﻿Positive 
  Electricity. 
  845 
  

  

  small 
  this 
  equation 
  is 
  approximately 
  

   md^y 
  lB.e 
  

  

  «o 
  that 
  if 
  / 
  is 
  the 
  distance 
  of 
  the 
  screen 
  from 
  0, 
  H 
  the 
  value 
  

   of 
  the 
  magnetic 
  force 
  at 
  a 
  distance 
  x 
  from 
  0, 
  

  

  = 
  — 
  fTl'^HJ^'l 
  dx 
  

  

  = 
  ~\\l-x)B.dx, 
  

   mv 
  L 
  ^ 
  

  

  Hence 
  the 
  quantity 
  we 
  have 
  to 
  determine 
  is 
  

  

  1-et 
  us 
  call 
  it 
  M. 
  

  

  D, 
  p 
  

  

  j: 
  

  

  (/-.i')HJ^; 
  

  

  Now 
  suppose 
  we 
  have 
  a 
  coil 
  of 
  triangular 
  section 
  

   DEr(DF 
  = 
  EF) 
  placed 
  with 
  its 
  vertex 
  at 
  the 
  screen 
  and 
  

   its 
  base 
  ED 
  at 
  the 
  hole 
  through 
  which 
  the 
  rays 
  emerge, 
  the 
  

   plane 
  of 
  the 
  coil 
  being 
  at 
  right 
  angles 
  to 
  the 
  magnetic 
  field. 
  

   The 
  magnetic 
  induction 
  through 
  the 
  coil 
  

  

  C^ 
  DE 
  r^ 
  DE 
  C^ 
  

  

  = 
  I 
  2H 
  PX, 
  dx 
  = 
  ^ 
  J^ 
  H 
  FN 
  dx 
  = 
  gl 
  J 
  (l-x) 
  H 
  dx 
  

  

  -OF^^- 
  

  

  Hence 
  if 
  we 
  measure 
  the 
  magnetic 
  induction 
  through 
  the 
  

   coil, 
  which 
  can 
  be 
  done 
  in 
  a 
  few 
  minutes 
  by 
  a 
  Grassot 
  

   fluxmeter, 
  we 
  can 
  at 
  once 
  determine 
  M. 
  

  

  This 
  method 
  takes 
  into 
  account 
  the 
  stray 
  field 
  and 
  is 
  

   applicable 
  whether 
  the 
  field 
  is 
  uniform 
  or 
  variable. 
  

  

  I 
  have 
  much 
  pleasure 
  in 
  thanking 
  Mr. 
  Kaye 
  and 
  Mr. 
  Everett 
  

   for 
  the 
  help 
  they 
  have 
  given 
  me 
  in 
  this 
  investigation. 
  

  

  