  type  in  series. 

This  has  been  done  by  the  Telefunken  Company  in  Germany 
with  marked  success.  A  diagram  of  the  quenched  spark,  com- 
prised of  several  minute  gaps  in  series  between  metal  discs,  is  shown 
in  Fig.  185.  The  face  of  one  of  these  discs,  which  are  of  copper, 
is  shown  in  the  upper  part  of  the  figure.  The  lower  part  of  the 
figure  shows  a  section  of  a  pile  of  these  discs,  placed  so  as  to  give 
several  of  the  gaps  in  series.  Between  each  pair  of  the  discs  is  a 


268 


WIRELESS  TELEGRAPHY 


Face  of  Copper  DiS'e 


Groove 
Flange 


Copper  Disc 
.Cooling 
Flange 

-Mica  Ring 


thin  mica  ring  with  a  width  extending  from  the  center  of  the 
protecting  grooves  out  beyond  the  face  of  the  disc.  The  distance 
between  two  adjacent  discs  is  about  .01  inch,  and  the  diameter 
of  the  discs  is  about  5  inches.  The  discharge  is  sent  through 

all  of  the  gaps  in  series.  The  re- 
sult is  a  quenched  spark  that  will 
operate  on  a  high  voltage,  which 
may  be  either  from  a  direct  current 
or  an  alternating  current  source. 
With  this  apparatus  the  Telefunken 
Company  claim  to  have  transmitted 
signals  to  enormous  distances  with  a 
very  small  consumption  of  energy.1 
Peukert's  Rotating  Quenched 
Spark  in  Oil.  —  Professor  W.  Peu- 
kert,2  of  Brunswick  in  Germany,  has 
devised  a  very  efficient  and  regular 
quenched  spark  in  oil  between  two 
parallel  discs  *&0  inch  apart.  One 
of  the  discs  is  stationary,  while  the 
other  is  rotated  with  a  speed  of  800 
revolutions  per  minute.  For  the 

purpose  of  keeping  the  discs  at  a  constant  distance  apart  the  mov- 
ing disc  is  carried  on  an  axis  mounted  in  conical  bearings.  For 
regulating  the  distance  between  the  plates  the  stationary  plate  is 
adjustable  axially.  Oil  is  fed  into  the  narrow  crevasse  between 
the  plates  by  a  tube  passing  through  the  fixed  plate.  The  rotation 
of  one  of  the  plates  throws  the  oil  out  centrifugally  and  thus  keeps 
a  constant  supply  of  fresh  oil  in  the  gap.  The  Peukert  gap 
operates  with  a  direct  current  source.  The  most  favorable  voltage 
of  the  source  is  between  600  and  700  volts,  but  400  or  500  volts 
will  also  give  good  results.  The  plates  which  constitute  the  elec- 
trodes should  be  of  pure  copper  or  of  copper  silvered  on  the  active 

1  For  further  information  in  regard  to  the  quenched  spark  of  this  type, 
the  reader  is  referred  to  Fleming:  Electrician,  Vol.  63,  June  11, 1909.    Telefun- 
ken Co.:  German  Patents,  No.  27,164,  filed  June  23,  1908;  No.  27,483,  filed 
Aug.  20,   1908;  No.  28,198,  filed  May  16,   1908.     Also  various  letters  and 
addresses  by  Count  Arco  in  London  Electrician,  Vol.  63,  1909. 

2  A  report  of  experiments  on  the  Peukert  gap  by  Dr.  A.  Wasmus  of  the 
Brunswick  Technische  Hochschule  is  contained  in  London  Electrician,  Vol.  64, 
p.  550,  1910.     The  apparatus  is  to  be  placed  on  the  market  by  the  Polyfre- 
quenz  Electricitats  Gesellschaft. 


FIG.  185.     Quenched-spark 
discharger. 


RECENT  METHODS  OF  EXCITING  ELECTRIC  WAVES      269 

surfaces.  One  gap  will  carry  efficiently  not  more  than  4  amperes. 
The  oscillations  occur  in  a  practically  continuous  train  and  are 
suitable  for  wireless  telephony.  To  give  a  tone  to  the  discharge, 
so  as  to  adapt  it  to  wireless  telegraphy  with  a  rectifier  and  tele- 
phone as  receiver,  one  of  the  discs  may  be  segmented. 

Some  Facts  in  Regard  to  the  Quenched  Spark.  —  Recurring  to 
the  curves  of  Fig.  184,  it  will  be  seen  wherein  consists  the  advantage 
of  a  properly  quenched  spark;  namely,  the  spark  is  active  only  long 
enough  to  allow  the  oscillations  of  the  antenna  circuit  to  build 
up  to  a  maximum  of  intensity.  The  number  of  oscillations  of  the 
primary  requisite  to  attain  this  is  the  fewer  the  closer  the  coupling 
between  primary  and  secondary.  The  intensity  of  the  secondary 
is  a  maximum  when  the  current  of  the  primary  is  a  minimum.  If 
the  spark  completely  loses  its  conductivity  at  this  point,  the  sub- 
sequent oscillations  of  the  secondary  induce  an  electromotive  force 
in  the  primary,  but  if  no  current  is  established  in  the  primary,  no 
energy  is  thereby  consumed,  and  all  of  the  energy,  which  is  now 
stored  in  the  secondary  circuit,  will  stay  there  until  radiated. 

If,  on  the  other  hand,  the  primary  spark  does  not  completely 
lose  its  conductivity  at  its  minimum,  the  e.m.f.  impressed  back  on 
the  primary  by  the  oscillations  in  the  secondary  will  reestablish 
current  in  the  primary.  This  current  in  the  primary,  flowing  as  it 
does  repeatedly  across  the  spark  gap,  heats  it,  and  dissipates  a 
considerable  part  of  the  energy  of  the  system  as  heat  in  the  gap. 
This  recommunication  of  energy  to  the  primary  is  worse  than  use- 
less because  in  addition  to  dissipating  energy*,  it  is  active  also  in 
burning  away  the  spark  gap  and  in  severely  straining  and  heating 
the  transmitting  condensers. 

In  addition  to  this  loss  of  energy  and  the  destructive  strain  on 
the  apparatus,  the  double  periodicity  of  the  vibration,  with  the 
use  of  the  unquenched  spark,  is  a  hindrance  to  discriminating 
tuning  of  the  receiving  station. 

The  quenched  spark  is,  therefore,  economical  in  transmitting 
energy,  and  is  favorable  to  sharp  tuning;  and,  by  obviating  a  use- 
less dissipation  of  energy  in  the  primary  circuit,  it  also  materially 
contributes  to  the  life  of  the  transmitting  apparatus. 

What  are  the  characteristics  of  a  spark  gap  in  order  that  it 
should  give  a  quenched  spark  ?  After  the  energy  has  left  the 
primary  circuit,  the  gap  should  very  rapidly  recover  its  high  resist- 
ance, so  that  oscillations  will  not  again  be  set  up  in  the  primary 
by  the  reaction  of  the  secondary.  This  the  author  found  to  be 


270  WIRELESS  TELEGRAPHY 

the  principal  characteristic  of  the  Hewitt  mercury  interrupter,1 
and  in  the  light  of  the  recent  investigations  of  Professor  Wien  and 
others  on  the  quenched  spark,  it  is  apparent  that  the  high  effi- 
ciency of  the  mercury  interrupter  in  exciting  oscillations  is  un- 
doubtedly due  to  its  action  as  a  quenched  spark.  Unfortunately 
Mr.  Hewitt's  mercury  interrupter  deteriorates  and  breaks  too 
easily  to  be  serviceable  in  its  ordinary  form  as  a  quenched  spark. 
It  is  possible  that  a  manner  of  constructing  this  apparatus  may  be 
discovered  that  will  remove  its  deficiency  of  short  life. 

Another  very  evident  quenched  spark  that  has  long  been  in 
use  in  America  is  the  gap  devised  by  Mr.  Kinraidy  for  operating 
his  Tesla  coil  for  therapeutic  use.  Mr.  Kinraidy's  gap  consisted 
of  two  water-cooled  flat  terminals  very  close  together  between 
which  the  discharge  occurred.  With  this  kind  of  a  gap  the  Kin- 
raidy coil  gives  extraordinarily  long  and  intense  Tesla  discharges 
with  the  expenditure  of  only  100  watts  in  the  primary. 

The  Lepel  arc,  the  Telefunken  series  of  Lepel  arcs,  the  Peukert 
gap  in  oil  between  a  fixed  and  a  rotating  disc,  are  very  efficient 
practical  forms  of  quenched  spark,  and  all  possess  in  common  the 
characteristic  of  a  very  short  spark  gap  provided  with  means  of 
rapid  cooling  so  as  to  effect  a  speedy  restoration  of  the  high  resist- 
ance of  the  gap  after  the  energy  has  left  the  primary  circuit. 
The  credit  for  foreseeing  ,the  importance  of  this  requirement  and 
of  indicating  means  for  attaining  it  belongs  to  Professor  Max 
Wien. 

1  G.  W.  Pierce:  Proc.  American  Academy  of  Arts  and  Sciences,  Vol.  39, 
No.  18,  February,  1904. 


CHAPTER  XXIV 

RESONANCE  OF  RECEIVING  CIRCUITS.     THE  POSSIBILITY  OF 
PREVENTING  INTERFERENCE 

How  does  the  current  induced  in  a  receiving  antenna  depend 
upon  the  height  of  the  receiving  antenna?  How  much  is  the 
strength  of  this  current  modified  by  tuning  the  antenna  ?  In  a 
coupled  receiving  circuit  what  resonant  relations  exist  between 
the  two  parts  of  the  coupled  system  ?  How  sharp  is  the  tuning 
at  the  receiving  station,  and  to  what  extent  can  interference  be 
prevented  ? 

It  is  proposed  in  this  chapter  to  present  a  brief  examination  of 
these  questions.1  For  this  purpose  some  experiments  are  described. 

DEPENDENCE  OF  RECEIVED  CURRENT  ON  HEIGHT  OF  RECEIVING 

ANTENNA 

IN  an  investigation  to  ascertain  the  dependence  of  received 
current  on  the  height  of  receiving  antenna,  a  direct  coupled 
transmitter,  like  that  illustrated  in  Figs.  152  and  165  was  used  to 
produce  the  electric  waves.  The  two  circuits  of  the  transmit- 
ting station  were  adjusted  to  resonance  with  each  other  by  the 
hot-wire  ammeter  method  of  Chapter  XXII.  The  dimensions 
of  the  transmitting  circuits  were  as  follows:  The  secondary  part 
S  of  the  helix  consisted  of  5  turns  of  wire  .208  cm.  in  diameter, 
wound  in  a  spiral  46  cm.  in  diameter,  with  a  pitch  of  5.08  cm.  The 
inductance  of  this  part  of  the  helix  was  1.56  X  10~~6  henrys.  The 
primary  part  P  of  the  helix  consisted  of  1.2  turns  and  had  an 
inductance  of  .151  X  10~5  henrys.  The  condenser  was  made  up 
of  sheets  of  copper  separated  by  miconite  plates.  The  antenna, 
with  dimensions  marked,  is  shown  in  Fig.  186.  The  station  sent 
out  two  waves,  —  one  of  wave  length  153  meters  and  the  other 
of  wave  length  129  meters. 

For  the  purpose  of  determining  what  relative  currents  are 

1  G.  W.  Pierce:  Physical  Review,  Vol.  19,  p.  196,  1904;  Vol.  20,  p.  220, 
1905;  Vol.  21,  p.  367,  1905;  Vol.  22,  p.  159,  1906. 

271 


272 


WIRELESS  TELEGRAPHY 


obtained  in  a  receiving  antenna,  I  set  up  an  experimental  receiving 
station  at  a  distance  of  550  feet  from  the  sending  station,  and  made 
some  comparative  measurements  of  the  current  received  when 
various  lengths  of  a  single  vertical  wire  (.208  cm.  in  diameter)  were 
used  as  a  receiving  antenna.  Provision  was  made  for  bringing 
the  receiving  antenna  back  into  resonance  with  the  incoming 
waves  after  each  change  of  length  of  the  antenna.  This  was  done 
in  two  different  ways:  (1)  by  an  inductance  inserted  in  the  antenna, 
and  (2)  by  a  shunt  capacity;  and  since  the  law  showing  the  relation 
of  current  to  height  of  receiving  antenna  was  different  in  the  two 
cases,  the  two  sets  of  results  will  both  be  briefly  presented. 


o  Height 
4above  coil 


59  CM 


Diam  Wire 
".208  CM 


Tube.8  CM 
Diameter 


FIG.  186. 


Antenna  of  experiments 
on  resonance. 


FIG.  187.     Antenna  with  variable 
inductance  for  tuning. 


Experiments  on  Received  Current  for  Various  Heights  of  Re- 
ceiving Antenna,  when  Tuning  was  Effected  by  an  Inductance  in 
Antenna.  —  The  form  of  receiving  circuit  employed  in  this  case 
is  shown  in  Fig.  187.  The  current-reading  instrument  shown  at  D 
was  the  high-frequency  dynamometer  described  on  page  113.  It- 
consisted  of  a  minute  coil  of  wire  through  which  the  oscillatory 
currents  were  passed;  near  this  coil  was  suspended  a  small  disc 
of  silver.  Oscillatory  currents  in  the  coil  induced  oscillations  in 
the  disc  and  caused  the  disc  to  deflect.  The  resistance  of  this  in- 
strument was  only  1.33  ohms.  Its  inductance  was  1.17  X  10~5 
henrys. 


RESONANCE  OF  RECEIVING  CIRCUITS 


273 


The  variable  inductance  used  for  tuning  the  circuit  consisted  of 
51  turns  of  wire,  .208  cm.  in  diameter,  wound  in  a  spiral  on  a 
vulcanite  drum.  Variations  of  inductance  were  made  by  turning 
the  drum,  and  thereby  causing  a  wheel-contact  to  move  along  the 
spiral.  The  inductance  of  the  whole  coil  was  16.5  X  10 ~5  henrys, 
and  the  inductance  of 
any  fraction  of  the  coil 
was  accurately  known. 

The  results  of  a  set  of 
measurements  are  given 
in  the  curves  of  Fig.  188. 
The  first  curve,  marked 
23.2  at  its  vertex,  was 
taken  with  a  vertical  re- 
ceiving antenna  23.2  me- 
ters long  (measured  from 
the  junction  with  the 
tuning  coil).  The  differ- 
ent points  on  this  curve 
were  obtained  as  deflec- 
tions of.  the  dynamome- 
ter for  different  values  of 
the  inductance  of  the 
tuning  coil.  When  the 
length  of  the  receiving 

antenna  was  changed  from  23.2  meters  to  20  meters,  the  curve 
marked  20  at  its  vertex  was  obtained.  In  the  same  way  the 
curves  marked  16,  12  and  8  were  obtained 
for  lengths  of  antenna  16,  12  and  8  meters 
respectively. 

Before  discussing  the  results  of  this  experi- 
ment I  will  present  data  obtained  with  a 
different  form  of  receiving  circuit. 

Similar  Experiments  with  Shunt-Capacity 
Method  of'  Tuning.  —  A  diagram  of  this 


Inductance 


10          12         14 

'5  Henry 


FlG.   188. 


Resonance  curves  with  circuit  of 
form  of  Fig.  187. 


OI 


FIG.  189.    Circuit 


An 


for    tuning   with  receiving  circuit  is  shown  in  Fig.   189. 
shunt  capacity.  ..      ,    _ .  ,  .  ,  ... 

adjustable  air  condenser  of  known  calibration 

in  terms  of  capacity  was  placed  in  shunt  to  the  receiving  instru- 
ment, /,  and  by  its  use  tuning  was  effected.  Different  lengths 
of  receiving  antenna  were  employed  and  the  resonance  curves 
of  deflections  against  capacity  were  plotted.  These  are  given 


274 


WIRELESS   TELEGRAPHY 


in  Fig.   190.     The  different  curves   correspond   to  the  different 
heights    of  antenna  as  marked    at  the  vertices  of    the  curves. 


50 


Capacity 


FIG.  190.     Resonance  curves  with 
shunt-capacity  tuning, 


.2468      10 
Capacity     x.lO-10Earad. 

FIG.  191.     Two  of  the  curves  on 
enlarged  scale. 


The  curves  taken  with  8  meters  and  4  meters  of  antenna  are 
plotted  separately  in  Fig.  191,  where  the  scale  of  deflections  is 
magnified  25  times. 

On  the  Form  of  the  Resonance  Curves.  —  The  two  sets  of 
curves  taken  with  the  two  different  methods  of  tuning  show  a 
marked  similarity  in  form.  The  two  maxima  corresponding  to 
the  two  different  waves  sent  out  from  the  transmitting  station 
are  clearly  apparent.  The  irregularities  near  the  summits, 
possessed  in  common  by  the  two  sets  of  curves,  evidently  belong 
to  the  wave  produced  at  the  sending  station  and  are  not  charac- 
teristic of  the  receiving  station.  These  irregularities  could  have 
been  eliminated  by  a  little  more  care  in  setting  up  the  sending 
stations. 

Comparison  of  Merits  of  the  Two  Methods  of  Tuning.  —  In 
passing,  it  is  interesting  to  compare  the  strength  of  signals  ob- 
tained with  the  shunt-capacity  method  of  tuning  with  those 


RESONANCE  OF  RECEIVING  CIRCUITS 


275 


obtained  with  the  series-inductance  method.  The  deflection  at 
resonance  for  the  two  different  methods  of  tuning,  for  different 
heights  of  antenna,  are  plotted  in  Fig.  192.  The  lower  curve  A 
was  obtained  with  the  inductance  method  of  tuning;  the  curve  B, 


50 


40 


10 


2      4      6      8     10    12    14    16    18    20    22    24 
Height  ot  Antenna,  Meters 

FIG.  192.     Deflection  as  a  function  of  the  height  of  antenna.     A,  circuit 
tuned  with  series  inductance;  B,  tuned  with  shunt  capacity. 

with  the  shunt-capacity  method.  It  is  seen  that  the  shunt- 
capacity  method  of  tuning  gives  larger  values.  In  comparing 
these  results  numerically  it  should  be  remembered  that  the  deflec- 
tions of  the  instrument  are  proportional  to  the  square  of  the 
current  received. 

Relation  of  Received  Current  to  Height  of  Receiving  Antenna.  - 
Coming  now  to  the  more  important  question  as  to  the  relation  of 
received  current  to  height  of  receiving  antenna  for  each  of  the 
methods  of  tuning,  we  get  the  interesting  result  that  the  law  is 
entirely  different  for  the  two  different  methods. 

In  order  to  make  the  relation  apparent,  the  scale  of  the  deflec- 
tions was  changed  by  a  constant  multiplier  so  as  to  make  the 
deflection  at  23.2  meters  unity.  The  simplified  relative  deflec- 
tions thus  obtained,  together  with  the  square  roots  and  the  fourth 
roots  of  these  deflections  are  plotted  in  Figs.  193  and  194.  It  is 
seen  that  in  the  series-inductance  case  (Fig.  193)  the  square- 
roots  of  the  deflections  lie  on  a  straight  line,  while  in  the  shunt- 
capacity  case  (Fig.  194)  it  is  the  fourth  roots  of  the  deflections 
that  lie  on  a  straight  line. 

Remembering  that  the  deflections  of  the  instrument  are  pro- 


276 


WIRELESS  TELEGRAPHY 


portional  to  the  square  of  the  current,  the  results  shown  by  the 
curves  may  be  stated  as  follows: 

I.  The  r.m.s.   current  in  a  vertical  receiving  antenna  is  proportional  to 
the  height  of  antenna,  when  this  antenna  is  brought  to  resonance  with  incident 
waves  by  an  appropriate  inductance  in  series  with  the  antenna. 

II.  The  current  in  an  inductive  part  of  the  circuit  (the  instrument)  shunted 
with  a  capacity  is  proportional  to  the  square  of  the  height  of  the  vertical  receiv- 
ing antenna,  when  the  circuit  is  brought  to  resonance  by  appropriate  adjust- 
ment of  the  shunt  capacity. 


FIG.  193. 


1.0 

9 

§8 

1' 

<D 

"§6 

£& 
34 

a. 

2 
1 

^ 

^ 

? 

X* 

X 

s' 

••''/ 

5 

^ 

x" 

s 

s 

A 

' 

X 

/ 

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/ 

$/ 

/ 

/ 

X 

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/ 

/ 

V 

/ 

/ 

s 

/ 

/ 

x 

/ 

zb 

^\ 

2     4      6      8     10     12    14    16    18    20    22    24 
Height  of  Antenna,  Meters 

Relation  of  received  current  to  height  of  antenna  when 
circuit  is  tuned  by  series  inductance. 


1.0 


8 


A/i 


6      8     10    12    14    16    18    20     22    24 
Height  of  Antenna,  Meters 

FIG.  194.     Relation  of  received  current  to  height  of  antenna  when 
circuit  is  tuned  by  a  shunt  capacity. 

These  laws  are  only  approximate,  as  shown  by  the  fact  that  the 
straight  lines  in  the  two  figures  do  not  pass  through  the  origin, 


RESONANCE  OF  RECEIVING  CIRCUITS 


277 


as  they  should  for  an  exact  proportion.  The  reason  of  this  de- 
parture from  proportionality  in  the  case  of  Law  II  may  be  found 
in  the  fact  that  the  lengths  of  antenna  were  measured  from  the 
instrument  to  the  top  of  the  antenna.  This  leaves  out  of  account 
the  part  of  the  antenna  between  the  instrument  and  the  ground, 
which  amounted  to  2  meters.  This  was  also  exposed  to  the  action 
of  the  waves,  and  should  perhaps  be  added  to  the  height;  this 
would  make  Law  II  almost  an  exact  statement  of  the  experimental 
result. 

It  is  entirely  possible  that  the  relations  I  and  II  here  stated  may 
fail  of  verification  when  tested  with  greater  heights  of  antenna. 
In  the  meanwhile  the  relations  may  be  taken  as  fair  approxima- 
tions to  the  truth. 


ii  in 


FIG.  195.     Transmitting  and  receiving  circuits  for  resonance  experiments. 
RESONANCE    IN    INDUCTIVELY    COUPLED    RECEIVING    CIRCUIT 

In  the  present  experiments  the  inductively  coupled  type  of 
circuits  was  employed  at  both  the  sending  and  the  receiving  sta- 
tions. These  circuits  are  shown  in  Fig.  195.  It  is  seen  that  the 


278 


WIRELESS  TELEGRAPHY 


complete  system  consists  of  four  circuits,  which  shall  be  referred 
to  in  what  follows  as:  I,  the  sending  condenser  circuit;  II,  the 
sending  antenna  circuit;  III,  the  receiving  antenna  circuit;  and 
IV,  the  receiving  condenser  circuit.  The  distance  between  the 
two  stations  is  187  meters  across  an  open  field.  Antennae  25  me- 
ters high  could  be  used. 

Coils.  —  Throughout  these  experiments  the  coils  of  the  four 
circuits  were  kept  constant  and  had  the  following  dimensions: 


Coil  No. 

No.  of  Turns. 

Diameter  of 
Wire,  cm. 

Length  of 
Solenoid,  cm. 

Inductance, 
Henrys. 

I. 

9 

.164 

1.71  X10~5 

II. 

240 

.104 

46 

125         X10-5 

III. 

240 

.104 

46 

125        X10-8 

IV.1 

17 

.164 

7.04X10-5 

1  Including  the  instrument. 

Condensers.  —  The  condensers  at  the  two  stations  were  ad- 
justable. The  condenser  at  the  sending  station  was  a  glass-plate 
condenser,  of  which  the  number  of  plates  could  be  varied.  At 
the  receiving  station  air  condensers  were  used.  They  were  four 
in  number,  made  of  concentric  tubes  of  brass.  The  capacity  in 
this  circuit  was  varied  by  throwing  in  or  out  these  air  condensers 
as  wholes  or  by  varying  any  one  of  them  by  withdrawing  the  inner 
cylinder  and  reading  on  a  scale  the  number  of  centimeters  of 
length  left  overlapping.  In  the  curves  presented  below,  the 
capacity  in  the  receiving  condenser  circuit  IV,  called  "  receiving 
capacity,"  is  given  in  centimeters  of  cylinder  overlapping  in  the 
air  condensers  —  1  cm.  being  equal  to  2.77  X  10~u  farads. 

Mercury  Interrupter.  —  The  oscillations  at  the  sending  station 
were  produced  by  the  discharge  of  the  glass  condenser  through  a 
Cooper  Hewitt  Mercury  Interrupter.1  The  mercury  interrupter 
was  submerged  in  oil  kept  at  95°  by  an  electric  heater  controlled  by 
an  automatic  thermal  regulator.  It  was  shown  in  a  previous 
research 2  that  a  mercury  interrupter,  in  which  no  residual  air  was 
left,  operated  most  effectively  at  that  temperature. 

Source  of  Current.  —  The  source  of  current  in  these  experi- 
ments was  a  step-up  transformer  operated  on  the  110-volt  alter- 

1  Pierce,  Proc.  Am.  Acad.  Arts  and  Sciences,  Vol.  39,  No.  18,  Feb.,  1904. 

2  Pierce,  Physical  Review,  Vol.  19,  p.  216. 


RESONANCE  OF  RECEIVING  CIRCUITS 


279 


nating  electric  light  circuit.  The  secondary  of  the  transformer 
was  connected  to  the  condenser  C,  Fig.  195.  The  switch  in  the 
primary  was  closed  and  opened  automatically  by  a  clockwork, 
so  that  the  signals  were  sent  every  35  seconds,  without  the  aid 
of  an  assistant.  Each  signal  lasted  for  5  seconds,  which  was  a 
little  greater  than  the  time  required  for  reading  the  receiving 
instrument. 

Receiving  Instrument.  —  The  receiving  instrument,  shown  at 
G,  Fig.  195,  was  again  the  high-frequency  dynamometer  (described 
on  p.  113),  with  a  resistance  of  1.33  ohms.  Such  an  instrument 
of  low  resistance  does  not  materially  modify  the  resonance  condi- 
tions, so  that  the  results  obtained  are  the  results  for  the  circuits 
themselves.  When  these  circuits  are  employed  with  the  com- 
mercial detectors  of  high  resistance,  it  is  necessary  to  ascertain 
how  far  the  resonance  relations  are  modified  by  the  detector. 
At  present,  however,  we  are  concerned  primarily  with  the  resonant 
behavior  of  the  circuits  themselves. 

Harmonic  Oscillation.  —  The  following  experiment  shows  the 
possibility  of  harmonic  resonance  of  the  inductively  coupled 


10       20       30 


50       60       70       80       90      100      110     120      130     140     150     160 
Receiving  Capacity 


FIG.  196.     Resonance  curves  obtained  by  taking  readings  of  the  dynamometer 
with  various  adjustments  of  the  sending  and  receiving  condensers. 


sending  and  receiving  circuits.  With  the  sending  and  receiving 
antennae  circuits  of  identical  dimensions,  different  values  were 
given  to  the  capacity  of  the  sending  station,  and  resonance  curves 
were  taken  by  variations  of  the  receiving  capacity.  The  curves 
of  Fig.  196  were  obtained.  Curves  1,  2,  3,  ...  7  were  with 
1,  2,  3,  ...  7  plates  of  condenser  at  the  sending  station.  It  is 


280 


WIRELESS  TELEGRAPHY 


seen  that  three  plates,  giving  the  resonance  curve  3,  appeared  to 
constitute  the  most  favorable  conditions  at  the  sending  station. 

But  a  close  examination  of  Fig.  196  shows  that  there  is  a  tend- 
ency of  the  resonance  curves  to  rise  again  out  in  the  region  near 
"  receiving  capacity"  70;  so  it  was  thought  advisable  to  go  on 


_W-    *T       ~  *T  T—    .  I  I  '_r.-j-r  '  -.  '  '  -     -  -      -I u          1  '  I  ...  I  _  _.! 

10        20       30       40        50       60        70       80       90       100      110      120     130      140      150 
Receiving  Capacity 

FIG.  197.     Resonance  curves  showing  the  existence  of  harmonic  oscillations. 

increasing  the  capacity  at  the  sending  station.  The  result  is 
shown  in  Fig.  197.  Increasing  the  plates  of  sending  condenser 
from  7  up  to  17  disclosed  the  fact  that  a  still  better  sending  station 
results  from  the  use  of  15  plates  of  condenser  at  the  sending  station. 
That  is,  the  sending  station  was  a  resonant  station  with  either 
3  or  15  plates  in  its  condenser  circuit.  This  was  without  any 


RESONANCE  OF  RECEIVING  CIRCUITS  281 

change  in  the  antenna  circuit.  The  reason  is  apparent.  The 
15  plates  set  the  antenna  vibrating  with  its  fundamental  period, 
while  the  3  plates  set  the  antenna  vibrating  as  a  first  odd  harmonic. 
The  plates  of  the  sending  condenser  were  not  all  equal,  so  we  must 
look  to  the  receiving  apparatus  for  a  verification  of  this  statement. 
This  verification  is  evident  from  the  optimum  values  of  the  reso- 
nant receiving  capacity;  namely,  approximately  108  and  12,  which 
are  in  the  ratio  of  9  to  1.  These  capacities  being  in  the  ratio  of 
9  to  1,  the  corresponding  periods,  which  are  proportional  to  the 
square  root  of  the  capacities,  are  in  the  ratio  of  3  to  1,  which  is  the 
ratio  of  fundamental  to  first  odd  harmonic. 

This  evidence  of  the  possibility  of  a  harmonic  excitation  of  the 
sending  antenna,  and  the  harmonic  response  of  the  receiving 
antenna,  shows  the  interesting  analogy  of  the  electrical  apparatus 
to  such  acoustic  apparatus  as  a  closed  organ  pipe. 

This  experiment  was  performed  with  the  receiving  antenna  cir- 
cuit an  exact  duplicate  of  the  sending  antenna.  For  the  purpose 
of  obtaining  information  somewhat  more  general,  it  is  proposed 
next  to  show  some  experiments  with  variations  of  the  length  of  the 
receiving  antenna,  and  to  study  the  resulting  effects  on  resonance. 

Resonance  Curves  with  Variation  of  the  Length  of  Receiving 
Antenna.  —  The  inductively  coupled  transmitting  station  S  of  Fig. 
195  was  employed  to  produce  the  waves.  The  sending  antenna 
used  was  the  four- wire  antenna  15.8  meters  long  of  Fig.  186.  The 
sending  condenser  circuit  was  carefully  adjusted  to  resonance  with 
the  antenna.  The  conditions  at  the  sending  station  were  kept 
constant. 

At  the  receiving  station,  which  was  also  inductively  coupled 
(cf.  R,  Fig.  195),  the  coils  of  the  inductive  coupling  were  kept 
constant.  The  problem  was  to  set  up  at  the  receiving  station 
various  heights  of  antenna,  make  various  adjustments  of  the  con- 
denser in  the  side  circuit  and  take  readings  of  deflections  of  the 
dynamometer  which  is  in  the  side  circuit. 

We  have  arriving  at  the  receiving  station  waves  of  constant 
period  and  approximately  constant  intensity,  and  we  are  to  seek 
the  conditions  under  which  the  receiving  instrument  shows  the 
largest  readings.  The  variables  are  the  height  of  the  receiving 
antenna  and  the  capacity  of  the  air  condenser,  which  is  in  the  side 
circuit  at  the  receiving  station. 

The  receiving  antenna  of  four  wires  was  started  at  a  height  of 
23.8  meters,  measured  from  the  coil  in  the  mast  circuit.  The 


282 


WIRELESS  TELEGRAPHY 


receiving  antenna  in  this  case  was  eight  meters  higher  than  the 
sending  antenna.  With  this  arrangement  Curve  1,  Fig.  198,  was 
obtained  by  reading  the  deflections  when  the  air  condenser  in  the 
receiving  side  circuit  was  set  at  various  values. 

Next,  the  receiving  antenna  was  shortened  by  cutting  off  3  meters 
from  the  parallel  portion,  making  the  height  20.8  meters.  Curve 
2,  Fig.  198,  was  obtained.  Decreasing  the  length  further  to  17.8, 
15.8,  14.8,  13.8  and  12.8  meters  gave  Curves  3,  4,  5,  6  and  7  respec- 
tively of  Fig.  198.  When  the  antenna  was  decreased  one-half 
meter  further,  the  deflections  were'  smaller  than  those  of  curve  7, 
and  increased  slowly  out  to  the  limit  of  my  available  receiving 
capacity  (180  cm.  cyl.),  so  that  the  maximum  could  not  be  located. 


10    20    30    40    50    60    70    80    90  100  110  120  130  1.40  150  160  170 

Receiving  Capacity 

FIG.  198.     Family  of  curves  obtained  by  taking  readings  of  the 
dynamometer  with  various  lengths  of  receiving  antenna. 


With  our  attention  fixed  upon  Curves  1  to  7  of  Fig.  198  let  us 
note  a  relation  between  the  height  of  the  antenna  and  the  capacity 
required  in  the  side  circuit  at  the  receiving  station  to  produce 
resonance. 

Empirical  Equation  for  the  Relation  of  Ha  to  C4.  —  The  above 
curves  show  that  with  a  fixed  frequency  of  incident  waves,  when 
the  height  of  the  receiving  antenna  Ha  was  decreased,  it  was  neces- 
sary to  increase  the  condenser  capacity  C^  in  order  to  obtain 
resonance. 

To  show  quantitatively  this  effect  Curve  A,  Fig.  199,  was  con- 
structed with  the  resonant  capacity  in  the  receiving  side  circuit 


RESONANCE  OF  RECEIVING  CIRCUITS 


283 


plotted  horizontally  and  the  height  of  the  antenna  plotted  ver- 
tically. Curve  A  was  found  by  trial  to  have  approximately  the 
equation 

(Ha  -  11.8)  (C4  -  84.6)  =  88,  (a) 


Meters,  Height  of  Receiving  Antenna 

»4tf».OjaOo5»£oiOo8l$Ji2 

A 

\ 

' 

\ 

^ 

V, 

k  —  - 

—  — 

•  •. 

. 

SA' 

"""  *• 

^ 

S, 

\\ 

V1 

\ 

A 

— 

— 

Uhs 

rai< 

erv* 
ilia 

>d 

fed 

1 

10  20 


40     50    60    70    80    90  100  110  120  130  140  150  160 
Receiving  Capacity 


FIG.  199.     Relation  of  resonant  receiving  capacity  to  height  of 
receiving  antenna. 


as  is  shown  by  the  following  comparison  of  observed  values,  with 
values  calculated  from  this  equation  (Table  XIV) : 


TABLE   XIV 

RELATION   BETWEEN  HEIGHT  OF  RECEIVING  ANTENNA  AND  RESONANT 
CAPACITY.     FOUR  WIRES   RECEIVING 


Curve  No., 
Fig.  196. 

Meters  Antenna 
Above  Coil,  Ha. 

Maximum 
Deflection,  cm. 

Resonant 
Capacity  Ob- 
served, C4 

Resonant 
Capacity  Cal- 
culated. 

1 

23.8 

64 

92 

91.9 

2 

20.8 

47 

94 

94.4 

3 

17.8 

43 

100 

99.3 

4 

15.8 

29.5 

106 

106.6 

5 

14.8 

21 

115 

113.9 

6 

13.8 

13 

130 

128.6 

7 

12.8 

7.5 

165 

172.6 

The  only  large  difference  between  the  observed  and  the  calcu- 
lated value  of  resonant  capacity  is  in  the  case  of  Curve  7,  where 


284  WIRELESS  TELEGRAPHY 

on  account  of  the  obtuseness  of  the  experimental  curve  its  maxi- 
mum could  not  be  accurately  determined. 

An  examination  of  equation  (a)  shows  that  if  we  make  Ha 
(height  of  receiving  antenna)  =  11.8  meters,  C4  would  become 
infinite.  This  is  the  interesting  fact  that,  with  the  particular 
fixed  inductance  coils  employed  in  this  experiment,  if  our  equation 
is  exact,  no  adjustment  of  the  side  condenser  would  enable  us  to 
receive  any  appreciable  amount  of  current  of  the  particular  wave 
length  arriving  from  the  sending  station.  The  experiment  showed 
this  to  be  approximately  true,  notwithstanding  the  fact  that  the 
receiving  antenna,  11.8  meters  of  4  wires,  was  not  very  different 
from  the  sending  antenna,  15.8  meters  of  4  wires. 

Let  us  look  at  equation  (a)  again  and  suppose  Ha  to  be  less 
than  11.8.  Let  Ha  be  10,  then  equation  (a)  shows  that 

-  1.8  (C4  -  84.6)  =  88, 

C4  -  84.6    =  -  49, 
C4  =  35.6; 

that  is,  having  lost  resonance  at  Ha  =  11 .8,  if  we  decrease  Ha  to 
10  meters  we  ought  to  find  the  resonance  •  again,  but  instead  of 
finding  it  out  near  where  we  lost  it  (beyond  C4  =  170)  the  reso- 
nance ought  to  reappear  at  a  comparatively  small  value  of  C4. 
This  was  tried  with  the  following  result: 

Search  for  the  Other  Branch  of  the  Curve.  —  As  the  height  of 
the  four  wires  of  the  receiving  antenna  was  decreased  by  small 
intervals  below  the  values  that  gave  Curve  7,  Fig.  198,  the  deflec- 
tions in  the  region  of  capacity  between  90  and  180  became 
smaller  and  smaller,  as  if  the  resonant  point  were  going  away  to 
infinity,  and  the  deflections  in  the  neighborhood  of  50  began  to 
grow,  until  when  the  height  of  the  antenna  was  made  10.5  meters, 
a  maximum  became  evident  for  about  50  cm.  of  the  receiving  con- 
denser. The  readings  by  which  this  maximum  was  obtained  are 
plotted  as  Curve  8  in  Fig.  198,  along  with  Curves  1  to  7.  Decreas- 
ing the  height  still  further,  Curves  9,  10,  11  and  12  were  obtained 
with  respectively  10,  9,  8  and  7  meters  as  the  height  of  the  antenna. 
These  curves  increase  in  intensity  up  to  Curve  10  and  fall  off  in  11 
and  12.  The  five  Curves  8  to  12  were  taken  with  sensitiveness  of 
the  receiving  instrument  about  five  times  as  great  as  the  sensitive- 
ness used  in  taking  Curves  1  to  7.  Curves  8  to  12  in  the  left- 
hand  group  of  Fig.  198  are  plotted  thus  magnified  five  times  in 
comparison  with  the  group  to  the  right,  numbered  1  to  7. 


RESONANCE  OF  RECEIVING  CIRCUITS 


285 


The  two  groups  when  plotted  with  resonant  receiving  capacity 
against  height  of  antenna  form  a  curve  of  two  branches  A,  A', 
Fig.  199.  Values  calculated  from  the  equation  (a)  are  plotted  as 
the  dotted  lines  in  Fig.  199.  The  heavy  curves  are  the  observed 
values.  From  a  comparison  of  the  observed  values  with  the  com- 
puted values,  we  see  that  our  equation,  although  it  led  us  to  look 
in  the  right  direction  for  the  resonance,  is  yet  an  imperfect  equa- 
tion. There  are  other  terms  in  it  beyond  those  here  set  down. 


\7 


FIG.  200.     Various  types  of  inductively  coupled  receiving  circuits. 

Applicability  of  these  Experimental  Results  to  Practice.  —  One 
may  ask,  what  is  the  use  of  this  experiment  in  which  the  receiving 
transformer  is  kept  constant  and  the  length  of  antenna  and  the 


286 


WIRELESS  TELEGRAPHY 


capacity  of  condenser  in  the  receiving  side  circuit  are  varied,  since 
we  are  not  going  to  vary  the  length  of  antenna  in  actual  practice? 
The  answer  is,  that  if  we  set  up  an  antenna  at  random  and  depend 
upon  variations  of  C*  alone  to  get  our  resonance,  we  may  have  our 
antenna  of  a  length  (capacity)  that  bears  to  the  waves  we  wish 
to  receive  the  same  relations  that  11.8  meters  of  four  wires  bear 
to  the  waves  of  my  experiment.  In  that  case  our  tuning  curve 
would  correspond  to  Curve  7  of  Fig.  198,  and  would  give  us  very 
little  current  and  very  dull  resonance.  The  remedy  is:  Tune  the 
antenna  circuit  as  well  as  the  side  condenser.  This  can  be  done  by 
having  (1)  a  variable  primary  of  the  receiving  transformer  or 
(2)  a  variable  inductance  in  series  with  the  primary,  or  (3)  a 
variable  condenser  shunted  about  the  primary,  or  (4)  a  variable 
condenser  in  series  with  the  primary.  The  several  methods  are 

shown  at  (1),  (2),  (3),  and  (4) 
of  Fig.  200,  respectively.  The 
methods  (1),  (2)  and  (3)  permit 
an  increase  of  the  wave  length 
of  the  antenna  and  adapt  it  to 
longer  waves.  The  method  (4) 
permits  a  decrease  of  the  wave 
length  of  the  antenna  and  adapts 
it  to  shorter  waves.  A  very  de- 
sirable arrangement  is  to  combine 
all  of  these  variables  in  one  appa- 
ratus, as  shown  in  Fig.  201.  Then 
we  can  make  such  adjustments  as 
are  necessary  for  obtaining  best 
resonance. 

In  order  to  see  further  the 
applicability  of  the  experimental 
curves  of  Fig.  198,  let  us  ex- 
press in  somewhat  more  general 
form  the  relation  which  we  have 
given  in  the  experimental  equa- 
tion (a). 

Approximate  Theoretical  Equation  for  Resonance  Relation  at 
Inductively  Coupled  Receiving  Station.  —  If  we  have  a  wave  of 
wave  length  X  arriving  at  an  inductively  coupled  receiving  station 
of  which  the  antenna  circuit  is  adjusted  to  wave  length  Xa,  and 
the  condenser  circuit  adjusted  to  wave  length  Xc;  then  theory 


FIG.  201.  An  inductively 
coupled  receiving  station 
with  several  variable 
elements. 


RESONANCE  OF  RECEIVING  CIRCUITS  287 

shows  that  the  following  is  approximately1  the  relation  between 
the  several  wave  lengths  in  order  to  produce  a  maximum  current 
in  the  condenser  circuit: 


in  which  r  is  the  coefficient  of  coupling  at  the  receiving  station. 
By  a  maximum  current  in  the  condenser  circuit  one  or  another 
of  the  maxima  of  the  twelve  different  curves  of  Fig.  198  is  meant. 
Not  all  of  these  maxima  are  equally  strong,  nor  is  the  resonance 
for  all  of  the  maxima  equally  sharp.  But  for  nearly  any  value 
of  Xa  we  can  get  a  valve  of  Xc  that  will  give  resonance  of  a  more 
or  less  pronounced  character. 

Let  us  try  a  few  numerical  examples  that  will  make  this  clear. 
Let  r  =  .20;  and  suppose  waves  are  arriving  of  wave  length  X  =  400 
meters.  Suppose  that  our  antenna  wave  length  is  set  at  Xa  =  300 
meters.  Then  we  have 

r   =  .20, 

X  =  400, 

Xa=  300, 

to    determine   Xc.     With    these   numerical    values    equation    (1) 
becomes 

(1  1      M      1  1      )  _  (0.20)2 


K2      (400)'}     M300)2      (400)2i 
Multiplying  by  (400)4  we  get 

(f  -MS- ')=•«• 

Whence  Xc  =  390  meters.  This  390  meters  is  the  wave  length 
at  which  we  must  set  our  receiving  condenser  (in  a  coupled  circuit) 
in  order  to  receive  a  400-meter  wave,  provided  our  antenna  is  set 
for  a  300-meter  wave. 

Carrying  through  similar  computations  for  other  values  of  the 
wave  length  of  the  incident  waves  we  obtain  the  results  recorded 
in  Table  XV. 

1  In  the  derivation  of  this  formula  the  small  effect  of  resistance  on  the 
wave  length  was  neglected;  also  the  capacity  of  the  antenna  was  considered 
localized  instead  of  distributed.  The  formula  (of  which  our  equation  (a)  is 
a  special  case)  is,  therefore,  inexact,  but  will  serve  to  illustrate  some  interest- 
ing facts  about  the  tuning  of  a  receiving  station. 


288 


WIRELESS  TELEGRAPHY 


TABLE  XV 

RESONANT  WAVE  LENGTH  ADJUSTMENT  OF  THE  CONDENSER 

SIDE-CIRCUIT  WHEN  THE  ANTENNA  IS  KEPT  FIXED 

AT  WAVE  LENGTH  \a  =  300  METERS 


Wave  Length  of 
Incident  Waves 
X. 

Resonant   Value  of  Re- 
ceiving Condenser  in 
Wave  Length,  Xc. 

100 

103 

200 

210 

250 

267 

280 

330 

290 

430 

300 

310 

207 

330 

302 

350 

332 

400 

390 

500 

493 

600 

598 

The  formula  does  not  apply  to  the  case  of  X  =  300  meters,  so 
this  value  is  omitted  from  the  calculations. 


500 


400 


=300 


^100 


100  200  300 

Wave-length  of  Incident  Waves 


400 


500 


FIG.  202.  Curves  showing  resonant  adjustment  of  wave  length  of 
the  condenser  circuit  for  different  values  of  incident  waves,  — 
the  antenna  wave  length  being  fixed  at  300  meters. 

The  results  recorded  in  Table  XV  are  shown  graphically  in 
Fig.  202. 


RESONANCE  OF  RECEIVING  CIRCUITS  289 

This  curve  shows  several  facts  of  interest.  It  shows,  for  example, 
that  when  we  have  been  receiving  a  wave  length  slightly  shorter 
than  our  antenna  wave  length,  and  a  wave  comes  in  slightly  longer 
than  our  antenna  wave,  we  must  actually  decrease  our  receiving 
capacity  to  bring  tho  longer  wave  into  resonance.  It  shows  also 
that  any  particular  adjustment  of  our  receiving  capacity  is  reso- 
nant for  two  different  waves.  For  example,  with  our  antenna  set 
at  wave  length  300  meters,  and  our  condenser  circuit  set  for  400 
meters,  we  are  really  in  tune  for  either  a  290-meter  wave  or  a  410- 
meter  wave,  not  in  the  best  tune,  it  is  true,  but  sufficiently  in 
tune  to  be  disturbed  if  the  interfering  signals  are  strong. 

Advantage  of  Varying  Coefficient  of  Coupling  in  Tuning.  — 
There  are  times  when  we  wish  to  be  in  tune  for  two  wave  lengths 
at  once,  because  the  station  we  are  receiving  usually  sends  out 
two  waves  at  once.  If  we  set  our  receiving  condenser  at  300 
meters,  we  are  in  tune  for  a  270-meter  and  a  330-meter  wave, 
and  these  might  well  be  sent  out  by  the  same  station.  They  will 
in  fact  be  sent  out  by  the  same  station,  if  it  has  the  same  coefficient 
of  coupling  as  our  receiving  station,  r  =  .20,  and  has  its  condenser 
circuit  and  antenna  circuit  tuned  to  300  meters. 

This  suggests  an  important  improvement  in  our  tuning  mech- 
anism at  the  receiving  station;  namely,  a  device  by  which  we  can 
change  the  coefficient  of  coupling  at  the  receiving  station  and  thus 
make  the  receiving  coefficient  of  coupling  identical  with  the  coeffi- 
cient of  coupling  of  any  particular  station  we  wish  to  receive. 
This  device1  is  employed  in  many  of  the  recent  receiving  sets, 
and  consists  of  an  adjustment  by  which  the  primary  coil  of  the 
receiving  transformer  may  be  either  moved  away  from  or  rotated 
with  respect  to  the  secondary  coil.  The  same  result  can  be  at- 
tained by  cutting  out  inductance  in  the  primary  of  the  transformer 
and  putting  it  in  series  where  it  will  not  be  in  inductive  relation 
with  the  secondary  coil. 

Effect  of  Variation  of  the  Coefficient  of  Coupling  on  Sharpness 
of  Resonance  and  on  Received  Energy.  —  Theory  shows  that 
diminution  of  the  coefficient  of  coupling  increases  the  sharpness 
of  resonance.  At  the  same  time  this  diminution  of  coefficient  of 
coupling  brings  with  it  a  decrease  of  energy.  I  tried  some  experi- 
ments to  see  what  improvement  in  sharpness  of  resonance  we  might 

1  On  account  of  the  high  resistance  of  the  detectors  the  proper  adjustment 
of  the  coefficient  of  coupling  is  not  one  of  exact  equality  with  the  coefficient 
of  coupling  of  the  sending  station,  but  must  be  determined  by  trial. 


290 


WIRELESS  TELEGRAPHY 


attain  by  this  method.  With  the  very  low-resistance  dynamo- 
meter as  a  measuring  instrument,  the  curves  of  Fig.  203  were 
obtained,  with  coefficients  of  coupling  at  the  receiving  station 
equal  to  .30  and  .07  respectively.  The  energy  received  in  the 
former  case  was  twenty  times  as  great  as  in  the  latter  case;  but 
to  compare  sharpness  of  resonance  the  two  curves  are  both  plotted 
with  amplitude  100. 

With  r  =  .30  the  deflection  falls  to  half  for  a  change  of  con- 
denser capacity  of  5%,  while  with  r  =  .07  the  deflection  falls  to 


100 


9C 


80 


70 


fl60 


40 


30 


20 


10 


V 


20       16       12        8        4*      0        40       i 

Change  of  Capacity 


12       16       20       24 


FIG.  203.     Sharpness  of  resonance  for  two  different  values  of  r,  the 
coefficient  of  coupling. 

half  for  a  change  of  capacity  of  2.5%.  The  deflection  is  propor- 
tional to  the  energy,  and  the  capacity  is  proportional  to  the  square 
of  the  wave  length,  so  we  may  say  that  the  energy  received  falls 
to  half  for  a  variation  of  2.5%  and  1.25%  of  the  wave  length  in  the 
two  cases. 

The  experiment  thus  confirms  the  theoretical  deduction  that 
with  a  decrease  of  the  coefficient  of  coupling  the  sharpness  of 
resonance  is  increased.  The  gain  in  sharpness  of  resonance  is, 
however,  paid  for  in  loss  of  energy,  —  the  energy  received  with 
r  —  .07  being  ^V  of  the  energy  received  with  r  =  .30. 


RESONANCE   OF  RECEIVING  CIRCUITS 


291 


EFFECT  OF  RESISTANCE  OF    DETECTOR  ON  RESONANCE    IN    COUPLED 
WIRELESS  TELEGRAPH  CIRCUITS 

Although  the  coefficient  of  coupling  of  the  coupled  circuits 
influences  somewhat  the  sharpness  of  resonance,  a  far  greater 
influence  in  the  case  of  the  practical  stations  is 
exercised  by  the  resistance  of  the  detectors 
which  are  used  in  the  reception  of  the  signals. 
These  detectors,  when  sufficiently  sensitive  to 
respond  to  weak  signals,  have  a  very  high  re- 
sistance. We  have  seen  in  Fig.  150  (p.  226) 
how  a  high  resistance  inserted  in  a  simple  circuit 
consisting  o'f  a  condenser  in  series  with  an  in- 
ductance renders  the  resonance  dull.  With  the 
coupled  circuits  the  effects  are  somewhat  more 
difficult  to  present,  and  it  is  necessary  to  examine 
the  resonance  curves  obtained  by  varying  both 
the  antenna  wave  length  and  the  condenser- 
circuit  wave  length  in  order  to  ascertain  the 
influence  of  resistance  on  the  sharpness  of 

resonance.  -c,      on/l    ~. 

FIG.  204.    Diagram 

I   have   submitted   the   problem  to  a  mathe-        of  circuit  provid- 


primary  and  sec- 
ondary by  vari- 
able condensers. 


matical  examination,  and  without  giving  the 
steps  of  the  reasoning,  I  take  the  liberty  of  pre- 
senting some  of  the  results.  The  form  of  re- 
ceiving circuits  to  which  the  discussion  applies 
is  shown  in  Fig.  204.  The  following  constants  of  the  circuits 
were  assumed  in  the  computations: 

L3  =  Self-inductance  of  the  antenna  circuit  =  .3  X  10~3  henry, 

L4  =  Self-inductance  of  the  condenser  circuit  =  .5  X  10~3  henry, 

M  =  Mutual  Inductance  =  .2  X  10"3  henry. 

r2=  Square  of  coefficient  of  coupling  =.267, 

X  =  wave  length  of  incoming  waves  =  472  meters. 

The  antenna  circuit  was  given  various  resistances,  J?3,  and  the 
condenser  circuit  various  resistances,  R*.  The  resistance  R± 
resides  chiefly  in  the  detector,  and  the  resistance  Rs  includes  the 
apparent  resistance  due  to  distributed  capacity  in  the  antenna. 

The  incoming  waves  were  supposed  to  be  a  persistent  train  of 
undamped  waves. 

Computations  were  made  for  two  cases:  I,  When  we  fix  the 
antenna  adjustments  at  their  best  values,  and  tune  with  C4; 


292 


WIRELESS  TELEGRAPHY 


II,  When  we  fix  C4  at  its  best  value,  and  tune  by  adjustments  of 
the  antenna  circuit.  The  appropriate  adjustments  for  the  two 
cases  and  the  sharpness  of  resonance  obtained  depend  in  an  inti- 
mate way  upon  the  values  of  Rs  and  R\.  The  results  for  the  two 
cases  are  here  briefly  presented. 

Case  I.  R3  =  10  Ohms,  R4  =  64,000  Ohms.  —  Reference  is 
made  to  the  curve  marked  "  #4  =  64,000  "  in  Fig.  205.  This 
curve  is  entirely  flat  on  top,  and  shows  that,  with  a  detector  of 
resistance  64,000  ohms  used  in  a  circuit  with  the  constants  we 


100  200  300 

Wave  Length  of  Condenser  Circuit, . 
.These  lines  show  the  value  of  X  3H 
for  resistances  R4  respectiveley  *         iff 


FIG.  205.  Curves  showing  effect  of  resistance  #4  on  resonance  with  a  coupled 
system  of  circuits  tuned  by  adjusting  C4.  Wave  length  of  incident 
waves  =  472  meters. 

have  assumed,  there  is  no  possibility  of  discriminating  between 
different  wave  lengths  by  any  adjustment  of  the  condenser  C4. 
In  this  case  we  may  as  well  leave  C*  set  at  any  value  above  that 
which  with  the  inductance  L4  gives  200  meters.  It  is  then  equally 
ready  to  detect  all  wave  lengths. 

In  this  case  the  calculations  show  that  the  antenna  circuit  must 
be  adjusted  to  the  wave  length  to  be  received;  namely,  472  meters 
in  the  numerical  example  under  consideration.  I  have  attempted 
to  indicate  this  fact  in  the  diagram  by  drawing  a  line  across  the 
wave-length  scale  at  472  meters  and  marking  it  64,000  ohms. 


RESONANCE  OF  RECEIVING  CIRCUITS  293 

Case  I  (Continued).  R3  =  io  Ohms,  R*  =  10,000  Ohms. - 
Suppose,  now,  that  the  detector  should  have  10,000  ohms  resistance 
instead  of  64,000  ohms.  With  this  reduced  resistance  the  curve 
marked  "  fl4  =  10,000  "  is  obtained.  With  this  value  of  R4,  tun- 
ing by  the  condenser  (\  is  possible,  but  the  resonance  is  dull  as 
is  indicated  by  the  obtuseness  of  the  curve. 

Appropriate  adjustment  of  the  antenna  in  this  case  is  at  the 
line  marked  "10,000"  on  the  bottom  margin;  namely,  X3  =  470 
meters. 

Case  I  (Continued).  RS  =  10  Ohms,  R^  =  1000  Ohms.  —  The 
curve  marked  "  #4  =  1000  "  is  obtained;  and  the  antenna  must 
be  shifted  to  the  line  on  the  bottom  margin  marked  "1000";  that 
is,  the  antenna  wave  length  must  be  set  at  460  meters  for  best 
resonance.  The  resonance  curve  "  -R4  =  1000  "  is  much  sharper 
than  those  obtainable  with  the  higher  resistances. 

Case    I    (Continued).     RS  =  10    Ohms,    Ri  =  100    Ohms. — 
Reference  is  made  to  the  curve  marked  "  #4  =  100,"  and  to  the 
line  at  the  bottom  margin  marked  "  100."     The  resonance  is 
sharper  than  with  the  higher  resistances,  and  the  appropriate 
adjustment  of  antenna  wave  length  has  shifted  to  X3  =  430  meters. 

Case  I  (Concluded).  RS  =  10  Ohms,  R*  =  10  Ohms.  —  Two 
resonance  positions  appear  in  this  case:  one  at  400  meters  (wave 
length  of  the  condenser  circuit),  with  appropriate  adjustment  of 
antenna  at  360  meters;  and  the  other  at  610  meters  (condenser 
circuit),  with  antenna  adjustment  at  810  meters.  The  resonance 
here  is  extremely  sharp,  especially  for  the  adjustment  of  condenser 
C4  in  the  neighborhood  of  400  meters. 

Case  II.  Let  us  now  suppose  a  detector  circuit  of  resistance 
10,000  ohms,  and  let  us  set  the  condenser  C4  of  this  detector 
circuit  at  its  resonant  value  in  the  neighborhood  of  135  meters 
(see  the  diagram  for  Case  I),  and  then  tune  with  the  antenna 
circuit;  for  example,  by  varying  the  condenser  C3.  The  results 
are  given  in  Fig.  206,  the  different  curves  corresponding  to  different 
values  of  R3  in  the  antenna  circuit.  From  these  curves  it  will  be 
seen  that  even  with  a  high-resistance  detector  (jR4  =  10,000  ohms) 
the  tuning  in  the  antenna  circuit  is  sharp,  provided  the  antenna 
effective  resistance  is  low  (curve  marked  "  R3  =  10  ").  With 
increase  of  antenna  resistance  the  resonance  becomes  less  sharp. 

In  practice  with  a  system  of  coupled  circuits  like  that  under 
discussion  and  with  the  high-resistance  detectors  in  use,  it  is 
difficult  to  realize  sharper  resonance  than  that  shown  in  the  curve 


294 


WIRELESS  TELEGRAPHY 


marked  "  Rz  =  50."  This  is  obtained  by  tuning  with  the  adjust- 
ment of  the  antenna  circuit.  These  curves  shift  almost  uniformly 
with  wave  length,  so  that  if  a  number  of  stations  are  sending 


§  ,8 

<x> 


ti   .6 


100 


200 


300          400  500  600          700 

A-ntenna  Wa.ve  Length,  in  Meters 


800 


900 


FIG.  206.     Curves  showing  effect  of  resistance  R3  on  resonance  with  a  coupled 
system  tuned  by  adjusting  C3. 


Incident  Wave-length 


100 


2CO          300          400          500          600         700 
Wave-length  in  Antenna  Circuit  in  Meters 


800 


900 


FIG.  207.     Curves  showing  the  extent  0f  interference  in  a  computed  case  of 

coupled  circuits. 

simultaneously,  the  series  of  resonance  curves  obtained  would  be 
like  that  of  Fig.  207. 

These  curves  are  computed  with  what  seems  to  be  about  the 
conditions  obtaining  in  good  practice.     It  is  seen  by  a  reference 


RESONANCE  OF  RECEIVING  CIRCUITS  295 

to  Fig.  207  that  if  a  receiving  station  is  attuned  for  a  500-meter 
wave,  it  will  receive  also  about  7%  as  much  energy  from  a  400- 
meter  or  a  600-meter  wave  as  it  does  from  the  500-meter  wave. 
From  a  station  emitting  a  300-meter  or  a  700-meter  wave  the 
disturbing  energy  will  amount  to  about  2%  of  the  energy  received 
from  the  500-meter  wave;  while  from  a  sending  station  emitting 
a  200-meter  or  a  800-meter  wave  the  disturbing  energy  will  be 
below  1%.  These  statements  are  on  the  assumption  that  all  of 
the  stations  would  give  the  same  received  energy  if  the  receiving 
station  were  in  tune  for  them. 

These  computations,  although  not  claiming  to  be  highly  accu- 
rate, will  give  a  crude  idea  of  about  the  extent  to  which  inter- 
ference can  be  prevented  by  the  use  of  the  coupled  circuits 
consisting  of  a  condenser  circuit  containing  the  receiving  instru- 
ment inductively  or  directly  coupled  to  an  antenna  circuit. 

There  are  other  methods  of  coupling  receiving  circuits  to  pre- 
vent interference  which  will  attain  better  discrimination  between 
desired  and  undesired  signals,  but  these  almost  always  greatly 
reduce  the  intensity  of  signals,  and  cannot  be  employed  for  the 
reception  of  signals  from  stations  at  a  great  distance  from  the 
receiving  station. 


CHAPTER  XXV 
DIRECTED    WIRELESS    TELEGRAPHY 

FOR  some  purposes  it  is  desirable  to  send  electric  waves  away  in 
all  horizontal  directions  from  the  sending  station,  and  to  receive 
electric  waves  coming  in  from  any  direction.  This  is  the  general 
mode  of  propagation  of  the  electric  waves,  and  permits,  for 
example,  the  establishment  of  communication  with  a  vessel  in 
an  unknown  location  at  sea. 

Such  a  general  diffusion  of  waves  is,  on  the  other  hand,  often 
very  undesirable  for  the  following  reasons:  (1)  It  is  wasteful  of 
transmitted  energy;  (2)  the  message  may  be  received  by  an  enemy 
or  an  unfriendly  neighbor  who  could  generally  be  prevented  from 
receiving  it  if  we  could  direct  the  waves;  (3)  when  we  wish  to 
communicate  in  one  direction  we  may  unnecessarily  disturb  or  be 
disturbed  by  an  operating  station  in  another  direction;  (4)  if  the 
receiving  apparatus  can  be  made  to  respond  selectively  to  electric 
waves  from  different  directions,  a  vessel  at  sea  can  get  its  bearings 
and  position  by  finding  its  direction  from  two  different  known 
stations.  For  these  and  other  reasons,  several  inventors  have 
given  attention  to  the  problem  of  emitting  or  receiving  electric 
waves  directively  and  have  made  some  progress  toward  a  solution. 

Hertz's  Parabolic  Metallic  Reflectors.  —  As  was  pointed  out 
in  Chapter  XII,  Marconi  in  his  early  experiments  tried  to  use  para- 
bolic metallic  reflectors  about  his  oscillator  and  receiver,  for  the 
purpose  of  transmitting  or  receiving  in  a  given  direction.  On 
account  of  the  difficulty  of  constructing  and  sustaining  mirrors 
sufficiently  large  to  have  proper  proportions  to  the  wave  lengths 
required,  this  device  has  not  been  successfully  used  in  practice. 

Braun's  Parabolic  Oscillator.  —  In  1902,  Ferdinand  Braun1 
proposed  the  use  of  an  oscillator  consisting  of  several  elements 
which  were  arranged  to  compose  a  parabolic  surface.  A  diagram 
of  this  form  of  oscillator  is  shown  in  Fig.  208.  Several  vertical 
metallic  strips  A\,  A%,  A3  .  .  .  were  arranged  to  lie  in  a  para- 

1  U.  S.  Patent,  No.  744,897,  filed  Feb.  19,  1902,  issued  Nov.  24,  1903. 

296 


DIRECTED  WIRELESS  TELEGRAPHY 


297 


bolic  cylindrical  surface  and  were  connected  to  a  spark  terminal  Si. 
Another  similar  set  of  strips  BI,  B2,  B3  .  .  .  below  the  first  set 
were  also  provided  with  a  spark  terminal  S2.  The  oscillations  are 
produced  by  a  discharge  across  the  spark  gap  SiS*.  This  arrange- 
ment, which,  according  to  the  inventor,  would  send  out  electric 
waves  in  one  direction,  does  not  seem  to  have  met  with  practical 
success. 

Braun's  Phase-difference  Oscillator.  —  Another  method  pro- 
posed by  Ferdinand  Braun1  makes  use  of  two  or  more  vertical 
oscillators  at  certain  distances  apart  provided  with  means  of 


FIG.  208.     Braun's  parabolic 
oscillator. 


FIG.  209.     Braun's  phase-difference  oscilla- 
tor for  directed  wireless  telegraphy. 


exciting  in  the  oscillators  waves  suitably  differing  in  phase.  For 
example,  if  the  two  antennae  A  and  B,  Fig.  209,  are  one  half  wave 
length  apart,  and  if  the  oscillations  in  the  two  antennae  are 
opposite  in  phase,  the  two  sets  of  waves  sent  out  will  add  in 
directions  in  the  plane  of  the  two  antennae  and  will  neutralize 
each  other  in  a  direction  at  right  angles  to  this  plane. 

Suitable  phase  difference  in  the  antennae  may  be  partially 
attained  by  the  use  of  a  condenser  circuit  coupled  with  the  an- 
tennae, as  shown  in  Fig.  209.  With  this  arrangement  the  problem 
is,  however,  complicated  by  the  occurrence  of  oscillations  of 
double  periodicity.  This  difficulty  has  been  removed  in  a  very 


U.  S.  Patent,  No.  776,380,  filed  July  26,  1904,  issued  Nov.  29,  1904. 


298 


WIRELESS  TELEGRAPHY 


interesting  method  of  excitation  devised,  at  Professor  Braun's 
suggestion,  by  Messrs.  Mandelstam  and  Papalexi,  and  is  described 
in  Physikalische  Zeitschrift,  Vol.  7,  p.  302,  1906,  to  which  the 
reader  is  referred. 

With  three  or  more  antennae  suitably  diffejing  in  their  phase  of 
excitation  and  situated  at  the  vertices  of  a  triangle  or  of  a  polygon, 
any  one  of  several  directions  may  be  selected  as  the  direction  of 
strongest  transmission.  In  a  similar  way,  by  employing  receiving 
stations  provided  with  a  multiplicity  of  antennas  separated  by 
suitable  fractions  of  a  wave  length,  and  by  using  proper  means  of 
combining  the  impulses  in  a  secondary  detector  circuit,  some  selec- 
tivity of  direction  from  which  the  waves  are  received  can  be 
attained. 

Marconi's  Directive  Antenna.  —  In  1906  Mr.  Marconi  pre- 
sented to  the  Royal  Society  an  account  of  some  experiments  which 

showed  that  an  antenna  having 

A  *O 

a  short  vertical  part  and  then 
extending  away  to  a  considerable 
distance  in  a  horizontal  direction, 
as  shown  in  Fig.  210,  emitted 
electric  waves  most  strongly  in  the 
direction  D  away  from  which  the 
free  end  of  the  antenna  points. 

Marconi's  experiments  showed 
for  a  given  distance  between 
the  receiving  station  and  the 
transmitting  station  the  relative 
intensities  in  different  directions 
which,  plotted  in  polar  coordi- 
nates, give  a  curve  of  the  form 
of  Fig.  211.  In  this  figure  the 
relative  intensities  in  different 
directions  are  the  lengths  of  the 
FIG'  MlrcSrf dTe^ Jdnltnynab°Ut  »dii  drawn  from  the  origin  to 

the  curve. 

In  like  manner  a  receiving  antenna  consisting  of  a  short  verti- 
cal part  and  a  long  horizontal  part  receives  more  strongly  waves 
arriving  from  the  direction  away  from  which  the  open  end  of 
the  antenna  points.  Mr.  Marconi  has  utilized  this  principle  in 
the  construction  of  his  powerful  stations  at  Wellfleet  and  at 
Poldu. 


"9- 

YE    • 

W/////M/////////^^ 

FIG.  210.     Marconi's  directive 
antenna. 

270 

300 


120 


60 


DIRECTED  WIRELESS  TELEGRAPHY 


299 


Explanation  of  Directive  Radiation  from  Marconi's  Bent 
Antenna.  —  Professor  Fleming,1  Dr.  Uller,2  Dr.  Zenneck,3  and 
others,  have  given  explanations  of  the  cause  of  the  directive  radia- 
tion from  the  Marconi  horizontal  antenna.  All  of  these  writers 
employ  the  theory  of  images  as  a  starting  point,  by  which  means 
the  antenna  and  ground  connection  of  Fig.  210  is  replaceable  by 
the  equivalent  system  of  Fig.  212. 

Fleming's  Explanation.  —  In  further  explanation,  Professor 
Fleming  takes  a  rectangular  circuit  of  the  form  shown  in  Fig.  213, 
and  imagines  a  current  flowing  around  the  rectangle  in  the  direc- 


A'  B' 

FIG.  212.     Marconi  directed  antenna  and  its  image. 

B 

h 

| 
Out 


h  H 

oo- 

In  Out 


FIG.  213.     Diagram  used  by  Professor  Fleming  in  explanation  of 
the  directive  action  of  the  Marconi  bent  antenna. 

tion  of  the  arrows.  This  current  creates  a  magnetic  field,  the 
direction  of  which  along  the  surface  of  the  earth  is  at  right  angles 
to  the  plane  of  the  paper;  and  at  equal  distances  from  the  center, 
the  magnetic  force  represented  by  H  is  toward  the  spectator  on 
both  sides.  Now,  suppose  a  wire  EF  equal  in  length  to  one  side 
of  the  rectangle  to  be  placed  contiguous  to  one  vertical  side,  and 
to  carry  a  current  opposite  in  direction  to  that  in  the  side  of  the 
rectangle  (left  hand)  to  which  it  is  in  proximity;  then  the  magnetic 
field  of  this  straight  current  is  h'  from  the  spectator  on  the  left- 
hand  and  h  toward  the  spectator  on  the  right-hand  side.  Accord- 
ingly, the  total  field  H  +  h  on  the  right  is  greater  than  the  total 
field  H  —  h'  on  the  left,  because,  according  to  Professor  Fleming, 
the  individual  fields  are  added  on  one  side  and  subtracted  on  the 
other.  Now,  since  the  two  oppositely  directed  currents  in  the 

1  J.  Fleming:  Phil.  Mag.,  Vol.  12,  p.  588-604,  1906. 

2  Carl  Uller:  Phys.  Zeitsch.,  Vol.  8,  p.  193,  1907. 

3  J.  Zenneck:  Phys.  Zeitsch.,  Vol.  9,  p.  553,  1908. 


300  WIRELESS  TELEGRAPHY 

adjacent  wires  may  be  imagined  to  come  so  close  together  as  to 
annul  each  other,  the  effect  is  the  same  as  if  a  circuit  of  parts 
A  BCD  were  used  with  the  parts  AD  and  EF  omitted. 

Objections  to  Professor  Fleming's  Explanation.  —  It  appears 
that  serious  objection  can  be  raised  to  Professor  Fleming's  expla- 
nation as  follows:  He  does  not  take  into  account  the  mode  of 
vibration  of  the  oscillator,  nor  does  he  take  account  of  the  time 
required  for  the  magnetic  field  to  travel  from  the  radiating  system 
to  the  point  under  consideration.  In  the  case  of  the  field  HH 
produced  by  the  closed  rectangular  circuit,  the  time  to  travel  to 
the  right  and  to  the  left  to  the  points  under  examination  will  be 
the  same,  and  the  two  H's  will  be  equal  and  in  the  same  direction, 
as  Professor  Fleming  explains,  only  provided  the  same  current 
flows  in  every  part  of  the  loop.  No  such  uniform  flow  of  current 
occurs  in  the  case  of  the  actual  oscillation.  Also,  in  the  case  of 
the  forces  h  and  In!  the  distances  from  EF  are  unequal,  and  therefore 
the  times  to  travel  to  the  points  under  examination  are  not  the 
same,  and  whether  the  fields  h  and  h'  will  be  opposite  to  each  other 
or  not  depends  on  the  mode  of  vibration  of  the  two  oscillators  and 
the  time  for  the  waves  to  travel  to  the  points  under  examination. 
The  whole  question  of  the  relative  strength  of  waves  emitted  in  the 
two  opposite  directions  is  avoided  by  Professor  Fleming  because 
of  his  substitution  of  a  system  that  can  never  represent  the  actual 
system;  and  after  we  have  examined  Professor  Fleming's  reasoning 
the  solution  of  the  actual  problem  is  still  completely  in  doubt. 

Explanation  of  Dr.  Uller.  —  Professor  Fleming l  had  earlier 
employed  a  different  method  of  attacking  the  problem  directly 
by  imagining  the  form  of  the  electric  field  of  force  about  the  direc- 
tive antenna.  This  method  was  revived  in  1907  by  Dr.  Uller,  who 
pictured  the  field  of  electric  force  about  the  Marconi  oscillator 
in  the  form  given  in  Fig.  214.  The  upper  half  of  this  diagram  would 
represent  the  mode  of  propagation  of  the  waves  over  the  surface 
of  a  good  conducting  plane.  Where  the  surface  of  the  earth  is 
not  a  good  conductor,  the  electric  force  would  be  inclined  near  the 
surface,  as  has  been  shown  in  Chapter  XV,  and  would  give  a  field 
of  force  slightly  different  from  that  here  represented. 

Zenneck's  Explanation.  —  Zenneck  has  modified  the  theory 
of  Uller  so  as  to  take  further  into  account  the  effect  of  im- 
perfect conductivity  of  the  earth's  surface.  As  we  have  seen  in 

1  Fleming:  Electric  Wave  Telegraphy,  p.  627,  1906. 


DIRECTED  WIRELESS  TELEGRAPHY 


301 


Chapter  XV,  the  electric  force  at  the  surface  of  the  earth,  where  - 
ever  it  is  not  a  good  conductor,  leans  forward,  so  that  we  can 
ascribe  to  the  electric  force  in  a  particular  case  a  mean  direction, 


FIG.  214. 


Dr.  Uller's  diagram  of  field  of  electric  force  about  the 
bent  antenna. 


E,  Fig.  215.  Now  the  direction  of  propagation  is  perpendicular 
to  E;  i.e.,  in  the  direction  S,  whence  there  is  penetration  of  the 
energy  into  the  earth's  surface  and  a  consequent  absorption,  so 


FIG.  215.  Diagram  used  by  Dr.  Zen- 
neck  in  explaining  directed  wireless 
telegraphy. 


Q 


FIG.  216.  Zenneck's  diagram  show- 
ing the  course  of  the  radiation 
from  A  to  R. 


FIG.  217.     Diagram  applying  to  Zenneck's  explanation. 

that  the  distant  receiving  station  is  reached  by  the  energy  that 
started  in  the  direction  AX,  Fig.  216,  and  not  by  the  energy  that 
started  along  the  surface  of  the  earth.  By  examination  of  Fig. 


302 


WIRELESS  TELEGRAPHY 


214  it  will  be  seen  that  a  bent  antenna  of  the  form  of  ABC,  Fig. 
217,  radiates  more  energy  in  the  direction  CP  than  in  the  direc- 
tion CQ,  and  therefore  attains  a  greater  distance  in  the  direction 
CD  than  in  the  direction  CE.  This  explanation  of  Zenneck 
would  indicate  that  the  directive  effect  of  the  bent  antenna  is 
much  greater  over  land  than  over  sea.  I  do  not  know  of  any 
experimental  confirmation  of  this  deduction. 

These  explanations  are  lacking  in  quantitativeness,  but  taken 
together  serve  to  give  a  tentative  reconciliation  of  some  of  the 
experiments  with  theory. 

Bellini  and  TosPs  Directive  Apparatus.  —  A  very  ingenious 
method  of  directively  transmitting  and  receiving  electric-wave 
signals  has  been  devised  by  Messrs.  Bellini  and  Tosi.1  A  diagram 


FIG.  218.     Bellini  and  Tosi's  directive  apparatus. 

of  a  receiving  station  embodying  their  invention  is  shown  in  Fig. 
218.  The  directive  aerial  system  consists  of  two  closed  or  nearly 
closed  oscillation  circuits  of  triangular  shape  ABBA  and  A1B1B1A1 
arranged  respectively  in  two  perpendicular  planes.  These  two 
antenna  circuits  contain  respectively  the  coils  m  and  n,  which  may 
be  circular  coils,  and  are  perpendicular  to  each  other  with  their 
windings  in  the  planes  of  the  antenna  circuits  respectively.  A 
third  coil  s  connected  to  a  wave  detector  and  a  condenser  c  is 


1  U.  S.  Patent,  No.  945,440,  filed  Oct.  1,  1907,  issued  Jan.  4,  1910. 


DIRECTED  WIRELESS  TELEGRAPHY  303 

placed  within  the  two  coils  m  and  n  and  is  capable  of  rotation 
about  an  axis  through  o. 

Electric  waves  coming  from  any  particular  direction  produce 
oscillation  in  the  two  antenna  circuits  with  intensities  respectively 
dependent  on  the  direction  from  which  the  waves  come.  The 
oscillations  thus  set  up,  passing  through  the  coils  m  and  n,  com- 
pound to  form  a  single  magnetic  field  with  a  direction  perpendicular 
to  that  from  which  the  waves  come.  The  strength  of  the  induced 
current  in  the  movable  coil  s  will  depend  on  its  orientation  with 
respect  to  the  resultant  magnetic  field,  and  will  be  a  maximum 
when  the  coil  s  is  in  a  position  to  embrace  as  many  as  possible  of 
the  lines  of  magnetic  force.  This  optimum  direction  is  perpen- 
dicular to  the  field,  and  therefore  parallel  to  the  direction  from 
which  the  waves  are  coming. 

It  is  therefore  possible  to  determine  the  direction  from  which 
the  waves  are  arriving  by  merely  providing  the  rotating  coil  s 
with  a  pointer  in  its  own  plane.  When  a  maximum  strength  of 
signals  is  received  the  pointer  is  directed  either  toward  or  away 
from  the  signaling  station.  The  final  ambiguity  as  to  whether 
the  signaling  station  is  in  the  direction  of  the  pointer  or  in  the 
opposite  direction  would  have  to  be  removed  by  some  additional 
general  knowledge  of  the  probable  location. 

A  sending  station,  devised  also  by  Bellini  and  Tosi,  and  capable 
of  directively  transmitting  signals,  consists  of  a  similar  aerial 
system  and  a  similarly  rotatable  interior  coil.  t  The  latter  is,  how- 
ever, connected  with  a  discharge  condenser  instead  of  with  the 
receiving  mechanism.  The  processes  involved  are,  then,  the 
reverse  of  those  entering  into  the  receiving  apparatus. 

Limitations  of  Directive  Wireless  Telegraphy.  —  The  several 
directive  devices  above  described  act  directively  only  in  a  general 
way;  that  is,  some  more  energy  is  sent  in  one  direction  than  in  other 
directions,  but  there  is  still  a  considerable  diffusion  of  energy  in  all 
directions.  The  economy  effected  in  the  energy  of  transmission 
does  not  seem  to  be  very  great,  particularly  because  the  closed 
loops,  or  nearly  closed  loops,  are  not  such  good  radiators  or  receiv- 
ers as  the  straight  vertical  antenna.  However,  whenever  the  bent 
antenna  is  installed  in  land  stations  the  orientation  to  effect  maxi- 
mum transmission  in  the  most  useful  direction  is  generally  chosen. 
Also,  it  has  been  proved  to  be  entirely  possible  with  each  of  the 
principal  systems  to  determine  the  direction  of  the  receiving  station 
from  the  sending  station.  This  achievement  does  not  seem  to  have 


304  WIRELESS  TELEGRAPHY 

been  of  sufficient  importance  up  to  the  present  to  warrant  special 
installations  for  the  purpose.  It  is,  however,  entirely  possible 
that  greater  attention  will  be  given  to  this  subject  when  the  art 
of  wireless  telegraphy,  which  is  now  embarrassed  by  novelty  in  so 
many  directions,  shall  have  become  a  little  more  standardized  in  its 
fundamental  requirements. 


CHAPTER  XXVI 
WIRELESS  TELEPHONY 

Sketch  of  the  Method  of  Wireless  Telephony  by  Electric  Waves. 

-  The  circuits  employed  in  wireless  telephony  by  electric  waves 
resemble  very  closely  those  used  in  wireless  telegraphy. 

The  transmitting  apparatus  for  wireless  telephony  makes  use 
of  a  persistent  train  of  electric  waves  of  high  frequency  sent  out 
from  an  antenna.  Instead  of  interrupting  these  electric  waves 
by  a  key,  as  in  telegraphy,  modifications  by  the  voice,  correspond- 
ing to  spoken  words,  are  impressed  upon  them.  These  modifica- 
tions by  the  voice  are  applied  to  the  electric  waves  by  means  of  a 
carbon  transmitter,  or  similar  instrument,  placed  in  the  sending 
circuit  or  connected  with  it. 

The  receiving  apparatus  is  indentical  with  that  employed  in 
wireless  telegraphy,  and  makes  use  of  a  receiving  antenna  coupled 
with  a  circuit  containing  some  type  of  rectifying  detector;  e.g.,  an 
electrolytic  detector,  a  crystal-contact  detector,  or  a  vacuum-tube 
rectifier.  About  the  detector  is  shunted  a  sensitive  telephone 
receiver. 

The  action  is  as  follows :  If  an  unmodified  train  of  electric  waves 
having  a  frequency  higher  than  the  limit  of  human  audibility 
(35,000  vibrations  per  second)  arrives  at  the  receiving  station,  the 
receiving  circuit,  if  properly  tuned,  will  sustain  electric  oscillations 
which,  passing  through  the  detector,  will  be  rectified  and  will  give 
a  series  of  rectified  impulses  to  the  receiving  telephone  circuit. 
These  impulses,  being  all  in  one  direction,  will  act  as  a  continuous 
pull  on  the  telephone  diaphragm,  —  a  continuous  pull  for  the 
reason  that  the  diaphragm  cannot  follow  the  rapid  successive 
impulses,  and  because  also,  on  account  of  the  inductance  of  the 
telephone  circuit,  these  impulses  are  modified  electrically  into  a 
practically  continuous  current  through  the  receiver. 

Having  in  mind  that  a  continuous  train  of  high-frequency  waves 
produces  a  continuous  pull  on  the  receiving  telephone  diaphragm, 
let  us  now  suppose  that  words  are  spoken  into  a  carbon  transmitter 
at  the  sending  station  in  such  a  manner  as  to  modify  the  emitted 

305 


306  WIRELESS  TELEGRAPHY 

train  of  waves.  These  modifications  of  the  emitted  waves  will 
produce  corresponding  modifications  in  the  pull  on  the  telephone 
diaphragm  at  the  receiving  station,  so  that  the  receiving  dia- 
phragm will  execute  vibrations  similar  to  those  of  the  transmitting 
diaphragm,  as  in  ordinary  telephony  over  wires. 

Methods  of  Producing  the  Persistent  Train  of  Waves.  —  Some 
of  the  details  of  the  process  will  now  be  presented.  To  produce 
the  persistent  train  of  oscillations  several  methods  are  available, 
of  which  three  will  be  mentioned,  to  wit:  1.  The  Singing-arc 
Method;  2.  The  High-frequency  Alternator  Method;  3.  The  Mer- 
cury-arc Method. 

The  first  of  these  methods  has  been  described  in  the  preceding 
chapter.  A  brief  description  of  the  other  two  methods  follows. 

The  High-frequency  Alternator  Method  of  Producing  Sus- 
tained Oscillations.  —  In  1901,  Professor  R.  A.  Fessenden1  applied 
for  a  patent  for  "  improvements  in  apparatus  for  the  wireless  trans- 
mission of  electromagnetic  waves,  said  improvements  relating  more 
especially  to  the  transmission  and  reproduction  of  words  or  other 
audible  signals. "  A  diagram  of  the  simple  apparatus  described 
in  this  application  is  shown  as  Fig.  219. 

In  the  diagram,  which  represents  the  transmitting  station,  D  is 
an  alternating-current  generator  of  high  periodicity;  for  example, 
50,000  per  second.  A  carbon  transmitter  is  shown  at  T.  The 
diaphragm  of  the  transmitter  is  marked  P.  A  is  the  sending 
antenna. 

One  of  Professor  Fessenden's  claims  is  as  follows: 

"  In  a  system  of  signaling  by  electromagnetic  waves,  the  com- 
bination of  means  for  the  practically  continuous  generation  of 
electromagnetic  waves  or  impulses,  means  for  modifying  or  chang- 
ing the  character  of  such  waves  or  impulses  without  interruption 
of  their  continuity,  and  an  indicating  means  or  mechanism  at  the 
receiving  station  operative  by  the  electromagnetic  waves  or  im- 
pulses, substantially  as  set  forth." 

In  carrying  out  the  invention  Professor  Fessenden,  in  1908,  con- 
structed a  high-frequency  alternator,  with  an  output  of  2.5  kilo- 
watts at  225  volts,  and  with  a  frequency  of  75,000  cycles  per 
second.  This  is  a  frequency  well  above  the  limit  of  audibility,  and 
in  fact  a  frequency  sufficiently  high  to  give,  when  the  generator 
is  connected  directly  or  inductively  to  an  antenna  in  resonance 

1  U.  S.  Patent,  No.  706,747,  applied  for  Sept.  28,  1901,  divided  July  22, 
1902,  issued  August  12,  1902. 


WIRELESS  TELEPHONY 


307 


with  it,  a  wave  length  suitable  for  wireless  telephony,  namely, 
3  X  108/75,000  =  4000  meters.  With  this  apparatus,  Professor 
Fessenden  reports  that  he  has  carried  on  telephonic  communica- 
tion between  Brant  Rock,  Massachusetts,  using  an  antenna  440 
feet  high,  and  New  York,  using  an  antenna  200  feet  high.  The 
distance  between  these  two  stations  is  about  200  miles.  Recently 
Professor  Fessenden  also  reports  successful  wireless  telephonic 
communication  between  Brant  Rock,  Massachusetts,  and  Wash- 
ington, D.  C.,  a  distance  of  about  600  miles. 


T    P 


FIG.  219.  Professor  Fessenden's  ap- 
paratus for  wireless  telephony  ^ 
using  high-frequency  generator  D 
and  a  microphone  transmitter  T. 


FIG.  220.     Diagram  of  Vreeland's 
mercury-arc  oscillator. 


The  Mercury-arc  Method  of  Producing  Sustained  Oscillations. 
—  In  1906  Mr.  Frederick  Vreeland1  described  a  very  interesting 
method  of  getting  practically  pure  sinusoidal  undamped  oscilla- 
tions from  a  direct-current  supply.  One  form  of  Mr.  Vreeland's 
apparatus  is  shown  in  Fig.  220.  T7  is  a  glass  vessel,  exhausted 
to  a  high  vacuum,  and  containing  a  mercury  cathode  K  and  two 
carbon  anodes  A  and  B.  E  is  a  small  auxiliary  electrode  used  in 
starting  an  arc  in  the  chamber.  The  arc,  when  established,  being 
fed  from  the  direct-current  source  D,  is  divided  into  two  branches 


Physical  Review,  Vol.  27,  p.  286,  1908. 


BOS  WIRELESS  TELEGRAPHY 

—  one  between  the  anode  A  and  the  cathode  K,  the  other  between 
the  anode  B  and  the  cathode  K.  A  resistance  R  and  two  choke 
coils  L  and  L'  serve  to  steady  and  maintain  the  two  arcs.  Now  an 
oscillation  circuit  consisting  of  a  condenser  C  and  two  coils  F  and 
F'  is  connected  between  the  two  anodes  A  and  B.  The  coils  F 
and  F'  in  the  oscillation  circuit  serve  as  field  coils  to  deflect  the 
arc  inside  the  vacuum  bulb,  and  to  cause  the  cathode  stream  of 
this  arc  to  oscillate  in  a  plane  perpendicular  to  the  axis  of  the  coils 
in  such  a  manner  that  this  oscillating  cathode  stream  impinges 
first  on  one  and  then  on  the  other  of  the  anodes  A  and  B.  The 
manner  in  which  this  deflection  of  the  cathode  beam  is  produced 
is  as  follows: 

At  the  start,  the  current  tends  to  divide  equally  between  the 
two  arcs  in  the  bulb,  but  there  are  always  some  variable  inequalities 
in  the  conductivities  of  the  two  paths.  These  irregular  fluctua- 
tions are  usually  sufficient  to  start  the  oscillations,  after  which 
they  give  place  to  the  periodic  fluctuations  controlled  by  the 
alternating  field.  The  action  of  the  magnetic  field  is  such  as  to 
produce  a  deflection  of  the  cathode  beam,  and  when  this  beam  is 
deflected,  say  from  the  anode  B  to  the  anode  A,  there  is  a  tendency 
for  the  current  to  pass  wholly  or  largely  from  the  anode  A  to  the 
cathode  K,  due  to  the  fact  that  the  path  from  B  is  interrupted  or 
increased  in  resistance.  As  the  choke  coils  L  and  L'  oppose  any 
change  in  the  current  passing  through  them,  this  results  in  the 
current  in  the  branch  L'  flowing  through  the  oscillating  circuit 
F'CF  from  right  to  left,  thus  traversing  the  field  coils  in  such 
direction  as  to  increase  the  deflection  of  the  cathode  beam  toward 
the  left,  thereby  augmenting  still  further  the  inequality  of  the 
two  paths  through  the  tube  and  increasing  the  current  through 
the  oscillating  circuit.  This  continues  until  the  condenser  C 
charges  to  a  certain  point,  when  it  begins  to  discharge,  reversing 
the  field,  and  causing  the  arc  to  be  deflected  in  the  other  direction, 
so  as  to  force  the  current  through  the  oscillating  circuit  from  A 
to  B.  This  process,  being  repeated  indefinitely,  results  in  feeding 
the  energy  into  the  oscillating  circuit  in  synchronism  with  the 
oscillations,  which  are  thus  maintained  at  constant  amplitude  and 
at  a  frequency  determined  by  the  self-inductance  and  capacity  of 
the  circuit.  A  photograph  of  the  completed  apparatus  is  shown 
in  Fig.  221. 

I  am  not  able  to  say  whether  or  not  Mr.  Vreeland  has,  up  to  the 
present,  been  able  to  get  the  frequency  of  his  oscillation  producer 


WIRELESS  TELEPHONY 


309 


up  to  the  pitch  required  for  wireless  telephony.     His  apparatus  is, 
however,  very  ingenious  and  full  of  promise. 

Method  of  Applying  the  Microphone  to  Modify  the  Oscilla- 
tions.—  Having  described  methods  of  producing   sustained   or 


FIG.  221.    View  of  Mr.  Vreeland's  apparatus. 


persistent  oscillations  I  wish  next  to  show  briefly  diagrams  of  con- 
nections by  which  the  carbon  microphone  may  be  applied  to  modify 
these  oscillations  in  accordance  with  the  vibrations  of  the  voice. 
In  most  of  these  diagrams  I  have  represented  the  source  of  the 
persistent  oscillations  as  a  singing  arc,  such  as  has  been  devised 
by  Simon,  Duddell,  and  Poulsen.  It  will  easily  be  seen  how  these 


310 


WIRELESS  TELEGRAPHY 


diagrams  should  be  modified  to  permit  of  the  use  of  Fessenden's 
high-frequency  generator  or  Vreeland's  mercury-arc  oscillator. 

Figure  222  shows  the  microphone  in  series  with  the  direct-current 
dynamo  of  the  feeding  system.  To  be  used  in  this  manner,  the 
microphone  must  have  high  current-carrying  capacity,  and  for 


FIG.  222.  One  method  of  applying  the  FIG.  223.  Showing  the  microphone 
microphone  M  to  wireless  telephone  M  in  a  circuit  inductively  coupled 
circuit.  with  the  feeding  circuit  of  an  arc 

used  in  wireless  telephony. 


FIG.  224.  Microphone  M  be- 
tween secondary  helix  and 
ground. 


FIG.  225.  Microphone  T  inductively  cou- 
pled with  secondary  helix  for  wireless 
telephony. 


this  purpose  some  inventors  have  proposed  to  use  several  micro- 
phones in  parallel,  —  all  of  the  diaphragms  facing  upon  a  common 
air  chamber  into  which  the  words  are  spoken. 

Figure  223  shows  the  microphone  transmitter  connected  in  cir- 
cuit with  the  primary  of  a  transformer  S,  the  secondary  of  which, 
L,  is  in  series  with  the  dynamo  and  the  arc.  In  this  case  the 
heavy  current  of  the  arc  does  not  go  through  the  microphone. 
In  common  with  the  method  of  Fig.  222  there  is  the  disadvantage 


WIRELESS  TELEPHONY  311 

that  the  microphonic  modifications  of  current  have  to  traverse 
the  generator  circuit,  and  hence  meet  with  high  impedance. 

Figure  224  shows  the  microphone  connected  in  series  with  the 
antenna  circuit,  between  the  secondary  of  the  oscillation  trans- 
former PS  and  the  ground  connection. 

Figure  225  shows  a  method  proposed  by  Mr.  Vreeland  and 
others  in  which  the  microphone  circuit  Is  inductively  connected 
with  the  secondary  S  of  the  oscillation  transformer. 

Other  methods  of  connecting  the  microphonic  transmitter  to 
the  oscillating  circuit  are  also  employed. 

Practical  Results  in  Wireless  Telephony. — I  have  briefly  pointed 
out  in  the  preceding  paragraphs  the  general  processes  employed 
in  wireless  telephony.  The  small  amount  of  space  here  devoted 
to  the  subject  is  not  to  be  taken  as  evidence  that  wireless  telephony 
is  a  simple  or  unimportant  branch  of  the  science  of  electric-wave 
transmission  of  intelligence. 

To  be  able  to  modulate  a  train  of  electric  waves  by  waves  of 
sound  existent  in  the  air  between  the  mouth  of  the  speaker  and  a 
transmitting  diaphragm,  and  to  be  able  to  receive  these  modulated 
electric  waves  at  a  distance  and  reconvert  them  into  sound  waves, 
is  a  very  remarkable  achievement  of  scientific  ingenuity,  even 
when  the  sending  and  receiving  stations  are  close  together.  Wire- 
less telephony  has,  however,  gone  far  beyond  this  stage;  and 
Fessenclen  in  America,  Poulsen  in  Denmark,  Majorano  in  Italy, 
and  Messrs.  Colin,  Jeance  and  Mercier  in  France,  have  severally 
reported  successful  wireless  telephonic  transmission  of  speech  to 
distances  ranging  from  40  to  600  miles.  Even  if  these  experi- 
ments have  been  lacking  in  some  details  of  perfection,  we  cannot 
doubt  that  practical  wireless  telephony,  especially  between  ships 
at  sea  at  a  considerable  distance  apart,  is  a  possibility  of  the 
present  time  or  of  the  immediate  future. 


CHAPTER  XXVII 


SOME  DETAILS  OF  CONSTRUCTION  OF  WIRELESS  TELEGRAPHIC 

APPARATUS 

IT  is  beyond  the  scope  of  an  elementary  treatise  to  erxter  exten- 
sively into  a  discussion  of  the  engineering  details  of  a  wireless 
telegraph  installation.  In  fact,  much  of  the  wireless  telegraphic 
engineering  of  the  present  time  is  done  by  methods  of  construction 
and  trial  rather  than  by  scientific  prognosis.  There  are,  however, 
certain  elementary  facts  that  may  be  of  service  to  amateurs 
engaged  in  constructing  or  operating  wireless  telegraphic  appara- 
tus, and  that  at  the  same  time  may  be  not  without  interest  for  the 


I 

Straight 


A 

Flat-top 


Fan-shaped 
FIG.  226.     Types  of  antenna. 


Umbrella 


general  reader.  Some  of  these  elementary  facts  regarding  con- 
struction are  here  presented. 

Antenna.  —  The  character  of  the  equipment  that  may  be 
employed  in  a  given  instance  depends  on  the  facilities  that  exist 
for  the  erection  of  an  antenna.  A  few  simple  types  of  antenna 
are  represented  in  the  diagrams  of  Fig.  226. 

The  Flat-topped  Antenna.  —  Of  these  types,  the  flat-topped 
antenna  usually  gives  the  best  results  for  a  small  installation. 
The  flat-topped  antenna  consists  of  the  nearly  vertical  portion  A  B 
and  the  nearly  horizontal  portion  BC.  The  horizontal  portion 
does  not  contribute  much  as  a  useful  radiating  member,  because 
waves  emitted  from  this  portion  have  their  electric  force  parallel 
to  the  earth's  surface,  so  that  the  part  of  this  radiation  that 

312 


CONSTRUCTION  OF  WIRELESS  APPARATUS     313 

travels  out  along  the  surface  of  the  earth  induces  currents  in  the 
earth  and  is  rapidly  absorbed.  The  remainder  of  the  energy 
radiated  from  this  horizontal  portion  travels  prevalently  upward 
and,  save  for  contributing  to  the  directiveness  of  transmission 
as  has  been  pointed  out  in  Chapter  XXV,  does  not  have  much 
effect  at  the  receiving  station  unless  it  is  desired  to  transmit 
to  a  balloon,  when  this  upward-traveling  component  is  most 
useful. 

The  horizontal  portion  of  the  flat-topped  antenna  is,  therefore, 
chiefly  serviceable  as  a  capacity  at  the  top  of  the  vertical  part, 
which  latter  is  the  chief  radiating  member.  As  to  the  amount  of 
the  capacity  it  is  interesting  to  note  that  a  single  wire  100  feet  long 
and  J  inch  in  diameter  when  alone  in  space  has  as  much  capacity 
as  an  isolated  flat  metallic  disc  16  feet  in  diameter.  (See  formulas 
for  calculation  in  Appendix  II.)  From  this  it  will  be  seen  that 
the  horizontal  top  to  the  antenna  is  a  far  more  economical  elevated 
capacity  than  any  kind  of  a  metallic  sheet  such  as  was  employed 
in  Marconi's  early  experiments. 

Comparison  of  Flat-topped  with  Straight  Antenna.  —  In  order 
to  illustrate  some  of  the  principles  involved,  let  us  next  compare 
the  radiation  from  a  single  vertical  wire  100  feet  long  and  say  J 
inch  in  diameter  with  that  from  a  flat-topped  antenna  consisting 
of  a  vertical  wire  100  feet  long  having  at  the  top  a  horizontal 
extension  of  the  same  length.  For  the  purpose  of  this  comparison 
we  shall  employ  the  experimental  curve  of  current  distribution 
found  in  Chapter  XIV  (Fig.  82).  In  the  first  place  the  flat-topped 
antenna,  because  of  its  greater  length  of  wire,  has  approximately 
twice  as  much  capacity  as  the  simple  vertical  antenna.  This 
means  that  if  we  charge  the  two  antennae  to  the  same  potential, 
about  twice  as  much  electricity  will  flow  during  one  oscillation  of 
the  flat-topped  antenna  as  during  one  oscillation  of  the  simple 
vertical  antenna;  but  the  time  of  the  oscillation  in  the  former  case 
will  be  about  twice  as  long;  therefore  the  maximum  current  flowing 
to  the  ground  will  be  about  the  same  in  the  two  cases.  Let  us 
now  plot  the  approximate  current-distribution  curves  for  the  two 
cases,  assuming  the  same  current  at  the  base;  and  in  doing  this 
we  shall  make  the  further  assumption  that  the  distribution  in  the 
bent  antenna  is  approximately  the  same  as  it  would  be  for  a 
straight  antenna  of  the  same  length.  The  curves  obtained  are 
given  in  Fig.  227.  In  these  curves  the  value  of  the  current  at  any 
point  of  the  length  of  the  antenna  is  plotted  as  a  distance  between 


314 


WIRELESS  TELEGRAPHY 


the  antenna  and  the  curve.  Careful  plotting  and  measurement  of 
these  curves  show  that  the  average  current  in  the  vertical  portion 
of  the  flat-topped  antenna  is  .88  of  the  maximum  current  at  the 
base;  whereas  the  average  current  for  the  vertical  antenna  is  only 
.62  of  the  current  at  the  base.  Dividing  .88  by  .62,  we  find  that 
the  average  current  in  the  vertical  portion  of  the  flat-topped 
antenna  is  1.41  times  the  average  for  the  simple  vertical  antenna. 
From  these  considerations  it  appears  that  we  have  gained  41%  in 
effective  current  by  the  use  of  the  flat-topped  extension.  We 
could  gain  approximately  the  same  by  extending  the  simple 
antenna  about  41  feet  upwards.  From  this  we  may  conclude  that 
two  poles  of  100  feet  in  height  and  100  feet  apart  supporting  a 


FIG.  227.  Comparison  of  current  distribution  on  a  flat-topped 
antenna  (left)  with  that  on  a  straight  antenna  (right)  of 
the  same  height. 

flat-topped  antenna  would  give  approximately  the  same  service  as 
a  single  pole  141  feet  high  supporting  a  single  vertical  antenna. 

On  account  of  difference  in  damping  and  on  account  of  the  effects 
on  radiation  introduced  by  the  difference  in  wave  length  in  the 
two  cases,  and  also  on  account  of  the  directive  emission  from  the 
flat-topped  antenna,  the  problem  is  not  so  simple  as  is  here  repre- 
sented; and  the  numerical  deductions  are  indeed  only  very  rough 
approximations,  which  serve  merely  to  show  wherein  consists  the 
efficacy  of  the  flat-topped  antenna;  namely,  in  the  increased 
average  current  in  the  vertical  part  due  to  the  capacity  of  the 
horizontal  part. 

Antenna  of  Several  Wires,  —  Instead  of  employing  a  single 
wire  in  the  antenna,  as  in  the  illustrative  example  here  given, 
several  wires  are  usually  employed.  It  should  be  noted,  however, 
that  n  wires  placed  side  by  side  have  not  anything  like  n  times  the 
capacity  of  a  single  wire;  because  the  charge  on  one  wire  repels 
the  charge  on  the  other  wires,  and  therefore  the  charge  that  the 
system  will  take  under  a  given  electromotive  force  applied  at  the 


CONSTRUCTION  OF  WIRELESS  APPARATUS 


315 


base  is  not  multiplied  in  the  ratio  that  the  number  of  wires  is 
multiplied. 

For  an  economical  installation  from  four  to  six  wires  may  well 
be  employed  in  the  antenna,  and  by  the  use  of  light  bamboo 
spreaders  they  can  easily  be  supported  three  feet  or  more  apart. 

Marconi  Antenna  at  Clifden.  —  An  example  of  the  use  of  the 
flat-topped  antenna  on  a  large  scale  is  afforded  by  the  Marconi 
high-power  station  at  Clifden,  Ireland.  The  horizontal  part  of 
the  antenna  of  this  station  consists  of  200  wires  1000  feet  long 
supported  180  feet  above  the  earth  The  wave  length  is  about 
4000  meters. 

The  Umbrella  Antenna.  —  When  only  one  supporting  pole  is 
available,  either  the  straight  type  or  the  umbrella  type  of  antenna 


FIG.  228.     Umbrella  type  of  antenna. 

is  usually  employed.  The  umbrella  type  meets  with  frequent  use 
in  small  amateur  stations  and  in  the  portable  stations  employed 
by  armies.  In  this  type  the  aerial  system  consists  of  a  vertical 
portion  terminating  above  in  a  system  of  wires  inclining  downward. 
These  inclining  wires  are  usually  the  guy  wires,  while  the  vertical 
part  may  be  either  a  wire  leading  to  the  top  of  the  pole,  or  the 
pole  may  itself  be  of  metal  and  serve  as  the  vertical  conductor. 
A  diagram  of  an  umbrella  type  of  antenna  with  a  metallic  pole 
serving  as  the  vertical  conductor  is  shown  in  Fig.  228.  The 


316  WIRELESS  TELEGRAPHY 

metallic  pole  used  in  a  small  installation  may  be  two  or  three 
sections  of  ordinary  tinned  gutter  pipe  or  of  one-inch  iron  water 
pipe.  To  keep  such  an  antenna  straight,  a  separate  set  of  guy 
wires  must  be  used  for  every  section  of  pipe  employed.  The 
bottom  of  the  metallic  pole  is  supported  on  an  insulating  base  B, 
which  is  protected  from  rain  by  a  shelter  S  placed -above  it  and 
connected  to  the  pole.  The  wire  W  leading  from  the  operating 
room  is  connected  directly  to  the  pole  near  its  base.  The  lower 
guy  wires  AAAA  are  preferably  insulated  from  the  pole  and  from 
the  housetop.  The  upper  parts  of  the  upper  guys  CCCC  are  con- 
nected to  the  top  of  the  pole,  and  these  serve  as  a  capacity  exten- 
sion to  the  antenna.  At  a  suitable  distance  from  the  top  of  the 
pole  high-tension  insulators  are  inserted  so  as  to  terminate  the 
antenna. 

With  this  form  of  antenna  it  will  be  observed  that  the  oscillation 
in  the  vertical  pole  and  that  in  the  inclined  extensions  CCCC  are 
partially  opposite  to  each  other,  and  therefore  partially  neutralize 
each  other  with  respect  to  radiation.  The  length  of  the  guy-wire 
extension  that  can  thus  be  used  with  advantage  will  depend  upon 
the  number  of  the  guys  and  their  inclination. 

The  Fessenden  Tower  at  Brant  Rock.  —  A  very  striking  example 
of  a  station  making  use  of  the  supporting  structure  as  antenna  is 
the  powerful  station  of  the  National  Electric  Signaling  Company 
at  Brant  Rock,  Massachusetts.  For  the  antenna  of  this  station 
there  is  provided  a  cylindrical  steel  tower  440  feet  high,  carefully 
insulated  at  the  base  and  provided  above  with  extension  capacity 
in  the  form  of  four  horizontal  arms  each  80  feet  long.  These 
arms  being  horizontal  do  not  offer  the  disadvantage  of  partially 
neutralizing  the  radiation  from  the  tower. 

When  it  is  remembered  that  this  very  tall  and  very  heavy  steel 
tube  must  be  sufficiently  insulated  from  the  earth  to  withstand 
the  enormous  potential  developed  in  a  very  high-power  wireless 
telegraph  sending  station,  it  will  be  seen  that  the  design  and 
erection  of  such  a  plant,  which  was  accomplished  by  Professor 
Fessenden,  is  a  very  considerable  feat  of  mechanical  and  electrical 
engineering. 

It  is  interesting  to  compare  the  capacity  of  this  large  tube  with 
that  of  a  small  wire.  With  the  aid  of  Formula  VII  of  Appendix  II, 
it  can  be  shown  by  calculation  that  a  tube  440  feet  (13,510  cm.) 
high  and  3  feet  in  diameter  (46  cm.  in  radius)  has  a  capacity, 
when  alone  in  space,  that  is  only  about  twice  as  great  as  the  capac- 


CONSTRUCTION  OF  WIRELESS  APPARATUS     317 

ity  of  a  wire  the  same  length  and  f  of  an  inch  (1  cm.)  in  diameter. 
Therefore,  so  far  as  concerns  capacity,  a  few  small  wires  five  or 
six  feet  apart  would  be  the  equivalent  of  this  large  steel  tube. 

The  Ground.  —  The  theory  of  the  action  of  the  ground  has 
been  discussed  in  Chapter  XIV.  In  practice,  for  a  small  station 
a  satisfactory  ground  can  be  obtained  by  a  connection  to  the  pipes 
of  a  water  supply.  Where  this  is  lacking,  a  good  arrangement  is 
to  bury  a  netting  or  network  of  wires  at  a  short  depth  below  the 
surface  of  the  earth.  This  may  be  supplemented  by  metallic  pipes 
driven  to  considerable  depths  into  the  earth,  and  also  by  wire 
netting  spread  out  on  the  surface  of  the  earth.  When  the  station 
is  located  near  the  sea  or  other  body  of  water,  the  wire  netting  or 
wires  provided  with  terminal  plates  may  be  led  into  the  body  of 
water.  On  board  ship,  the  grounding  is  usually  effected  by  a 
heavy  wire  attached  to  the  metallic  hull  of  the  ship.  In  the  high- 
power  land  stations,  netting  and  wires  are  made  to  ramify  the 
surface  of  the  earth  for  many  acres. 

We  have  seen  in  Chapter  XIV  that  a  properly  resonant  artifi- 
cial conductor  supported  without  contact  with  the  earth  serves 
as  a  very  good  ground.  The  difficulty  about  the  artificial  ground 
is  the  fact  that  the  artificial  ground  should  be  tuned  along  with  the 
aerial  system  in  order  to  get  resonance  with  different  wave  lengths. 

Sending  Condensers  for  a  Coupled  Transmitting  Station. - 
The  details  of  construction  of  the  simple  Marconi  apparatus  of 
1896  need  not  be  given.  When  a  sending  station  of  the  inductively 
coupled  or  direct  coupled  type  is  to  be  employed,  the  sending  con- 
densers must  be  electrically  strong  in  order  to  permit  the  storage 
of  the  large  quantities  of  electricity  used  in  producing  the  waves. 
Among  the  types  of  condenser  employed  for  this  purpose  the  bank 
of  Leyden  jars  or  of  flat  glass  plates  provided  with  metallic  coat- 
ings are  most  familiar.  The  use  of  tinfoil,  for  the  coating  of 
Leyden  jars  or  flat-plate  condensers  for  use  in  wireless  telegraphy, 
has  been  largely  discontinued.  In  the  case  of  the  flat-plate  con- 
densers copper  or  brass  sheets  between  the  plates  in  the  place  of 
the  tinfoil  that  was  formerly  much  used  gives  a  much  smaller 
loss  of  energy,  and  consequently  much  smaller  heating  of  the  con- 
denser. Ordinary  window  glass,  when  selected  free  from  flaws,  is 
electrically  stronger  than  plate  glass  for  making  glass-plate  con- 
densers. When  high  power  is  to  be  used,  the  flat-plate  condensers 
should  be  submerged  in  castor  oil  to  prevent  brush  discharge. 

In  the  case  of  the  Leyden  jars,  when  used  in  stations  of  large 


318 


WIRELESS  TELEGRAPHY 


power,  glass  of  especially  high  electric  breaking  strength  is  em- 
ployed and  the  tinfoil  of  other  days  is  now  usually  replaced  by  a 
coating  of  silver  or  copper  electrolytically  deposited  on  the  inner 
and  outer  surface  of  the  jars.  A  photograph,  lent  me  by  Mr. 
Pickard,  of  some  jars  coafced  m  this  way  by  the  Wireless  Specialty 
Apparatus  Company  of  New  York,  is  shown  in  Fig.  229. 

Air  Condensers,  formed  of  metallic  plates  with  air  between  as 
dielectric,  are  said  to  be  employed  as  sending  condensers  in  Mr. 
Marconi's  high-power  stations  at  Poldu,  Clifden  and  Wellfleet. 

Condensers  employing  compressed  air  as  dielectric  have  been 
employed  by  Mr.  Fessenden  in  his  Brant  Rock  station  and  in 


FIG.  229.     Copper-plated  Ley  den  jars. 

some  of  the  ship  installations  supplied  by  The  National  Electric 
Signaling  Company  to  the  United  States  Navy.  The  dielectric 
constant  of  the  compressed  air  is  about  the  same  as  that  of  air 
at  atmospheric  pressure.  The  purpose  in  compressing  the  air  is 
to  increase  its  disruptive  strength  so  as  to  enable  the  condenser 
to  stand  higher  potentials.  The  disruptive  strength  is  approxi- 
mately proportional  to  the  gas  pressure. 

Amount  of  Capacity  to  be  Used  at  a  Given  Station.  —  The 
amount  of  capacity  to  be  used  at  a  given  coupled-type  of  sending 
station  depends  upon,  (1)  the  amount  of  power  to  be  supplied  to 
the  condenser;  (2)  the  number  of  sparks  per  second,  and  (3)  the 


CONSTRUCTION  OF  WIRELESS  APPARATUS     319 

voltage  at  which  the  discharge  occurs.  As  a  specific  example,  let 
us  suppose  that  the  power  is  to  be  supplied  by  an  alternating 
current  source  of  n  cycles  per  second.  By  means  of  a  transformer 
with  its  primary  connected  to  the  source  of  power  and  its  secondary 
attached  to  the  condenser,  we  may  step  up  the  potential  to  the 
value  required  to  produce  the  required  spark.  Let  us  suppose  the 
transformer  to  supply  P  kilowatts  of  power  to  the  condenser,  and 
let  us  choose  the  condenser  and  the  spark  gap  to  be  such  that 
the  condenser  charges  to  a  sparking  potential  only  once  during 
each  half-cycle ;  that  is,  2  n  times  per  second. 

Now  to  charge  a  condenser  once  to  a  potential  of  V  volts  requires 
an  amount  of  energy, 

W  =  \QV  joules,  (1) 

where  Q  is  the  number  of  coulombs  of  electricity  required  and 
|  V  is  the  average  potential  during  the  charge.     (See  Appendix  I.) 
And,  from  the  definition  of  capacity, 

Q  =  CV,  (2) 

where  C  is  the  capacity  of  the  condenser  in  farads. 

Substituting  the  value  of  Q  from  equation  (2)  in  equation  (1),  we 
have 

W  =  J  CV2  joules,  (3) 

V  being  the  potential  in  volts  to  which  the  condenser  is  charged. 

In  our  supposed  case  the  condenser  is  charged  2  n  times  per 
second;  therefore  the  energy  expended  per  second,  which  is  the 
power  supplied,  is 

W=  2  n  X  \  CV2  =  nCV2  joules  per  second.  (4) 

But  1  joule  per  second  is  1  watt,  and  1000  watts  make  a  kilowatt; 
therefore  if  P  is  the  power  in  kilowatts, 

P.  ^kilowatts.  (5) 

In  interpreting  this  formula,  it  must  be  remembered  that  V  is  the 
potential  to  which  the  condenser  is  charged  at  the  time  that  the 
spark  begins. 

The  formula  (5)  is  very  useful  in  practical  computations.  By 
a  simple  transposition  of  terms,  equation  (5)  may  be  put  in  the 
form 

1000  X  Power  in  Kilowatts 

C  =  -  T79  Co; 

nV2 


320  WIRELESS  TELEGRAPHY 

From  this  we  can  calculate  the  capacity  required  in  a  given  case, 
provided  we  know  the  power  to  be  employed,  the  number  of  cycles, 
and  the  voltage  to  which  the  condenser  is  to  be  charged.  For  a 
given  source  of  power  we  can  employ  either  a  large  condenser 
charged  to  a  low  potential  or  a  smaller  capacity  charged  to  a 
higher  potential.  A  simple  computation,  which  is  not  here  given, 
shows  that  approximately  the  same  volume  of  dielectric  (e.g.,  glass) 
will  have  to  be  used  in  the  condenser  in  either  case. 

In  estimating  the  amount  of  capacity  to  be  employed  to  con- 
sume a  given  amount  of  power,  according  to  formula  (6),  it  is  well 
to  estimate  about  15,000  volts  to  the  centimeter  of  spark  length; 
for  this  is  about  the  value  of  the  potential  when  the  spark  gap  is 
heated  and  ionized  by  continuous  sending.  On  the  other  hand,  in 
estimating  the  amount  of  dielectric  to  use  for  sufficient  strength 
to  stand  the  charge  without  breaking,  it  is  well  to  estimate  about 
39,000  volts  to  the  centimeter;  for  the  voltage  will  rise  to  this  value 
when  the  station  is  first  started  up. 

The  Charging  Transformer.  —  After  the  dimensions  and  capac- 
ity of  the  condenser  for  the  sending  station  have  been  settled 
upon,  the  transformer  must  be  designed  to  be  in  resonance  with  the 
condenser.  The  proper  proportioning  of  the  primary  and  second- 
ary inductance  and  the  mutual  inductance  of  the  charging  trans- 
former of  the  sending  station  is  one  of  the  most  troublesome  factors 
arising  in  connection  with  wireless  telegraphy  design  and  con- 
struction, and  cannot  be  adequately  discussed  in  an  elementary 
treatise.  The  fact  to  be  kept  in  mind  is  that  the  transformer  for 
this  purpose  must  have  entirely  different  properties  from  those  pos- 
sessed by  an  ordinary  closed  iron-core  lighting  transformer,  because 
the  lighting  transformer  is  designed  to  supply  more  and  more 
power  as  the  load  is  made  of  lower  and  lower  resistance;  while 
with  the  wireless  telegraph  transformer  the  load  is  a  condenser, 
which  will  attain  a  maximum  charge  for  a  certain  resonant  relation 
of  the  constants  of  the  transformer  to  the  capacity  of  the  con- 
denser. A  spark  will  then  pass.  This  spark  amounts  to  a  short- 
circuit  of  the  secondary  of  the  transformer.  Under  this  condition 
an  ordinary  closed-core  transformer  would  supply  a  maximum 
amount  of  power  right  across  the  short-circuited  gap,  so  that  this 
gap  would  sustain  an  arc,  and  the  condenser  would  then  not  charge 
up  again.  This  is  not  desired.  What  is  desired  is,  that  when  the 
discharge  of  the  condenser  occurs  and  short-circuits  the  secondary 
of  the  transformer,  the  transformer  should  be  so  designed  that  it 


CONSTRUCTION  OF  WIRELESS  APPARATUS 


321 


will  draw  a  very  small  amount  of  power,  and  allow  the  spark  to 
extinguish  promptly  after  the  discharge  of  the  condenser. 

A  mathematical  examination  of  this  problem  shows  that  this 
result  can  be  obtained  with  a  proper  adjustable  resistance  placed 
in  the  primary  circuit  of  the  transformer,  if  a  common  closed-core 
transformer  is  used.  The  same  result  can  be  more  economically 
obtained  by  the  use  of  an  adjustable  inductance  in  series  with  the 


D— i 


Ground 


Secondary 
Condenser 


a                                          Sending  Helix 

r^ 
I   Cut-over 

r 

i 

i 
i 

i^ 

,* 
i 

i 
i 

i 

o 

J—l 

C 

\     Switch 

r==i3 

t-^             _JSa<gU 

< Line--- 

FIG.  230.     Diagram  of  transmitting  and  receiving  installation. 


primary.  It  can  also  be  attained  by  an  adjustable  inductance  in 
series  with  the  secondary  of  the  closed-core  transformer. 

With  an  open-core  type  of  transformer  and  an  adjustable  induc- 
tance in  the  primary  circuit  considerably  greater  flexibility  in 
attaining  resonance  with  condensers  of  different  capacities  is  pos- 
sible, and  many  engineers  prefer  the  open-core  transformer. 

Sending  Helix.  —  The  construction  of  the  sending  helices  of 
the  direct-coupled  and  the  inductively  coupled  type  is  shown  in 
the  photographs  of  Figs.  166  and  168  respectively. 

Sending  Key.  —  With  power  not  exceeding  5  kilowatts  at  a 


322 


WIRELESS  TELEGRAPHY 


voltage  not  higher  than  150  volts,  the  primary  circuit  can  be  inter- 
rupted with  an  ordinary  Morse  telegraph  key  provided  with  heavy 
platinum  or  silver  contacts.  For  larger  values  of  the  power  some 
form  of  relay  key  by  which  the  current  is  broken  between  large 
contacts  under  oil  is  generally  employed. 

Diagram  of  Sending  and  Receiving  Circuit  with  Cut-over 
Switch.  —  In  the  diagram  of  Fig.  230,  which  shows  the  connections 
for  a  complete  station,  the  sending  apparatus  is  shown  at  the 


FIG.  231.     View  of  installation. 

right,  the  receiving  apparatus  at  the  left  and  the  cut-over  switch 
for  throwing  from  sending  to  receiving  is  shown  near  the  center. 
This  switch  usually  has  three  blades,  mounted  on  a  hard  rubber 
axis,  and  sufficiently  far  apart  to  avoid  sparking  between  the 
blades  of  the  switch  or  from  the  sending  to  the  receiving  apparatus. 
The  switch  is  shown  in  the  position  for  sending;  two  of  the  blades 
join  respectively  the  antenna  and  the  ground  to  the  sending  helix, 
and  the  third  blade  closes  the  line  circuit  to  the  key  and  trans- 
former. When  the  switch  is  thrown  to  the  left,  the  antenna  and 


CONSTRUCTION  OF  WIRELESS  APPARATUS 


323 


ground  are  joined  to  the  primary  of  the  inductively  connected 
receiving  transformer,  and  the  line  circuit  is  opened  so  as  to  avoid 
a  possible  accidental  discharge  of  the  high-potential  circuit  while 
receiving. 

A  photograph  of  a  station  with  approximately  the  arrangement 
of  circuits  here  indicated  is  shown  in  Fig.  230. 

I  will  next  describe  some  of  the  parts  of  the  receiving  appa- 
ratus, and  shall  employ  in  the  description  the  designations  used  in 
Fig.  231. 

Receiving  Condensers.  —  The  series  condenser,  which  is  em- 
ployed in  the  antenna  circuit  between  the  primary  of  the  receiving 
transformer  and  the  ground,  should  be  an  air  condenser  of  the 
semicircular  plate  type,  like  that  shown  in  the  photograph  of 
Fig.  81.  The  introduction  of  this  condenser  has  the  effect  of 
shortening  the  wave  length  of  the  antenna,  so  as  to  adapt  an 
antenna  of  long  wave  length  to  receive  short  waves.  Tuning  by 
means  of  this  condenser  gives  a  better 
discrimination  of  signals  according  to 
their  wave  lengths  than  can  be  obtained 
by  the  use  of  adjustments  in  the  detec- 
tor circuit;  nevertheless  this  series  con- 
denser can  often  be  dispensed  with. 

The  secondary  receiving  condenser,  in 
circuit  with  the  detector,  cannot  be  dis- 
pensed with.  This  condenser  may  also 
be  of  the  semicircular  air  type,  but  its 
capacity  should  usually  be  larger  than 
can  be  attained  with  a  single  condenser 
of  this  type.  If  the  secondary  of  the 
receiving  transformer  is  adjustable  as 
to  inductance,  the  secondary  condenser 
does  not  require  to  be  capable  of  fine 
adjustment,  and  a  condenser  with  mica 
plates  as  dielectric,  and  provided  with 
step-by-step  adjustment,  may  be  used. 
In  fact,  with  adjustable  inductances  in 
the  transformer,  the  value  of  the  secon- 
dary condenser  may  well  be  entirely  fixed. 

Receiving  Transformer.  —  A  photograph  of  one  type  of  receiv- 
ing transformer  is  given  in  Fig.  232.  The  secondary  coil  of  this 
transformer  is  shown  near  the  top  of  the  apparatus.  The  primary 


FIG.  232.     A  receiving 
transformer. 


324  WIRELESS  TELEGRAPHY 

is  the  long  solenoid  of  a  single  layer  of  wire  wound  on  a  cylindrical 
paper,  glass,  or  vulcanite  drum.  The  inductance  of  this  primary 
coil  can  be  varied  by  the  sliding  contact.  After  this  adjustment 
has  been  made,  the  secondary  may  be  moved  as  a  whole  down  or 
up,  so  as  to  bring  it  into  proper  inductive  relation  to  the  primary. 

The  reader  will  easily  see  how  this  construction  may  be  varied; 
for  example,  both  the  primary  and  the  secondary  coils  may  be 
wound  on  long  glass  tubes  of  different  diameters.  One  of  the 
coils  may  then  be  mounted  inside  of  the  other,  and  the  inductance 
of  both  coils  can  then  be  varied  by  sliding  contacts,  —  the  contact 
to  the  inner  coil  beingf  carried  by  a  rod  that  protrudes  through 
the  head  of  the  coils. 

The  Detector  and  Potentiometer.  —  The  detector  usually  em- 
ployed at  the  present  time  is  either  an  electrolytic  detector  or  a 
crystal-contact  rectifier.  Details  in  regard  to  these  detectors  have 
already  been  partially  given.  For  the  electrolytic  detector,  plati- 
num wire  from  two  to  four  ten-thousandths  of  an  inch  drawn  so  as 
to  form  the  core  of  a  larger  wire  of  silver  is  usually  employed  for 
the  most  sensitive  receiver  of  the  electrolytic  type.  The  electrolyte 
used  is  generally  20%  nitric  acid.  The  fine  platinum  wire  must 
be  capable  of  delicate  adjustment  up  and  down  so  as  to  bring  it 
into  minute  contact  with  the  electrolyte.  In  attaining  this  result 
the  wire  with  the  silver  on  it  is  attached  to  an  arm  movable  by  a 
micrometer  screw.  It  is  then  dipped  into  the  electrolyte  to  a 
depth  of  about  yV  of  an  inch,  and  a  current  somewhat  stronger 
than  the  operating  local  current  is  sent  through  it  from  the  fine 
point  to  the  electrolyte  so  as  to  remove  electrolytically  the  silver 
coating  from  the  point.  When  this  has  been  accomplished,  the 
point  is  raised  until  it  is  in  very  minute  contact  with  the  liquid, 
the  voltage  in  the  local  circuit  through  the  detector  and  telephone 
is  reduced  until  the  hissing  noise  in  the  telephone  made  by  the 
local  current  through  the  detector  just  ceases.  The  detector  is 
then  in  delicate  adjustment  for  receiving.  The  sensitiveness  of 
the  detector,  and  its  readiness  to  receive  signals,  may  further  be 
tested  by  a  buzzer  device  for  that  purpose. 

The  accurate  adjustment  of  the  local  voltage  is  achieved  by 
the  use  of  a  potentiometer,  which  is  shown  in  the  diagram  of  the 
complete  station,  Fig.  230.  This  potentiometer  consists  of  a  coil 
of  resistance  wire  of  about  500  ohms,  connected  to  about  three 
Leclanche  cells  or  three  dry  cells.  This  resistance  coil  is  wound 
on  a  tube  and  provided  with  a  sliding  contact.  The  adjustable 


CONSTRUCTION  OF  WIRELESS  APPARATUS 


325 


voltage  for  the  local  circuit  is  taken  from  this  resistance  by  two 
leads,  one  to  the  end  of  the  resistance  and  the  other  to  the  sliding 
contact.  The  exterior  of  a  potentiometer  in  which  the  resistance 


FIG.  233.     View  of  a  potentiometer. 

is  wound  on  a  circular  collar  and  the  sliding  contact  carried  by  a 
rotating  arm  is  shown  in  Fig.  233. 

Two  electrolytic  detectors,  mounted  on  a  common  base  with 
this  potentiometer,  are  shown  in  Fig.  234. 

With  some  of  the  crystal-contact  detectors  a  small  voltage  in 


FIG.  234.    Two  electrolytic  detectors  with  potentiometer. 

the  local  circuit  may  be  an  advantage.  The  potentiometer  in  this 
case  need,  however,  employ  only  one  dry  cell  or  one  Leclanch£  cell. 
Reliance  on  Principles  Rather  than  on  Details.  —  The  details 
of  construction  here  given  appertain  primarily  to  what  is  at  the 
present  time  the  most  usual  type  of  wireless  telegraph  station. 
Progress  in  this  respect  is,  however,  very  rapid,  and  it  is  not  at  all 


326  WIRELESS  TELEGRAPHY 

unlikely  that  the  reader  who  is  engaged  in  the  practical  study  of 
wireless  telegraphy  may  readily  see  how  to  make  improvements 
on  any  of  the  constructions  here  suggested. 

In  order  to  get  the  greatest  interest  and  benefit  out  of  practical 
experiments  in  any  science,  the  reader  is  advised  to  seek  carefully 
for  the  meaning  of  any  novelty  that  arises  in  his  experience  and  to 
attempt  to  interpret  his  results  in  terms  of  the  general  principles 
of  the  subject. 


CHAPTER  XXVIII 
CONCLUSION 

THERE  are  at  present  many  thousand  wireless  telegraph  stations 
in  daily  operation  in  the  world.     The  map  of  Fig.  235  shows  the 


FIG.  235.     Map  showing  location  on  the  coast  of  North  America  of  wireless 
telegraph  stations  belonging  to  the  United  States  and  to  Great  Britain. 

location  of  some  of  the  stations  on  the  Atlantic  Coast  of  North 
America.  On  this  map  only  the  stations  of  the  United  States 
Navy  on  the  coasts  of  the  United  States,  Cuba,  Porto  Rico  and 

327 


328  WIRELESS  TELEGRAPHY 

Panama,  and  the  stations  of  the  British  Government  in  its  North 
American  provinces,  are  represented.  For  lack  of  room  and  infor- 
mation all  of  the  commercial  and  amateur  stations  have  been 
omitted.  When  it  is  remembered  that  of  the  stations  on  the  map 
many  are  capable  of  being  heard  at  night  for  a  distance  equal  to 
half  the  length  of  the  whole  coast,  an  idea  can  be  formed  as  to  the 
earnestness  with  which  electric-wave  telegraphy  has  been  seized 
upon  as  a  method  of  communicating  intelligence.  By  sending  out 
daily  time  signals,  weather  reports  and  storm  warnings,  ancl  by 
responding  to  calls  for  aid  from  ships  in  distress,  these  governmen- 
tal equipments  have  been  of  inestimable  service  to  mariners,  and 
have  already  saved  thousands  of  human  lives. 

The  history  of  this  development  is  a  striking  example  of  the 
manner  in  which  the  labors  of  scientists  in  fields  of  pure  research 
apparently  unrelated  to  commercial  applications  may  result  in 
discoveries  of  the  utmost  material  importance.  Maxwell  in  his 
search  for  a  rational  grasp  of  the  undulatory  theory  of  light  and 
Hertz  in  his  experimental  effort  to  establish  a  relation  between 
electromagnetic  force  and  the  dielectric  polarization  of  insulators 
were  unwittingly  laying  the  foundation  for  radiotelegraphy,  which 
is,  in  fact,  after  all  only  a  single  development  from  among  a  host 
of  other  consequences  of  perhaps  even  greater  significance  that 
have  grown  out  of  the  remarkable  discoveries  of  Maxwell  and 
Hertz. 


APPENDIX  I 

ELEMENTARY  FACTS  ABOUT  ELECTRICITY  AND 
DEFINITIONS  OF  UNITS 

Two  sets  of  electrical  units,  based  directly  on  the  centimeter, 
gram  and  second,  are  in  use  for  the  measurement  of  electrical 
quantities.  These  two  systems  are  both  called  centimeter-gram- 
second  units  (abbreviated  c.g.s.  units). 

One  of  these  sets  of  c.g.s.  units  (called  the  electrostatic  units)  is 
obtained  from  the  laws  of  attraction  and  repulsion  between  charged 
bodies. 

The  other  set  of  c.g.s.  units  (called  electromagnetic  units)  is 
obtained  by  a  consideration  of  the  laws  of  eleetromagnetism. 

In  addition  to  these  two  sets  of  c.g.s.,  or  absolute  units,  there  is 
also  in  international  use  a  set  of  units  of  a  size  somewhat  better 
adapted  to  practical  measurements,  which  are  designated  the  prac- 
tical units. 

In  defining  these  several  units,  we  shall  make  use  of  some  of 
the  fundamental  principles  of  electricity,  which  are  here  reviewed. 

ON  ELECTROSTATIC  ATTRACTION  AND   REPULSION 

Measurement  of  Electrostatic  Forces.  —  The  force  of  attraction 
or  repulsion  between  two  electrified  bodies  may  be  measured 
directly.  One  method  is  to  attach  one  of  the  bodies  in  an  unelec- 
trified  state  to  one  arm  of  a  delicate  balance,  and  counterbalance 
it  with  a  weight  suspended  from  the  other  arm  of  the  balance. 
Another  body  may  now  be  brought  up  under  the  first ;  both  bodies 
may  be  electrified,  and  their  attraction  be  measured  by  the  coun- 
terbalancing weight  that  must  be  added  to  bring  the  system  again 
into  equilibrium.  This  is  the  method  employed  in  Sir  William 
Thomson's  absolute  electrometer. 

Another  method,  which  is  more  sensitive  for  measuring  electric 
forces,  makes  use  of  the  torsion  balance,  in  which  a  light  lever  is 
suspended  by  a  fine  fiber  so  as  to  be  free  to  rotate  by  twisting 
the  fiber.  One  of  the  electrified  bodies  is  attached  to  one  arm  of 
the  lever  and  counterbalanced;  the  other  electrified  body  is  brought 

329 


330  WIRELESS  TELEGRAPHY 

up  near  the  first  in  a  horizontal  plane,  so  that  the  force  of  attraction 
or  repulsion  tends  to  twist  the  fiber.  By  determining  the  torsional 
rigidity  of  the  fiber,  and  the  amount  of  twist  given  it  by  the  electric 
attraction,  the  force  of  the  attraction  may  be  determined.  This 
method  was  employed  by  Coulomb  in  measuring  the  force  of 
attraction  or  repulsion  between  electric  charges. 

On  the  Proof  of  the  Law  of  Inverse  Square  of  the  Distance.  — 
Coulomb,  by  the  use  of  the  torsion  balance,  proved  that  the  attrac- 
tion or  repulsion  of  two  given  charges  of  electricity  is  inversely 
proportional  to  the  square  of  the  distance  between  the  charges. 

A  very  sensitive  method  of  testing  this  law  was  devised  by 
Cavendish,  and  is  based  on  the  result  obtained  by  Faraday  in  his 
so-called  "  ice-pail  experiment."  Faraday's  experiment  showed 
that  electricity  at  rest  on  a  closed  conducting  body  resides  only 
on  the  outside  surface  of  the  body.  Cavendish  proved  mathe- 
matically that  the  only  law  of  repulsion  between  like  electrical 
charges  that  will  produce  this  distribution  of  electricity  on  a  con- 
ductor is  the  law  of  repulsion  inversely  proportional  to  the  square 
of  the  distance. 

Attraction  or  Repulsion  Proportional  to  the  Product  of  the 
Charges.  —  With  the  aid  of  the  torsion  balance,  Coulomb  showed 
that,  if  the  distance  between  two  charged  bodies  be  kept  constant, 
and  the  two  bodies  be  charged  with  quantities  of  electricity  Q  and 
Q',  respectively  (measured  in  arbitrary  units),  the  force  of  attrac- 
tion between  the  two  bodies,  if  they  have  unlike  signs,  or  the  force 
of  repulsion  between  them,  if  they  have  the  same  sign,  is  propor- 
tional to  the  product  of  the  charges. 

Combination  of  Quantity  Law  with  Distance  Law.  —  A  com- 
bination of  the  two  laws  above  enunciated  gives 

QQ' 


in  which  F  is  the  force  of  repulsion  between  the  two  charges. 
If  the  two  charges  have  unlike  signs,  their  product  will  be  negative 
and  the  force  becomes  a  negative  force  of  repulsion;  that  is,  a 
force  of  attraction. 

ELECTROSTATIC  UNIT  OF  QUANTITY  AND  OF  CURRENT 

The  C.G.S.  Electrostatic  Unit  of  Quantity.  —  The  law  of  repul- 
sion of  electrical  charges,  stated  in  the  preceding  paragraph,  sug- 
gests a  rational  unit  in  which  to  measure  quantity  of  electricity. 


APPENDIX   I  331 

The  law  is  that  the  repulsion  is  proportional  to  the  product  of  the 
two  quantities  divided  by  the  square  of  the  distance  between 
them.  The  rational  unit  is  chosen  to  make  the  attraction  not 
only  proportional  to,  but  equal  to,  the  product  of  the  charges 
divided  by  the  square  of  the  distance  (if  the  charges  are  in  vacuo)  ; 
that  is,  in  rational  units,  in  vacuo, 


That  this  may  be  true  F  must  be  1  when  Q,  Q'  and  r  are  all  made 
equal  to  1,  as  may  be  seen  by  substitution  of  the  value  1  for  Q,  Q' 
and  r.  This  leads  to  the  following  definition: 

Definition.  The  c.g.s.  Electrostatic  Unit  of  Quantity  is  that  quantity  of 
electricity  which,  when  placed  at  a  distance  of  one  centimeter,  in  vacuo,  from 
an  equal  quantity  of  electricity,  repels  it  with  a  force  of  one  dyne.1 

The  c.g.s.  Electrostatic  Unit  of  Current  is  that  current  that  delivers  one 
electrostatic  unit  quantity  of  electricity  per  second. 

ELECTROMAGNETIC  UNIT  OF  CURRENT  AND  OF  QUANTITY 

By  the  use  of  the  torsion  balance  and  by  reasoning  similar  to 
that  employed  in  the  experiments  on  electrostatics  described  above, 
it  has  been  shown  that  two  like  magnetic  poles  repel  each  other 
with  a  force  proportional  to  the  product  of  the  strengths  of  the 
poles  and  inversely  proportional  to  the  square  of  their  distance 
apart.  This  leads  to  the  following  definition  of  a  unit  magnetic 
pole. 

A  Unit  Magnetic  Pole  is  that  pole  that,  placed  at  a  distance  of  one  centimeter 
from  an  equal  pole  (in  vacuo),  repels  it  with  a  force  of  one  dyne. 

Now  it  has  been  shown  in  Chapter  III  that  if  a  suspended 
magnet  is  placed  parallel  to  a  conductor  of  electricity  and  a  cur- 
rent is  sent  through  the  conductor,  one  pole  of  the  magnet  is 
driven  one  way  and  the  other  pole  of  the  magnet  is  driven  in  the 
opposite  way,  so  that  the  magnet  tends  to  set  itself  at  right  angles 
to  the  conductor. 

By  measuring  the  force  exerted  by  the  current  on  the  magnet, 
the  laws  according  to  which  the  force  acts  in  this  case  have  also 
been  discovered,  and  these  laws  have  led  to  the  selection  of  a  set 
of  units  called  the  electromagnetic  units. 

1  In  a  medium  of  dielectric  constant  k  the  repulsion  between  two  like 

charges  is,  F  =  ^  ,  where  Q  and  Q'  are  in  c.g.s.  electrostatic  units,  r  in  cm., 

KT 
and  F  in  dynes. 


332  WIRELESS  TELEGRAPHY 

The  force  acting  between  the  electric  current  and  a  magnetic 
pole  depends  on  the  strength  of  the  current,  the  strength  and  posi- 
tion of  the  magnetic  pole,  and  the  shape  of  the  electric  circuit. 
In  defining  the  electromagnetic  unit  of  current,  the  form  of  the 
circuit  selected  is  the  circle  of  unit  radius,  and  the  pole  used  is  the 
unit  magnetic  pole. 

Definition.  The  c.g.s.  Electromagnetic  Unit  of  Current  is  that  current 
that,  flowing  in  a  circle  of  one  centimeter  radius,  exerts  on  a  unit  magnetic  pole 
placed  at  the  center  of  the  circle  a  force  of  one  dyne  for  each  centimeter  of 
arc  of  the  circle. 

The  c.g.s.  Electromagnetic  Unit  of  Quantity  is  that  quantity  of  electricity 
delivered  in  one  second  by  an  Electromagnetic  Unit  of  Current. 

Relation  of  the  Electromagnetic  Unit  to  the  Electrostatic  Unit 
of  Quantity.  —  Experimental  determinations  of  the  ratio  of  the 
two  c.g.s.  units  of  electrical  quantity  have  shown  that 

The  Electromagnetic  Unit  of  Quantity  IQ 

The  Electrostatic  Unit  of  Quantity 

This  is  the  velocity  of  light  in  centimeters  per  second.  The 
fact  that  the  ratio  of  these  two  units  is  equal  to  the  velocity  of 
light  was  also  derived  theoretically  by  Maxwell  from  his  electro- 
magnetic theory  of  light. 

Ratio  of  the  Absolute  Units  of  Current.  —  From  the  fact  that 
the  unit  of  current  is  a  unit  quantity  of  electricity  per  second,  it 
follows  that 

The  Electromagnetic  Unit  of  Current  _ 

The  Electrostatic  Unit  of  Current 


ON  ELECTRICAL  WORK,  AND  THE  C.G.S.  UNITS  OF  POTENTIAL 

Having  defined  the  units  of  current  and  quantity  in  both  systems 
of  c.g.s.  units,  we  shall  next  proceed  to  a  consideration  of  electrical 
work  and  potential. 

Work  and  Potential  Energy.  —  When  a  body  is  moved  against 
a  force  tending  to  prevent  the  motion,  work  is  done.  The  amount 
of  work  done  is  defined  as  the  product  of  the  force  overcome  by 
the  effective  displacement  of  the  point  of  application  of  the  force, 
the  effective  displacement  being  that  component  of  the  displace- 
ment which  is  parallel  to  the  force.  As  an  illustration,  let  us  take 
the  case  of  the  force  of  gravitation  due  to  the  attraction  of  the 
earth  for  a  body  near  the  surface  of  the  earth.  This  force  is 


APPENDIX  I 

perpendicular  to  the  surface  of  the  earth.  If  now  the  body,  which 
is  attracted  with  a  force  F,  is  raised  a  vertical  height  h,  the  work 
done  is  F  X  h',  and  this  work  is  the  same  whether  the  body  is 
raised  straight  up  a  height  h,  or  whether  it  goes  up  a  staircase  or 
along  an  inclined  plane  or  along  any  other  line,  provided  its  final 
position  is  somewhere  in  a  horizontal  plane  at  a  distance  h  above 
a  horizontal  plane  through  its  initial  position.  In  carrying  a  body 
upward  against  the  vertical  force  of  attraction  F  from  one  horizon- 
tal plane  to  another  at  a  distance  h  apart,  the  amount  of  work 
F  X  h  is  done.  If  the  body  is  allowed  to  descend  again,  the  body 
can  do  the  same  amount  of  work  against  any  other  force  con- 
veniently arranged;  so  the  body  is  said  to  have  potential  energy; 
that  is,  the  capacity  to  do  work  by  virtue  of  its  position.  The 
body  has  more  potential  energy  in  the  higher  position  by  the 
amount  F  X  h. 

Electrical  Work  and  Electrical  Potential.  —  Similar  ideas  are 
made  use  of  in  the  study  of  electricity.  These  ideas,  like  those  of 
the  gravitational  problem,  are  obtained  from  experience.  Suppose 
we  have  a  body  E  charged  with  positive  electricity,  and  suppose 
a  second  body  Ef,  also  charged  positively,  to  be  moved  up  toward 
the  first  against  the  force  of  repulsion  between  the  bodies.  Work 
is  done,  and  as  a  consequence  of  the  law  of  repulsion  between  the 
charged  bodies,  it  can  be  shown  by  a  theoretical  method  not  given 
here  that  the  work  done  in  carrying  a  given  charge  from  B  to  A 
is  independent  of  the  path. 

The  work  done  in  carrying  a  unit  quantity  of  electricity  from 
B  to  A  is  called  the  difference  of  potential  between  A  and  B. 

A  point  at  an  infinite  distance  from  a  system  of  charged  bodies 
has  a  potential  zero,  and  the  potential  at  any  point  P  is  the  work 
done  in  bringing  a  unit  quantity  of  electricity  from  an  infinite 
distance  up  to  the  point  P. 

Consistent  with  these  principles  we  have  the  two  following  c.g.s. 
units  for  measuring  potential  and  difference  of  potential. 

The  c.g.s.  Electrostatic  Unit  of  Potential. — Two  points  have  a  Difference 
of  Potential  of  one  Electrostatic  Unit  of  Potential  when  the  work  done  in 
carrying  an  Electrostatic  Unit  Quantity  of  Electricity  from  one  point  to 
the  other  is  the  c.g.s.  unit  of  work  (one  erg). 

The  c.g.s.  Electromagnetic  Unit  of  Potential.  —  The  two  points  have  a 
Difference  of  Potential  of  one  Electromagnetic  Unit  of  Potential  when  the 
work  done  in  carrying  the  Electromagnetic  Unit  Quantity  of  Electricity  from 
one  point  to  the  other  is  one  erg. 


334  WIRELESS  TELEGRAPHY 

Method  of  Computing  Electrical  Work.  —  According  to  the 
definitions  and  principles  given  above,  the  work  W  done  in  moving 
Q  units  of  electricity  from  one  point  to  another,  of  which  the 
average  difference  of  potential  during  the  transfer  is  V,  is 

W  =  QV, 

in  which  W  is  measured  in  ergs,  and  Q  and  V  are  either  both  in 
Electrostatic  or  both  in  Electromagnetic  units. 
Ratio  of  the  Units  of  Potential.  - 

The  Electromagnetic  Unit  of  Potential      1  1 


The  Electrostatic  Unit  of  Potential          v      3  X  1010 

This  relation  follows  from  the  ratio  of  the  units  of  quantity  given 
on  page  332,  together  with  the  fact  that  the  product  of  quantity 
by  potential  (work  in  ergs)  must  give  the  same  result  in  both  sets 
of  units. 

Another  Aspect  of  Potential,  Relating  Potential  Gradient  to 
Electric  Force.  —  The  exact  definition  of  potential  given  above 
is  based  on  the  idea  of  work.  We  may  slightly  change  the  aspect 
of  this  definition  and  obtain  from  it  the  fact  that  the  potential 
of  a  point  represents,  in  a  way,  the  tendency  of  electricity  to 
flow  from  the  point,  and  that  the  difference  of  potential  between 
two  given  points  measures,  jn  a  way,  the  tendency  of  electricity 
to  flow  from  the  point  of  higher  potential  to  the  point  of  lower 
potential.  Let  us  illustrate  this,  and  obtain  a  more  exact  state- 
ment. 

If  two  points  have  the  same  potential,  no  work  is  done  in  carry- 
ing a  unit  charge  from  one  to  the  other;  therefore,  since  work  is 
force  times  effective  distance,  there  is  no  electric  force  acting  from 
one  point  to  the  other,  and  no  tendency  of  a  charge  to  move  from 
one  point  to  the  other. 

On  the  other  hand,  if  two  fixed  points  have  a  difference  of  poten- 
tial, work  is  done  in  carrying  the  charge  from  one  of  the  points  to 
the  other;  there  is,  therefore,  a  force  acting  and  tending  to  send  a 
charge  from  the  point  of  high  potential  to  that  of  low  potential; 
and  the  greater  the  difference  of  potential,  the  greater  the  work 
of  carrying  a  unit  charge  the  same  distance,  and  therefore  the 
greater  the  force  acting  from  the  point  of  high  potential  to  that  of 
low  potential.  It  thus  looks  as  if  potential  difference  were  pro- 
portional to,  if  not  synonymous  with,  electric  force. 

However,  another  example  will  show  how  this  idea  needs  to  be 


APPENDIX  I  335 

slightly  modified  in  order  to  give  the  exact  relation  of  potential 
difference  to  electric  force.  Let  us  suppose  that  two  points  A  and 
B  have  a  certain  difference  of  potential,  and  that  two  other  points 
A'  and  B'  farther  apart  than  A  and  B  have  the  same  difference  of 
potential  as  A  and  B ;  the  work  done  in  carrying  a  unit  charge  from 
A'  to  Be  is  the  same  as  that  done  in  carrying  a  unit  charge  from 
A  to  B  (since  potential  difference  is  the  same) ;  but  since  the  work 
is  force  times  distance,  and  the  latter  distance  is  the  smaller,  the 
corresponding  electric  force  will  be  greater.  An  examination  of 
this  proposition  shows  that  the  force  acting  on  a  unit  charge  in 
either  case  is  proportional  to  the  fall  of  potential  per  unit  length. 

This  latter  quantity  is  called  potential  gradient,  and  we  have 
the  result  that  the  force  driving  any  given  charge  in  a  given  direction 
is  proportional  to  the  potential  gradient  in  that  direction. 

This  is  true  whether  we  are  concerned  with  electric  forces  tend- 
ing to  send  a  current  through  the  conductor  or  tending  to  move 
charged  bodies  as  a  whole.  The  electromotive  force  of  a  circuit, 
which  is  the  work  done  in  carrying  a  unit  quantity  of  electricity 
completely  around  the  circuit,  is  measured  in  terms  of  the  same 
units  as  potential  and  difference  of  potential. 


UNITS    OF    RESISTANCE,    INDUCTANCE    AND    CAPACITY 

Resistance.  —  In  accordance  with  Ohm's  Law  (current  in  a 
steady  state  equals  electromotive  force  divided  by  resistance) , 

The  Unit  of  Resistance  is  defined  as  that  resistance  through  which  a  unit 
electromotive  force  constantly  applied  will  produce  a  unit  current. 

Inductance.  —  In  accordance  with  the  definitions  of  mutual 
inductance  and  self-inductance  given  in  Chapter  III,  the  unit  in 
which  each  of  these  quantities  is  measured  is  defined  as  follows: 

The  Unit  of  Inductance  is  an  inductance  in  which  a  unit  electromotive  force 
is  generated  by  a  current  changing  at  the  rate  of  one  unit  current  per  second. 

Capacity.  —  Finally,  from  the  definition  of  capacity, 

The  Unit  of  Capacity  is  the  capacity  of  a  condenser  that  is  charged  to  a  unit 
difference  of  potential  by  a  unit  quantity  of  electricity. 

Either  one  of  these  definitions  is  true  in  any  set  of  units  provided 
each  of  the  three  magnitudes  in  a  given  definition  is  in  the  same 
set  of  units. 


336 


WIRELESS  TELEGRAPHY 


PRACTICAL  UNITS 


The  following  table  contains  the  practical  units,  together  with 
their  equivalents  in  terms  of  the  two  sejts  of  c.g.s.  units : 


C.G.S.  Units. 

Unit  of 

Practical  Unit. 

Electro- 
magnetic. 

Electrostatic. 

Quantity  
Current  
Potential  
Resistance  .  .  . 
Capacity 

1  Coulomb    = 
1  Ampere     = 
1  Volt 
1  Ohm 
1  Farad        — 

10-1= 

lO-1^ 
108    = 
109    = 
10-9- 

10-i  X  v=  3  X109 
10-i  X  v  =  3  X  109 
108  -r-  v     =  $  X  10-2 
109  -  v2    =  ^  X  ID"11 
10~9  X  v2—  9  X  1011 

Inductance  .  . 

1  Henry       = 

109    = 

109  -5-  v2   =  |  X  10-11 

APPENDIX  II 

CONCERNING  THE  CALCULATION  OF  RESISTANCE,  SELF- 
INDUCTANCE  AND  CAPACITY 

Formulas  for  High-frequency  Electric  Resistance.  —  The  resist- 
ance of  a  circuit  to  the  passage  of  a  high-frequency  electric  cur- 
rent through  it  is  greater  than  its  resistance  to  a  steady  current. 
This  is  due  to  the  fact  that  the  high-frequency  current,  instead  of 
distributing  itself  uniformly  throughout  the  conductor,  tends  to 
concentrate  in  the  outer  layers  of  the  conductor.  In  a  qualita- 
tive way,  the  following  considerations  will  explain  the  tendency 
of  rapidly  varying  current  to  flow  on  the  outside  surface  of  the  con- 
ductor. Let  us  take  the  case  of  a  straight  cylindrical  wire,  and  let 
us  suppose  that  there  is  at  first  a  steady  current  flowing  with  a 
uniform  distribution  throughout  the  conductor.  Let  us  now  sup- 
pose a  rapid  variation  to  be  made  in  the  current;  this  variation 
will  reproduce  a  variation  in  the  magnetic  field,  and  consequently 
will  call  into  play  an  electromotive  force  tending  to  prevent  the 
change  of  current.  This  induced  electromotive  force  will  not  be 
the  same  throughout  the  whole  cross  section  of  the  conductor, 
but  will  be  greatest  near  the  center  of  the  wire,  because  the  center 
of  the  wire  is  on  the  average  nearer  to  every  part  of  the  cross  sec- 
tion of  the  wire  than  is  any  other  point  within  the  wire.  We  have 
thus,  during  any  periodic  variation  of  a  current  in  a  cylindrical 
wire,  a  greater  back  electromotive  force  near  the  center  of  the 
wire,  and  consequently  less  current  will  flow  in  the  central  portion 
of  the  wire  than  near  the  surface.  Such  a  distribution  of  current, 
which  utilizes  only  partly  the  carrying  facility  of  the  wire,  ex- 
periences a  higher  resistance  than  does  a  current  uniformly  dis- 
tributed throughout  the  entire  cross  section  of  the  wire. 

Lord  Rayleigh  has  derived  the  following  formula  for  the  resist- 
ance of  a  straight  cylindrical  wire  carrying  an  alternating  current 
of  high  frequency: 

f-1+5-  2io  +'etc" 

337 


338 


WIRELESS  TELEGRAPHY 


in  which  R'  =  the  resistance  for  the  high-frequency  current, 
R  =  the  resistance  of  the  wire  for  a  steady  current, 

k  =  an  abbreviation  for -> 


d  =  the  diameter  of  the  wire  in  centimeters, 

n  =  the  number  of  complete  oscillations  per  second, 

p  =  the  specific  resistance  of  the  material  of  the  wire  in 

terms  of  absolute  c.g.s.  electromagnetic  units, 
TT  =  3.1416.  .  .  ,  and 
H  =  the  magnetic  permeability  of  the  material  of  the 

wire,  and  is  unity  for  nonmagnetic  wires. 

The  formula  (A)  is  convenient  for  computation  when  k  is  less 
than  1.  When  k  is  greater  than  5  or  6,  the  following  formula,  also 
derived  by  Lord  Rayleigh,  is  more  accurate  and  convenient  for  calcu- 
lation: 


The  succeeding  table  contains  some  computed  values  of  resistance 
R'  for  1,000,000  oscillations  per  second  in  terms  of  R,  for  different 
diameters  of  copper  and  German-silver  wire. 

TABLE  FOR  RATIO  OF   §-'  . 

H 

R'  =  Resistance  for.1,000,000  oscillations  per  second, 
R  =  Steady-current  resistance. 


Diameter  in  Cm. 

Copper 
p  =  1600. 

German    Silver 
p  =  20,900. 

.01 

1.008 

1.000 

.02 

1.117 

1.000 

.03 

1.32 

1.000 

.05 

1.95 

1.000 

.1 

3.88 

1.005 

.2 

7.85 

1.09 

.3 

11.8 

.4 

15.7 

4.30 

.5 

19.7 

5.38 

.6 

23.6 

6.5 

.7 

27.5 

7.5 

.8 

31. 

8.6 

.9 

35. 

9.7 

1.0 

39. 

10.7 

1.5 

59. 

16. 

2.0 

79. 

21.5 

By  reference  to  this  table  it  will  be  seen  that  the  resistance  of  a 
copper  wire  2  centimeters  in  diameter  is  79  times  as  great  with  the 


APPENDIX  II  339 

rapidly  oscillating  current  as  it  is  with  a  steady  current.  With 
decrease  in  the  diameter  of  the  wire  the  effect  of  the  high  frequency 
in  diminishing  the  resistance  decreases,  and  with  a  wire  of  copper 
1  millimeter  in  diameter  the  resistance  for  current  making  one 
million  oscillations  per  second  is  only  3.88  times  as  great  as  the 
resistance  for  steady  current.  For  diameters  below  one-tenth  of  a 
millimeter:,  the  high-frequency  resistance  of  copper  does  not  differ 
from  the  steady-current  resistance.  For  radii  greater  than  .5 
millimeter  the  resistance  of  a  circular  copper  wire  is  very  nearly 
inversely  proportional  to  the  radius  of  the  wire,  while  the  steady 
resistance  is  inversely  proportional  to  the  square  of  the  radius. 

If  we  pass  now  from  the  case  of  copper  to  that  of  German  silver, 
which  has  a  specific  resistance  about  14  times  as  great  as  copper, 
it  is  seen  that  the  departure  between  high-frequency  resistance  and 
steady-current  resistance  is  not  so  great  as  for  copper.  For  Ger- 
man-silver wires  less  than  1  millimeter  in  diameter  the  high-fre- 
quency resistance  differs  by  not  more  than  |  of  1%  from  the 
steady  resistance.  Above  one  millimeter  in  diameter  the  ratio  of 
Rf  to  R  for  German  silver  increases  progressively  with  increase 
of  diameter. 

The  formulas  here  given  apply  only  to  approximately  straight 
conductors,  and  should  not  be  used  to  apply  to  wires  wound  into 
coils. 

Formulas  for  Calculation  of  Capacity.  —  The  following  formulas 
serve  for  the  calculation  of  capacity  in  some  simple  cases.  The 
linear  dimensions  are  to  be  measured  in  centimeters  and  k  is  the 
dielectric  constant  of  the  dielectric  between  the  plates.  The  dielec- 
tric constant  of  air  or  other  gas  at  ordinary  atmospheric  pressure 
is  approximately  1.  Approximate  values  of  k  for  some  other 
dielectrics  are  given  in  the  table  on  page  341. 

I.  Capacity  of  a  condenser  of  two  parallel  flat  plates  oppositely 
charged. 

kA 

C  =  — —  c.g.s.  electrostatic  units, 
4-rra 

in  which  A  is  the  area  of  one  of  the  plates  overlapped  by  the  other 
plate,  d  is  the  distance  of  the  plates  apart  in  centimeters. 

This  formula  holds  accurately  only  when  the  distance  apart  of 
the  plates  is  small  in  comparison  with  the  length  and  breadth  of 
the  plates. 


340  WIRELESS  TELEGRAPHY 

II.   Capacity  of  two  concentric  cylinders  oppositely  charged. 

kl 
C  =  -  —  c.g.s.  electrostatic  units, 


in  which  I  =  the  overlapping  length  of  the  cylinders, 
R2  —  the  radius  of  the  outer  cylinder, 
RI  =  the  radius  of  the  inner  cylinder. 

III.  Capacity  of  a  length  I  of  two  practically  infinite  parallel 
wires  of  the  same  radius,  —  the  wires  being  oppositely  charged. 

kl 
C  =  --  -  c.g.s.  electrostatic  units, 

41og.J 

d  =  distance  apart  of  the  wires, 

R  =  radius  of  either  wire  in  centimeters. 

IV.  Capacity  of  two  concentric  spheres,  oppositely  charged. 

7?  7? 

C  =  —  -r-2  c.g.s.  electrostatic  units, 
d 

in  which  R2  =  the  radius  of  the  outer  sphere, 
RI  =  the  radius  of  the  inner  sphere, 
d  =  RZ  —  RI. 

V.  Capacity  of  a  single  sphere  alone  in  space. 

C  =  R  c.g.s.  electrostatic  units, 
in  which  R  =  radius  of  the  sphere. 

VI.  Capacity  of  a  circular  disc,  or  thin  plate. 

9  7? 
C  =  --  c.g.s.  electrostatic  units, 

7T 

in  which  R  =  the  radius  of  the  disc. 

VII.  Capacity  of  a  single  cylindrical  wire  alone  in  space. 

C  =  -      —  c.g.s.  electrostatic  units, 


in  which  I  =  the  length  of  the  wire  in  centimeters, 

R  =  its  radius. 

Rule  for  Several  Condensers  in  Parallel.  —  If  several  con- 
densers of  capacities  Ci,  C2,  C3,  .  .  .  are  connected  in  parallel, 
the  combined  capacity  C  is 

C  =  d  +  C2  +  C3  +  . 


APPENDIX  II 


341 


Rule  for  Several  Condensers  connected  in  Series. — 

-  =—+-+-  +  . 

C      Ci      Cz     C$ 
TABLE  OF  DIELECTRIC  CONSTANTS 


Substance. 

Dielectric  Constants. 

Empty  space  

1 

IAir  or  other  gas    1 
under  atmospherics 
pressure                    1 

1  approx 

Glass 

6  to  10 

Mica  
Hard  Rubber  
Kerosene  Oil  
Castor  Oil    

6.6 
2.7 
2.0 

4.78 

Water 

80 

Formulas  for  Calculating  Inductance.  —  The  lengths  are  to  be 
measured  in  centimeters,  and  the  results  are  in  c.g.s.  electro- 
magnetic units.  The  medium  surrounding  the  conductors  is 
supposed  to  be  nonmagnetic. 

I.  The  mutual  inductance  between  a  long  single-layer  solenoid 
and  a  lumped  secondary  wound  about  it. 

M  =.4:TrniNzA  c.g.s.  electromagnetic  units, 
in  which 

wi  =  the  number  of  turns  per  cm.  length  on  primary  coil, 
_/V2  =  the  total  number  of  turns  on  secondary  coil, 
A   =  the  area  of  cross  section  included  within  the  primary  coil. 

II.  Self-inductance  of  a  single-layer  solenoid. 

2  a4  +  a*V      8 


L  =  4w2n2 


c.g.s.,  electromagnetic  units, 


in  which 

a  =  the  mean  radius  of  the  solenoid, 
n  =  the  number  of  turns  per  cm.  of  length, 
I  =  the  length  in  centimeters. 
This  is  accurate  to  better  than  }  of  1%  when  I  is  not  less  than  4  a.1 

III.  Self-inductance  of  a  length  I  of  two  practically  infinite 
parallel  wires  in  which  the  current  is  flowing  in  opposite  direc- 
tions (i.e.,  a  return  circuit). 

L  =  4  I  •  loge  -    c.g.s.  electromagnetic  units, 

R 
1  Cohen,  Bulletin  of  the  Bureau  of  Standards,  Vol.  4,  p.  385,  1907-08. 


342  WIRELESS  TELEGRAPHY 

in  which  d  =  the  distance  between  centers  of  the  two  wires, 
R  =  the  radius  of  each  wire,  supposed  equal, 
I    =  the  length  of  the  pair  of  wires. 

This  formula  assumes  that  the  current  is  flowing  only  on  the  out- 
side surfaces  of  the  two  wires,  as  is  the  case  with  oscillations  of  high 
frequency.  (See  next  formula.) 

IV.  Self-inductance  of  a  return  circuit  like  that  of  III,  with, 
however,  a  uniform  distribution  of  current  throughout  the  wires 
instead  of  merely  on  the  surfaces. 

L  =  4 1  ]  loge  -  +  -  [  c.g.s.  electromagnetic  units, 
\.  ) 

in  which  I  =  the  length  of  the  pair  of  wires, 

d  =  the  distance  between  centers  of  the  two  wires, 
R  =  the  radius  of  the  wires,  supposed  equal, 
M  =  the  magnetic  permeability  of  the  material  of  the 
wires,  —  the  permeability  of  the  medium  between 
the  wires  being  assumed  unity  (i.e.,  nonmagnetic). 

V.  Self-inductance  of  a  length  Z  of  two  concentric  tubes. 

D 

L  =  2  \oge  17  c.g.s.  electromagnetic  units, 
RI 

in  which  R%  =  the  inner  radius  of  the  outer  tube, 
#1  =  the  outer  radius  of  the  inner  tube. 

In  this  case  the  distribution  of  current  is  supposed  to  be  only  on 
the  adjacent  surfaces  of  the  tubes. 

VI.  Self -inductance  of  a  single  wire  of  length  I. 

(         27          ) 
L  =  21}  loge  — -  —  1  >  c.g.s.  electromagnetic  units, 

(        R         } 

in  which  I  =  the  length  of  the  wire, 
R  =  its  radius. 

In  this  case  the  wire  is  supposed  to  be  of  small  diameter  in  com- 
parison with  its  length. 


INDEX 


Abraham,  M.,  theoretical  value  of 
wave  length,  116. 

Absorption  of  electric  waves.  By 
soil,  127,  131;  by  ionized  air,  137. 

Air.  Absorption  by,  137;  Conduc- 
tivity of,  137. 

Air  condenser,  318;  of  Korda,  114. 

Alternator,  high-frequency,  306. 

Amesbury,  Mass.,  experiments  at, 
134. 

Analogy.  Of  self-inductance  to  in- 
ertia, 22,  28;  of  capacity  to  me- 
chanical quantities,  26,  28. 

Anatase,  177. 

Antenna.  Dependence  on  height  of, 
271;  resonance  with  various  lengths 
of,  281;  theory  of  directive,  299; 
types  of,  312,  313. 

Antenna  circuit,  determination  of 
wave  length  of,  246. 

Apparatus,  construction  of,  312. 

Arc.  Mercury,  307;  singing,  253; 
talking,  254;  pulsating,  255;  in 
steam,  259;  period  of  singing,  260. 

Armagnat,  characteristic  of  electro- 
lytic detector,  203. 

Arons  tube,  72. 

Atlantic  cable,  63. 

Atomic  structure  of  electricity,  8. 

Attenuation  of  electric  waves,  By  ab- 
sorption, 127;  by  divergence,  129. 

Attraction,  electrostatic,  329. 

Audibility,  limit  of,  148. 

Audion,  214. 

Austin,  L.  W.  Sensitiveness  of  tele- 
phone receiver,  140;  detector,  160, 
198;  electrolytic  detector  a  recti- 
fier, 203. 

Balloons,  89. 

Barretter,  154;  liquid,  203. 


Bell,  Graham,  telegraphy  by  conduc- 
tion through  water,  77. 

Bellini,  directive  wireless  telegra- 
phy, 302. 

Bjerkness,  waves  on  wires,  70. 

Blondlot,  waves  on  wires,  68,  71,  72. 

Bolometer,  72,  153. 

Bornite,  134,  161. 

Bose,  short  waves,  60. 

Boys,  C.  V.,  radiomicrometer,  129. 

Brandes,  H.,  characteristics  of  de- 
tectors, 171. 

Branly,  E.,  coherer,  80,  143. 

Brant  Rock,  Mass.,  tower  at.  316. 

Braun,  Ferdinand.  Coupled  cir- 
cuits, 101;  artificial  ground,  121; 
cathode  tube,  151,  181;  directed 
wireless  telegraphy,  296,  297. 

Break  key,  90. 

British  stations,  326. 

Brookite,  177,  187. 

Calibration  of  wave  meter,  22,  117. 

Calzecchi-Onesti,  coherer,  80. 

Capacity.  Electrostatic,  22;  of  con- 
denser, 24;  of  earth,  24;  analogy, 
26;  measured  by  wave  meter,  224; 
amount  at  sending  station,  318; 
formulas  for,  339,  340. 

Cape  Race,  106,  107. 

Capillary  electrometer,  142. 

Carbon  microphone.  As  detector, 
158;  applied  to  wireless  teleph- 
ony, 309. 

Carbon-steel  detector,  158,  198. 

Carborundum.  Detector,  160;  ex- 
periments with,  162;  unilateral 
conductivity,  164;  current- voltage 
curves  of,  164,  165,  167,  169,  171; 
oscillograms  of,  187. 

Cathode  tube,  151,  181. 


343 


344 


INDEX 


Cavendish  Laboratory,  9. 

Cavendish,  laws  of  repulsion,  330. 

Cay,  C.  H.,  letter  to,  5. 

Characteristic.  Of  crystal  contact, 
164,  165,  167,  169,  171,  180,  181; 
rising,  171;  falling,  171;  of  arc, 
255,  258. 

Claims  of  Marconi's  1896  patent,  90. 

Clifden,  Ireland,  antenna  at,  315. 

Coal  gas,  arc  in,  258. 

Coefficient  of  coupling.  Denned,  236, 
250;  advantage  of  varying,  289; 
effect  on  resonance,  289. 

Coefficient  of  .induction.  Self,  19; 
Mutual,  18. 

Coherer,  80,  85,  143;  and  circuit,  81, 
86;  applied  to  study  of  electric 
waves,  81;  theory  of,  144. 

Cole,  A.  D.,  short  waves,  59. 

Compressed-air  condenser,  318. 

Conduction,  electric,  in  gases,  9. 

Conductivity.  Unilateral,  164,  170, 
172;  high-frequency,  337. 

Conductor,  earth  not  a  perfect,  125. 

Condenser.  Discharge  of,  1,  2,  3,  26, 
28,  31;  definition  of,  24;  energy  and 
e.m.f.  of,  25;  work  of  charging,  27; 
air,  114;  sending,  317;  receivmg, 
323. 

Condenser  circuits.  Resonance  be- 
tween, 42;  measurement  of  wave 
length  of,  246. 

Conrad,  F.,  wave  length  of  oscil- 
lator, 116. 

Construction  of  apparatus,  312. 

Continuity  of  arc-oscillations,  260. 

Corpuscular  theory  of  electricity, 
8,  10. 

Coulomb,  torsion  balance,  330. 

Coulomb,  24. 

Coupled  circuits,  93;  two  types,  95; 
introduction  of,  97;  reasons  for 
using,  228;  oscillations  in,  228; 
detuning  of,  251. 

Coupled  pendulums,  232. 

Coupling.  Close  and  loose,  241 ;  per- 
fect, 241;  coefficient  of,  see  coef- 
ficient. 

Criterion  of  oscillation,  30, 


Crystal  detectors.  See  Rectifiers, 
Crystal. 

Crystal  rectifiers,  156,  157,  175. 
See  Rectifiers,  Crystal. 

Cuba,  U.  S.  Stations  in,  326. 

Current  distribution  in  oscillator,  108, 
111,  115. 

Current- voltage  characteristic.  Of 
carborundum,  164,  171;  of  molyb- 
denite, 180;  of  electrolytic  detector, 
203;  of  arc,  255,  258. 

Curvature  of  earth,  effect  of,  128. 

Cut-over  switch,  90,  322. 

Cycle,  hysteresis,  149. 

Cyclic  change,  21. 

Cymometer,  221. 

Damped  discharge,  34. 

Damping,  30,  146,  291. 

Dawn,  effect  of,  on  transmission, 
134,  136. 

Daylight,  effect  of,  133. 

Decohering  devices,  85. 

De  Forest.  Audion,  214;  arc  in 
steam,  259. 

De  la  Rive.  Spark  in  oil,  57;  Hertz's 
experiments,  68. 

Detectors,  140;  why  needed,  142; 
classification  of,  143;  magnetic, 
145;  thermal,  153;  crystal,  156,  157, 
175;  rectifiers  as,  171;  why  recti- 
fiers act  as,  173;  effects  of  resistance 
of,  291;  electrolytic,  201;  vacuum, 
212;  and  potentiometers,  324. 

Detuning,  251. 

Dielectric,  definition  of,  24. 

Dielectric  constant,  25 ;  relation  to  in- 
dex of  refraction,  40;  table  of,  341. 

Diffusion,  of  electric  current,  63. 

Direct-coupled  circuits,  95,  96. 

Directive  antenna.  Marconi,  298; 
intensity  about,  298;  theory  con- 
cerning, 299. 

Directive  wireless  telegraphy,  296; 
limitations  to,  303. 

Discharge.     See  Condenser. 

Displacement  assumption,  37. 

Displacement  current,  37;  about  an 
oscillator,  39. 


INDEX 


345 


Dissipation,  of  charge  by  ionized  air, 
138. 

Distance.  Law  of,  130;  of  trans- 
mission over  different  soils,  131. 

Doenitz,  Johann,  wave  meter,  216. 

Dolbear,  Amos,  wireless  telegraphy 
of,  77. 

Double  oscillation,  spark  photograph 
of,  248. 

Drude,  Paul.  Calibration  of  wave 
meter,  117;  resonance  method  of 
measuring  wave  length,  216;  wave 
meter,  216. 

Duane,  velocity  of  waves  on  wires, 
68,  69. 

Duddell,  W.  Law  of  distance,  129; 
thermo-galvanometer,  129,  154; 
singing  arc,  254,  255. 

Dunwoody,  H.  H.  C.,  carborundum 
detector,  160. 

Duplicity  of  vibration  of  coupled  cir- 
cuits, 235. 

Dynamometer,   high-frequency,    113. 

Earth.  Propagation  of  electric  waves 
over,  122,  125.  See  Ground. 

Edward  VII,  message  from  Pres. 
Roosevelt  to  King,  107. 

Einthoven  galvanometer,  141,  263. 

Electric  force,  related  to  potential 
gradient,  334. 

Electric  waves.  Maxwell's  theory  of, 
5,  36,  38;  Hertz's  experiments  on, 
5,  43,  50,  51,  66;  properties  of,  40, 
48;  interference  of,  45,  46,  47,  48; 
refraction  of,  40,  55;  velocity  of, 
in  air,  40,  50,  69;  of  short  wave 
length,  51,  56,  59,  60;  polarization 
of,  54;  table  of,  60;  on  wires,  62, 
66,  74;  velocity  of,  on  wires,  66, 
68,  69,  70,  74;  from  grounded  os- 
cillator, 124. 

Electricity.  Theories  as  to  nature  of, 
6;  and  magnetism,  12;  elementary 
facts  about,  329. 

Electrolytic  detector,  201;  current- 
voltage  characteristic,  203;  oscillo- 
graphic  study,  205;  conclusions  re- 
garding, 211. 


Electromagnetic  theory  of  light,  5, 
36,41. 

Electrometer.  Capillary,  142;  abso- 
lute, 329. 

Electromotive  force.  Of  condenser, 
25;  definition  of,  334. 

Electron,  mass  and  charge  of,  9. 

Electroptatics,  23,  329. 

Energy.  Relation  of  magnetic  field 
to,  21;  and  e.m.f.  of  charged  con- 
denser, 25. 

Engineering  details,  312. 


Fahie.    History,  75,  82;  letter  to,  158. 

Falling  characteristic,  171. 

Farad,  24,  336. 

Faraday,  Michael.  Electrolysis,  8, 
9;  current  from  magnetic  field,  16; 
electrostatics,  23;  dielectric,  24; 
basis  of  Maxwell's  theory,  36. 

Feddersen,  rotating-mirror  photo- 
graphs, 3. 

Fessenden,  R.  A.  Barretter,  154; 
electrolytic  detector,  201,  203; 
high-frequency  alternator,  306; 
tower  at  Brant  Rock,  316. 

Field  of  electric  force  about  oscilla- 
tor, 49,  124. 

Field  of  magnetic  force,  13,  50. 

Fizeau,  velocity  of  electric  propaga- 
tion, 62. 

Fleming,  J.  A.  Dynamometer,  113; 
note  on  Zenneck's  theory,  125, 
133;  oscillation  valve,  212;  wave 
meter,  220;  method  of  measuring 
capacity,  224;  on  directive  antenna, 
299. 

Formulas.  For  current  during  dis- 
charge, 31;  period  of  discharge,  35; 
for  two  wave  lengths  in  coupled 
circuits,  236;  for  period  of  arc, 
260;  for  sending  capacity,  319;  for 
high-frequency  resistance,  337;  for 
calculating  capacity,  339;  for  cal- 
culating inductances,  341. 

Franklin,  Benjamin,  theory  of  elec- 
tricity, 7. 

Frequency   meter.     See  Wave  meter. 


346 


INDEX 


Galvanometer.  Principle  of,  13;  sen- 
sitiveness of,  141;  Einthoven's 
string,  141. 

Geissler  tube  detector,  70,  216. 

Glace  Bay,  Nova  Scotia,  107. 

Gounelle,  Velocity  of  electric  pro- 
pagation, 62. 

Graph  of  current,  32. 

Grating  used  in  showing  polariza- 
tion, 54. 

Ground.  Quarter-wave,  121;  prac- 
tical, 317. 

Grounded  circuits,  83,  108,  118. 

Guided  electric  waves,  124. 

Hack,  F.,  effect  of  sub-surface  water, 
133. 

Harmonic  oscillation,  279. 

Heat,  21,  60. 

Heaviside,  Oliver,  theory  of  waves  on 
wires,  63,  65. 

Height  of  antenna,  dependence  on, 
271,  275. 

Helix,  321,  344,  345. 

Helmholtz,  on  elementary  portions 
of  electricity,  9. 

Henry,  Joseph.  Magnetism  by  Ley- 
den-jar  discharge,  2;  obtained  cur- 
rent from  varying  magnetic  field,  16. 

Hertz,  Heinrich.  Existence  of  elec- 
tric waves  in  air,  5,  41,  42,  43; 
oscillator,  44;  resonator,  44;  re- 
flection of  electric  waves,  44; 
attempt  to  measure  velocity  of 
electric  waves,  50;  short  waves, 
51 ;  waves  on  wires,  66. 

Hertz  oscillator.  Displacement  cur- 
rent about  a,  39;  distribution  of 
current  and  potential  in  a,  108. 

Hewitt.  Mercury  interrupter,  270, 
278. 

Hot-wire  ammeter,  use  in  tuning, 
249. 

Hughes,  D.  E.  Microphonic  detec- 
tor, 158. 

Hydrocarbon  gas,  arc  in,  259. 

Hydrogen,  arc  in,  258. 

Hysteresis,  21;  dielectric,  25;  sup- 
pression of,  148. 


Identity  of  electric  waves  and  light, 
56. 

Images,  electrical,  122. 

Image  theory  of  ground,  118,  122. 

Index  of  refraction,  40. 

Indicating  instruments,  140. 

Inductance.  Mutual,  defined,  18; 
self,  19,  20;  tuning  by,  93,  272; 
formulas  for  calculating,  341. 

Induction.  Electromagnetic,  17;  mu- 
tual and  self,  17,  18;  resem- 
blance to  inertia,  20. 

Inductively  coupled  circuits,  95. 

Inertia,  20,  21. 

Infra-red  and  electric  waves,  60. 

Installation,  322. 

Interference,  45;  possibility  of  pre- 
venting, 271;  curves  showing  ex- 
tent of,  294. 

Interrupter.  Mercury,  270,  278;  vi- 
brator used  with  persistent  oscil- 
lations, 262. 

lonization  of  air,  137,  138. 

Irish  Channel,  experiments  on,  129. 

Iron  wires,  velocity  on,  69. 

Jackson,  H.  B.,  transmission  over 
various  surfaces,  133. 

Johnson,  K.  S.,  assistance  with  rec- 
tifier, 176. 

Kelvin,  Lord.  See  Thomson,  Sir 
Wm. 

Kennelly,  A.  E.  ,  explanation  of  day- 
light effect,  139. 

Key,  321. 

Kinraidy,  spark  gap,  270. 

Kirchhoff,  theory  of  waves  on  wires, 
63. 

Kites,  89,  91. 

Klemencic,  thermal  junction,  59,  154. 

Korda,  air  condenser,  114. 

Lampa,  short  waves,  60. 
Lebedew,  short  waves,  59. 
Lecher,  waves  on  wires,  70. 
Lepel,  Baron  von,  arc,  265. 
Leyden  jar  discharge.  See  Condenser. 
Leyden  jars,  copper-plated,  317. 


INDEX 


347 


Light.  Electromagnetic  theory  of,  3, 
41;  identity  of  electric  waves  and, 
56;  table,  60;  effect  on  transmis- 
sion, 133. 

Lindsay,  J.  B.,  signaling  through 
water,  76. 

Loadstone,  12. 

Lodge-Muirhead-Robinson,  coherer, 
144. 

Lodge,  Sir  Oliver.  Resonance  ex- 
periment, 42,  215;  use  of  coherer, 
81;  patent  of  resonant  circuits,  93; 
system  of  wireless  telegraphy,  97. 

Loops  of  potential  and  current,  45, 
46,  47,  48,  111. 

Lyman,  Theodore,  ultra-violet  light, 
60. 

Macdonald,  wave  length  of  oscil- 
lator, 116. 

Madelung,  E.,  on  magnetic  detec- 
tor, 151. 

Magnet,  12. 

Magnetic  detector,  145,  146,  147, 
151,  153. 

Magnetic  Field,  13,  14,  15,  16;  about 
a  Hertz  oscillator,  50. 

Magnetism,  relation  between  elec- 
tricity and,  12. 

Magnetization  by  condenser  dis- 
charge, 2. 

Mandelstam,  phase-difference  exci- 
tation, 298. 

Map  of  stations,  326. 

Marconi,  Guglielmo,  80;  first  patent, 
83;  1896  apparatus,  83;  grounded 
circuits,  85;  coherer,  85;  deco- 
hering devices,  85;  "  claims,"  90; 
achievements  between  1896  and 
1898,  91;  coupled  circuits,  103; 
duplex  apparatus,  105;  achieve- 
ments in  1901-1902,  106;  effect  of 
daylight,  133;  company,  139;  mag- 
netic detector,  146;  reflectors,  296: 
directive  antenna,  298. 

Maurain,  C.,  suppression  of  hys- 
teresis, 149. 

Maxwell,  James  Clerk,  electro-mag- 
netic theory,  5,  36,  41. 


Medium,  influence  of  intervening,  23. 

Mercury-arc  oscillator,  307. 

Method  of  wireless  telephony,  305. 

Microfarad,  24. 

Microphone.  As  detector,  158;  ap- 
plied to  wireless  telephony,  309. 

Mirrors,  cylindrical  metallic,  51. 

Molybdenite,  161,  177,  178;  oscillo- 
grams  of,  186;  thermo-electric  prop- 
erties, 189. 

Monarch,  repair  ship,  129,  131. 

Morse,  S.  F.  B.,  telegraphy  by  con- 
duction through  water,  75. 

Mounting  for  molybdenite  detector, 
179. 

Muirhead,  coherer,  144. 

Nasmyth,  G.  W.,  period  of  arc,  260. 
National  Electric  Signaling  Co.,  139. 
Navy,  U.  S.,  Stations  on  Atlantic 

Coast,  326. 

Nodes,  45,  46,  47,  48,  111. 
Northrup,  E.  F.,  dynamometer,  113. 

Oersted,  H.  C.,  relation  of  electricity 
to  magnetism,  13,  14. 

Oil,  castor,  condensers  submerged  in, 
317. 

Oil,  vaseline,  spark  in,  57. 

Optics  of  electric  oscillations,  Righi, 
56. 

Oscillation.  Spark-photograph  of,  3; 
period  of,  35;  number,  87;  nature 
of,  108;  of  coupled  systems,  228; 
photograph  of  double,  248;  har- 
monic, 279. 

Oscillator.  Hertz,  44;  field  about, 
49;  rectilinear,  51;  for  short  waves, 
59;  Marconi,  83;  wave-length  of, 
116;  mercury-arc,  307. 

Oscillatory  discharge.  See  Conden- 
ser. 

Oscillographic  study,  of  crystal  recti- 
fiers, 181;  of  electrolytic  de- 
tector, 205,  208;  of  pendulum 
motion,  233. 

Paalzow,    waves   on   wires,    72,    73; 

bolometer,  154. 
Panama,  U.  S.  Stations  in,  326. 


348 


INDEX 


Parallel,  capacities  in,  340. 

Pendulum,  coupled  or  sympathetic, 
232. 

Persistent  oscillations,  262;  train  pro- 
duced by,  306. 

Period.  Of  oscillation,  35;  relation 
of  wave  length  to,  50;  of  singing 
arc,  260. 

Peukert,  rotating  quenched  spark,268. 

Phase-difference  oscillator,  297. 

Pickard,  G.  W.  Daylight  effect,  134; 
crystal  detectors,  161;  copper- 
plated  jars,  317. 

Pierce,  G.  W.  Spark  potential  along 
glass,  56;  number  of  oscillations, 
87;  current  distribution,  111;  dyna- 
mometer, 113;  wave  length  of 
oscillator,  116;  test  of  image  theory, 
119;  crystal-contact  detectors,  162; 
anatase,  brookite,  molybdenite, 
177;  sound  measurements,  178; 
oscillographic  study,  181,  205; 
thermoelectric  properties,  189 ;  con- 
clusions regarding  crystal  recti- 
fiers, 199;  electrolytic  detector, 
205,  211;  wave  meter,  221;  photo- 
graph of  double  oscillation,  248; 
mercury  interrupter,  270,  '278; 
resonance,  271,  281. 

Platinized  contacts,  169. 

Poincare",  H.,  concerning  error,  50. 

Polarization.  Of  dielectric,  37;  of 
electric  wave,  53. 

Poldu,  England,  106;  directive  an- 
tenna at,  298. 

Popalexi,    phase-difference    excita- 
tion, 298. 

Popoff,  receiving  apparatus,  81. 

Porto  Rico,  U.  S.  Stations  in,  326. 

Potential,  332;  distribution  in  oscil- 
lator, 108;  gradient,  334. 

Potentiometer,  324. 

Poulsen,  V.  Singing  arc,  256;  in- 
terrupter at  receiving  station,  263. 

Power,  228,  318. 

Practical  units,  336. 

Preece,  Sir  Wm.  Telegraph  by  elec- 
tromagnetic induction,  78;  co-oper- 
ation with  Marconi,  91. 


Principles,  reliance  on,  325. 
Pupin,  M.  J.     Loaded-line  telephony, 
65;  electrolytic  rectifier,  202,  204. 

Quenched  spark,  253,  266,  267,  268, 
269. 

Radiant  heat  and  electric  waves,  60. 

Radium,  renders  gases  conductive,  9. 

Rathenau,  telegraphy  by  water  con- 
duction, 77. 

Ratio  of  units,  40,  332. 

Rays  and  shadows,  52. 

Receiving  circuits.  Resonance  of, 
271;  forms  of,  285. 

Receiving  condensers,  323. 

Receiving  transformer,  323. 

Recording  device,  263. 

Rectification.     Of  A.  C.  by  crystal 
contact,  170;  facts  adverse  to  ther- 
.  mo-electric  explanation  of,  196;  in- 
sufficient   heating    of    contact    to 
account  for,  198. 

Rectifiers.  Crystal,  145,  157,  162; 
with  or  without  battery,  172;  why, 
act  as  detectors,  173;  permanence 
of,  176;  molybdenite,  178;  oscillo- 
graphic study  of  crystal,  181 ;  sum- 
mary of  conclusions  regarding 
crystal,  199;  electrolytic  detector 
a,  211. 

Reflectors,  metallic,  51,  296. 

Refraction,  40,  55. 

Reichmann,  Fritz,  dynamometer,  113. 

Relay,  sensitiveness  of,  140. 

Repulsion,  electrostatic,  329. 

Resistance.  Contrast  of  inductance 
with,  21;  damping  by,  34;  pro- 
tective, 88;  temperature  coefficient, 
194;  effect  of,  on  sharpness  of 
resonance,  225;  of  detectors  and 
effect  on  resonance,  291;  high- 
frequency,  337. 

Resonance.  Between  condenser  cir- 
cuits, 42;  of  circuits,  92;  electrical, 
215;  effect  of  resistance  on,  225; 
of  sending  station,  243;  of  receiving 
circuits,  271;  curves,  281;  relation 
in  coupled  system,  286;  sharpness 
of,  with  coupled  circuits,  291. 


INDEX 


349 


Resonator.  Hertz's  circular,  44;  rec- 
tilinear, 51;  Righi's,  56;  with 
thermal  junction,  59. 

Righi,  Augusto,  apparatus,  56. 

Rising  characteristic,  171. 

Robinson,  coherer,  144. 

Robison,  S.  S.  Manual  of  Wireless 
Telegraphy,  219. 

Roentgen  rays  make  gases  conduc- 
tive, 9. 

Rogers,  telegraphy  through  water,  76. 

Roosevelt,  President,  message,  107. 

Rubens.  Waves  on  wires,  72,  73; 
telegraphy  by  water  conduction, 
77;  bolometer,  154. 

Rutherford,  E.,  magnetic  detector, 
145. 

Sarasin.  Repetition  of  Hertz's  ex- 
periments, 68;  spark  in  oil,  57. 

Saunders,  velocity  of  waves  on  wires, 
68. 

Schloemilch,  electrolytic  detector,  201, 
203. 

Schmidt-Wilkes  telephone  receiver, 
sensitiveness  of,  140. 

Schumann,  V.,  ultra-violet  light,  60. 

Seawater,  propagation  of  electric 
waves  over,  125,  131. 

Sending  station.  Tuning  of,  243; 
construction  of,  312. 

Shadows,  cast  by  metallic  screens,  52. 

Shoemaker,  electrolytic  detector,  203. 

Shunt  capacity,  tuning  by,  273. 

Shunted  telephone,  used  with  detec- 
tor, 135. 

Silicon,  161;  steel,  198. 

Silver,  removal  of,  324. 

Simon,  H.  Th.,  talking  arc,  254. 

Singing  arc,  253,  260,  264. 

Singing  spark,  253. 

Skin  effect,  70,  337. 

Soil,  propagation  over,  126,  131. 

Spark.  In  oil,  57,  268;  potential,  29, 
56;  photographs,  3,  248;  quenched, 
253,  266,  269;  singing,  253. 

Spectrum  of  electric  waves,  60. 

Station,  diagram  of  circuits  of  com- 
plete, 322. 


Stationary  waves,  48;  on  wires,  74. 
Steam,  arc  in,  259 
Steel-carbon  detector,  158,  198. 
St.  John,  waves  on  wires,  73,  74. 
St.  Johns,  Newfoundland,  106. 
Strecker,  telegraphy  by  water    con- 
duction, 77. 

Submarine  telephony,  limit  to,  65. 
Sunset,  effect  of,  134,  136. 
Sun's  rays,  ionization  by,  138. 
Surface  travel,  69. 
Sympathetic  pendulums,  232. 
Syntonic  circuits.     See  Resonance. 

Table.     Of  wave  lengths,  60;  of  di- 
electric constants,  341 ;  of  units,  336. 
Talking  arc,  254. 

Taylor,  J.  E.,  law  of  distance,  129. 
Telefunken  Co.     Arcs  in  series,  259; 

quenched  spark,  267. 
Telegraphy,  by  wires,  63. 
Telephone  receiver,  sensitiveness  of, 

140. 
Telephony.     Line,  65;    wireless,  265, 

305. 

Tellurium  detector,  160,  198. 
Tesla  coil,  93. 

Thermal  detectors,  59,  153,  154. 
Thermal  junction,  use  in  receiver,  59, 

154. 

Thermo  electric,  177,  189,  196. 
Thomson,   Elihu.     Transformer,   93; 
dynamometer,      113;      continuous 
spark,    253;    singing    spark,    265. 
Thomson,  J.  J.,  electricity  and  mat- 
ter, 7,  9,  10. 

Thomson,  Sir  Wm.  Proof  of  oscilla- 
tory discharge,  3;  criterion,  30; 
period  of  oscillation,  35;  waves  on 
wires,  63;  absolute  electrometer, 
329. 

Torsion  balance,  329. 
Tosi,    directive   wireless   telegraphy, 

302. 
Transformer.  High-frequency,  93,  95; 

charging,  320;  receiving,  323. 
Trowbridge,  John,  68,  69,  76. 
Tube.   Geissler,  70, 216;  cathode,  151. 
Tuning.    See  Resonance. 


350 


INDEX 


Uller,  Carl,  on  directive  antenna,  300. 
Ultra-violet  light  and  electric  waves, 

9,  60,  137,  139. 

Unilateral  conductivity,  164, 170, 172. 
Units,  24,  329,  336. 

Vacuum  detectors,  70,  212,  216. 

Vail,  telegraphy  by  conduction 
through  water,  76. 

Varley,  S.  A.,  coherer,  80. 

Velocity  of  electric  waves,  40;  attempt 
to  determine,  in  air,  50;  relation  of, 
to  wave  length  and  period,  50;  on 
wires,  62;  on  wires  same  as  in  sur- 
rounding medium,  68;  same  as  of 
light,  69. 

Visible  spectrum,  61. 

Volt,  unit  of  e.m.f .,  24. 

Vreeland,  F.  K.,  oscillator,  307. 

Wasmut,  A.,  on  Peukert's  spark,  268. 

Water.  Propagation  over,  125;  sub- 
surface, 133. 

Waves,  electric.     See  Electric  Waves. 

Wave  length.  Determined  by  sta- 
tionary system,  45;  relation  of,  to 
period,  50;  of  electric  waves  and 
light,  60;  of  Hertz  oscillator,  116: 
absorption  conditioned  oii,  13&;  in 
coupled  systems,  235;  three  pos- 
sible, 251. 


Wave  meter.  Calibration  of,  117, 
222;  description,  216;  observa- 
tions with,  219;  method  of  using, 
223;  used  in  measuring  capacity, 
224;  applied  to  tuning  sending 
station,  244. 

Wave-system,  stationary,  48. 

Webb,  wave  length  of  oscillator,  116. 

Webster,  A.  G.,  electricity  and  mag- 
netism, 63. 

Wellfleet,  Mass.,  107,  298. 

Wheatstone,  velocity  of  electricity, 
62. 

Wien,  Max.  Detuning,  251 ;  quenched 
spark,  266,  270. 

Wireless  telegraphy.  Before  Hertz, 
75;  by  Hertzian  waves,  80;  by 
resonant  circuits,  92. 

Woodman,  wave  length  of  oscilla- 
tor, 116. 

Work.  Done  in  charging  a  con- 
denser, 27;  definition,  332;  method 
of  computing  electrical,  334. 


Zenneck,  J.  Propagation  over  earth, 
125;  absorption  a  function  of  wave 
length,  133;  on  directive  antenna, 
300. 

Zincite,  134,  161. 


OF   THE 
OF 


f) 


14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 

LOAN  DEPT. 

This  book  is  due  on  the  last  date  stamped  below,  or 

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AUG    91963 


LD  21A-50m-12,'60 
(B6.221slO)476B 


YC   19344 


TK  574-1