﻿Relations 
  between 
  JEther, 
  Mattel', 
  and 
  Electricity. 
  371 
  

  

  required 
  to 
  give 
  one 
  millimetre 
  deflexion 
  when 
  the 
  resistance 
  

   of 
  the 
  galvanometer 
  (for 
  coils 
  in 
  series) 
  was 
  50 
  ohms 
  and 
  

   the 
  scale 
  was 
  distant 
  one 
  metre. 
  The 
  test-current 
  was 
  

   derived 
  from 
  a 
  dry 
  cell 
  of 
  1'4 
  volts 
  E.M.F. 
  cut 
  down 
  by 
  

   shunts 
  of 
  10/1400 
  and 
  100/10,000 
  and 
  then 
  passed 
  through 
  

   10,000 
  ohms 
  in 
  series 
  with 
  the 
  galvanometer. 
  The 
  coil 
  on 
  

   the 
  experiment 
  wheel 
  was 
  always 
  in 
  series 
  when 
  the 
  sensi- 
  

   bility 
  was 
  tested, 
  the 
  resistance 
  being 
  negligible 
  in 
  comparison 
  

   with 
  the 
  10,000 
  ohms. 
  The 
  testing 
  system 
  was 
  kept 
  con- 
  

   nected 
  so 
  that 
  it 
  could 
  be 
  used 
  at 
  frequent 
  intervals 
  during 
  

   the 
  progress 
  of 
  the 
  readings 
  and 
  while 
  the 
  wheel 
  was 
  running. 
  

   The 
  sensibility 
  during 
  the 
  last 
  and 
  best 
  of 
  the 
  readings 
  was 
  

   kept 
  at 
  10 
  _lu 
  . 
  At 
  this 
  sensibility 
  the 
  galvanometer 
  was 
  

   " 
  dead-beat 
  " 
  and 
  the 
  time 
  required 
  for 
  a 
  single 
  throw, 
  or 
  

   one-fourth 
  of 
  a 
  complete 
  period, 
  was 
  about 
  fifteen 
  seconds. 
  

   The 
  sensibility 
  could 
  be 
  increased 
  beyond 
  this 
  by 
  further 
  

   cutting 
  down 
  the 
  field. 
  Occasionally 
  10~ 
  n 
  and 
  on 
  one 
  trial 
  

   4' 
  5 
  x 
  10 
  -12 
  was 
  reached, 
  but 
  the 
  time 
  required 
  for 
  a 
  single 
  

   throw 
  was 
  increased 
  to 
  two 
  or 
  even 
  three 
  minutes 
  ; 
  and 
  the 
  

   time 
  demanded 
  for 
  a 
  complete 
  reading 
  seldom 
  elapsed 
  without 
  

   a 
  variation 
  in 
  the 
  thermal 
  current 
  or 
  some 
  magnetic 
  dis- 
  

   turbance 
  from 
  outside, 
  

  

  At 
  these 
  high 
  sensibilities 
  it 
  is 
  interesting 
  to 
  note 
  that 
  the 
  

   throw 
  due 
  to 
  a 
  small 
  instantaneous 
  induction-current 
  depended 
  

   directly 
  upon 
  the 
  current, 
  but 
  was 
  practically 
  independent 
  of 
  

   the 
  sensibility 
  of 
  the 
  galvanometer, 
  showing 
  that 
  the 
  inertia 
  

   of 
  the 
  system 
  was 
  negligible 
  compared 
  with 
  the 
  damping. 
  

  

  The 
  first 
  attempt 
  to 
  discoA'er 
  an 
  electromotive 
  force 
  due 
  to 
  

   the 
  longitudinal 
  motion 
  of 
  a 
  wire 
  was 
  made 
  with 
  a 
  coil 
  of 
  

   rectangular 
  cross 
  section, 
  of 
  No. 
  30 
  copper 
  wire, 
  wound 
  in 
  a 
  

   slot 
  cut 
  in 
  the 
  side 
  of 
  the 
  rim 
  of 
  a 
  wheel. 
  The 
  rim 
  was 
  slit 
  

   radially 
  to 
  avoid 
  currents 
  in 
  the 
  wheel 
  itself. 
  The 
  mean 
  

   radius 
  of 
  the 
  coil 
  was 
  6*25 
  cms. 
  and 
  the 
  speed 
  was 
  70 
  turns 
  

   per 
  second. 
  The 
  length 
  of 
  wire 
  was 
  about 
  42,000 
  cms., 
  with 
  

   a 
  resistance 
  of 
  146 
  ohms. 
  This 
  wheel 
  was 
  rotated 
  in 
  both 
  

   directions 
  at 
  a 
  time 
  when 
  a 
  deflexion 
  of 
  one 
  millimetre 
  might 
  

   have 
  been 
  detected 
  ; 
  but 
  no 
  reversible 
  deflexion 
  of 
  this 
  

   amount 
  was 
  obtained, 
  though 
  some 
  irregular 
  deflexions 
  

   occurred, 
  which 
  were 
  due, 
  doubtless, 
  to 
  slight 
  variations 
  in 
  

   the 
  magnetic 
  field 
  through 
  the 
  coil. 
  The 
  continuity 
  of 
  the 
  

   circuit 
  was 
  frequently 
  tested 
  by 
  moving 
  a 
  magnet 
  near 
  the 
  

   coil 
  and 
  noting 
  the 
  throw 
  due 
  to 
  the 
  induced 
  current. 
  The 
  

   sensibilitv 
  of 
  the 
  galvanometer 
  was 
  1*(5 
  x 
  10~ 
  9 
  and 
  the 
  resist- 
  

   ance 
  of 
  the 
  circuit 
  200 
  ohms, 
  so 
  that 
  an 
  E.M.F. 
  of 
  3'2 
  x 
  10~ 
  7 
  

   volts 
  in 
  the 
  circuit, 
  corresponding 
  to 
  K 
  = 
  3x 
  10 
  -15 
  volts 
  (K 
  

   being 
  the 
  hypothetical 
  E.M.F. 
  generated 
  in 
  one 
  centimetre 
  

  

  