﻿Magnetization 
  on 
  the 
  Resistance 
  of 
  Graphite. 
  165 
  

  

  To 
  obtain 
  the 
  magnetic 
  fields 
  the 
  latest 
  large 
  type 
  1911 
  

   model 
  of 
  the 
  du 
  Bois 
  half-ring 
  electromagnet 
  was 
  used. 
  To 
  

   obtain 
  the 
  highest 
  fields 
  at 
  ordinary 
  temperatures 
  special 
  

   prism-shaped 
  pole 
  end-pieces 
  (XV.) 
  were 
  generally 
  used 
  — 
  

   12 
  mm. 
  long 
  and 
  1*6 
  mm. 
  wide 
  : 
  with 
  these 
  parallel 
  end- 
  

   pieces 
  0*7 
  mm. 
  apart 
  and 
  a 
  pair 
  of: 
  extra 
  polar 
  coils 
  a 
  field 
  

   of 
  50 
  kilogauss 
  could 
  be 
  easily 
  attained 
  ; 
  this 
  proved 
  prac- 
  

   tically 
  uniform 
  over 
  an 
  equatorial 
  length 
  of 
  4 
  mm. 
  and 
  

   uniform 
  within 
  1 
  per 
  cent, 
  over 
  8 
  mm. 
  For 
  observations 
  at 
  

   low 
  and 
  high 
  temperatures 
  the 
  same 
  thermomagnetic 
  ar- 
  

   rangement 
  (XI.) 
  was 
  used 
  as 
  that 
  adopted 
  bv 
  du 
  Bois 
  and 
  

   Wills*. 
  The 
  polar 
  distance 
  was 
  1*5 
  mm., 
  the 
  diameter 
  of 
  

   the 
  frontal 
  planes 
  7 
  mm., 
  and 
  the 
  semi-angle 
  56°; 
  as 
  the 
  

   frontal 
  planes 
  were 
  not 
  sufficiently 
  parallel 
  the 
  field 
  proved 
  

   uniform 
  within 
  about 
  1 
  per 
  cent. 
  only. 
  For 
  the 
  determina- 
  

   tion 
  of 
  the 
  transverse 
  effect 
  at 
  ordinary 
  temperatures 
  at 
  the 
  

   lower 
  fields, 
  pole-shoes 
  (VII.) 
  (10-5 
  mm. 
  apart) 
  and 
  then 
  

   pole-end 
  pieces 
  VIII., 
  with 
  their 
  frontal 
  planes 
  adjusted 
  to 
  be 
  

   exactly 
  parallel 
  (2 
  mm. 
  apart) 
  were 
  nsed, 
  so 
  that 
  over 
  the 
  

   space 
  occupied 
  by 
  the 
  graphite 
  a 
  practically 
  uniform 
  field 
  

   was 
  obtained. 
  

  

  The 
  uniformity 
  of 
  the 
  field 
  was 
  tested 
  by 
  means 
  of 
  dif- 
  

   ferential 
  measurements 
  with 
  an 
  exploring 
  coil 
  of 
  only 
  1*3 
  mm. 
  

   in 
  diameter. 
  The 
  magnetic 
  fields 
  were 
  measured 
  by 
  means 
  

   of 
  an 
  exploring 
  coil 
  and 
  a 
  ballistic 
  galvanometer 
  in 
  the 
  usual 
  

   wayf- 
  It 
  was 
  assumed 
  that 
  the 
  fields 
  were 
  appreciably 
  the 
  

   same 
  at 
  all 
  the 
  temperatures 
  used 
  for 
  a 
  given 
  current 
  through 
  

   the 
  coils 
  of 
  the 
  electromagnet 
  J. 
  

  

  II. 
  Experiments 
  at 
  ordinary 
  temperature 
  (18° 
  C). 
  

   About 
  twenty 
  specimens 
  of 
  Ceylon 
  graphite 
  I. 
  were 
  in- 
  

   vestigated, 
  which 
  all 
  gave 
  variations 
  of 
  resistance 
  of 
  a 
  very 
  

   high 
  order. 
  The 
  increase 
  in 
  a 
  transverse 
  field 
  of 
  20 
  kilo- 
  

   gauss, 
  e. 
  g., 
  varied 
  . 
  between 
  300 
  and 
  500 
  per 
  cent, 
  of 
  the 
  

   resistance 
  in 
  zero 
  field. 
  Considering 
  the 
  difficulty 
  of 
  obtain- 
  

   ing 
  specimens 
  of 
  graphite 
  of 
  definite 
  crystalline 
  structure, 
  

   and 
  having 
  regard 
  to 
  the 
  impurities 
  occurring 
  in 
  the 
  natural 
  

   substance, 
  the 
  variations 
  in 
  the 
  magnitude 
  of 
  the 
  effect 
  are 
  

   not 
  surprising. 
  About 
  five 
  specimens, 
  which 
  gave 
  the 
  greatest 
  

   variation 
  of 
  resistance, 
  were 
  investigated 
  more 
  particularly; 
  

  

  * 
  H. 
  du 
  Bois, 
  Zeitschr.fur 
  Instr.kunde, 
  xxxi. 
  p. 
  369, 
  fig. 
  9 
  (1911). 
  

  

  t 
  H. 
  du 
  Bois, 
  ' 
  The 
  Magnetic 
  Circuit 
  in 
  Theory 
  and 
  Practice,' 
  p. 
  300. 
  

   London, 
  1896. 
  

  

  X 
  P. 
  Weiss 
  and 
  H. 
  Kamerlingh 
  Onnes, 
  Versl. 
  Kon. 
  Akad.Amd. 
  xviii. 
  

   p. 
  794 
  (1910;, 
  find 
  21 
  percent, 
  for 
  the 
  increase 
  of 
  the 
  saturation 
  value 
  

   on 
  cooling 
  from 
  +20° 
  C. 
  to 
  — 
  253° 
  C. 
  

  

  