﻿Magnetic 
  Shells 
  equivalent 
  to 
  Circular 
  Coils. 
  323 
  

  

  careful 
  experimenter. 
  The 
  method 
  is 
  accurate 
  and 
  sensitive, 
  

   its 
  accuracy 
  depending 
  (1) 
  on 
  the 
  accuracy 
  of 
  the 
  comparison 
  

   and 
  adjustment 
  of 
  resistances 
  ; 
  (2) 
  on 
  the 
  slide-apparatus 
  

   being 
  well 
  enough 
  made 
  to 
  give 
  very 
  nearly 
  true 
  axial 
  

   motion 
  to 
  the 
  coils 
  with 
  a 
  minimum 
  of 
  friction 
  ; 
  (3) 
  on 
  the 
  

   delicacy 
  of 
  motion 
  given 
  by 
  the 
  slow-motion 
  screws 
  to 
  the 
  

   sli 
  ding-bars. 
  

  

  The 
  accurate 
  measurement 
  of 
  the 
  displacement 
  of 
  a 
  coil 
  

   offers 
  no 
  difficulty. 
  

  

  In 
  regard 
  to 
  (1), 
  the 
  arrangement 
  used 
  by 
  Lord 
  Rayleigh 
  

   and 
  Mrs. 
  Sidgwick 
  to 
  compare 
  the 
  mean 
  radii 
  of 
  the 
  coils 
  of 
  

   their 
  current-balance 
  leaves 
  nothing 
  to 
  be 
  desired. 
  The 
  

   apparatus 
  I 
  have 
  described 
  is 
  the 
  one 
  I 
  have 
  used, 
  and 
  is 
  not 
  

   necessarily 
  of 
  the 
  most 
  suitable 
  design. 
  The 
  slow-motion 
  

   attachments 
  to 
  the 
  sliding-bars 
  are 
  not 
  altogether 
  a 
  success, 
  

   and 
  will 
  have 
  to 
  be 
  modified. 
  The 
  motion 
  realized 
  was 
  not 
  

   delicate 
  enough 
  and 
  was 
  rather 
  jerky, 
  so 
  that 
  I 
  could 
  rarely 
  

   adjust 
  the 
  displaced 
  coil 
  in 
  an 
  exact 
  equilibrium 
  position. 
  The 
  

   necessity 
  for 
  a 
  sensitive 
  slow 
  motion 
  will 
  appear 
  from 
  the 
  

   following. 
  

  

  Two 
  ordinary 
  galvanometer-coils 
  were 
  being 
  measured, 
  

   one, 
  the 
  larger, 
  of 
  36 
  turns 
  and 
  resistance 
  nearly 
  1 
  ohm, 
  the 
  

   other 
  of 
  30 
  turns 
  and 
  resistance 
  a 
  little 
  over 
  half 
  an 
  ohm. 
  

   The 
  battery-power 
  used 
  was 
  three 
  secondary 
  cells 
  with 
  a 
  

   resistance 
  of 
  20 
  ohms 
  included 
  in 
  the 
  circuit. 
  When 
  the 
  

   small 
  coil 
  was 
  very 
  near 
  the 
  equilibrium 
  position 
  (h* 
  from 
  the 
  

   large 
  coil), 
  a 
  motion 
  of 
  *016 
  cm. 
  of 
  the 
  small 
  coil 
  caused 
  a 
  

   motion 
  of 
  the 
  needle 
  represented 
  by 
  14 
  millimetres 
  on 
  a 
  scale 
  

   40 
  cm. 
  from 
  the 
  mirror. 
  The 
  little 
  magnet 
  was 
  swinging 
  in 
  

   the 
  earth's 
  field 
  slightly 
  neutralized 
  so 
  that 
  it 
  made 
  one 
  swing 
  

   in 
  2 
  seconds. 
  Thus 
  1 
  mm. 
  motion 
  of 
  the 
  scale 
  in 
  the 
  telescope 
  

   corresponded 
  very 
  nearly 
  to 
  a 
  motion 
  of 
  one-hundredth 
  of 
  a 
  

   millimetre 
  of 
  the 
  coil, 
  and 
  at 
  a 
  distance 
  of 
  40 
  cm. 
  from 
  the 
  

   mirror 
  a 
  half-millimetre 
  scale 
  could 
  be 
  used 
  with 
  comfort, 
  and 
  

   each 
  half 
  millimetre 
  divided 
  into 
  tenths. 
  

  

  In 
  order 
  to 
  indicate 
  the 
  degree 
  of 
  accuracy 
  obtainable, 
  as 
  

   well 
  as 
  the 
  necessity 
  of 
  having 
  the 
  motion 
  of 
  the 
  slide-bars 
  

   as 
  Motionless 
  as 
  possible, 
  I 
  will 
  quote 
  the 
  following 
  results 
  

   for 
  the 
  equivalent 
  radius 
  of 
  a 
  small 
  galvanometer-coil 
  obtained 
  

   with 
  a 
  rough 
  preliminary 
  apparatus. 
  

  

  In 
  this 
  apparatus 
  the 
  slow-motion 
  screw 
  could 
  only 
  apply 
  

   a 
  thrust 
  to 
  the 
  slide-bar 
  at 
  the 
  end 
  near 
  the 
  observer, 
  the 
  

   bar 
  being 
  drawn 
  back 
  by 
  hand, 
  when 
  it 
  was 
  necessary 
  

   to 
  do 
  so. 
  

  

  The 
  two 
  equilibrium 
  positions 
  of 
  the 
  small 
  coil 
  are 
  indicated 
  

  

  Y2 
  

  

  