﻿174 
  Mr. 
  D. 
  E. 
  Roberts 
  on 
  the 
  Effect 
  of 
  Temperature 
  and 
  

  

  No. 
  301, 
  by 
  Hirtmann 
  and 
  Braun, 
  and 
  extends 
  to 
  —259° 
  C; 
  

   this 
  shows 
  in 
  the 
  neighbourhood 
  of 
  —200° 
  a 
  weak 
  extended 
  

   inflexion 
  ; 
  this 
  curve 
  is 
  given 
  in 
  fig. 
  3, 
  and 
  is 
  denoted 
  byBi. 
  

  

  Tznf 
  

  

  All 
  the 
  samples 
  tested 
  of 
  the 
  variety 
  G 
  I. 
  behaved 
  similarly 
  

   although 
  exhibiting 
  appreciable 
  quantitative 
  differences, 
  as 
  

   is 
  shown 
  by 
  the 
  values 
  of 
  R 
  ]8 
  /R 
  in 
  Table 
  I. 
  Moreover, 
  

   sometimes 
  changes 
  in 
  R 
  18 
  or 
  H 
  were 
  observed 
  after 
  the 
  

   graphite 
  had 
  been 
  subjected 
  to 
  extreme 
  temperatures. 
  

  

  The 
  temperature 
  during 
  a 
  series 
  of 
  readings, 
  the 
  graphite 
  

   being 
  between 
  the 
  poles 
  of 
  the 
  electromagnet, 
  was 
  deter- 
  

   mined 
  as 
  follows. 
  Before 
  commencing, 
  the 
  current 
  required 
  

   to 
  be 
  sent 
  through 
  the 
  electromagnet 
  in 
  the 
  reverse 
  direction 
  

   in 
  order 
  to 
  reduce 
  the 
  residual 
  field 
  to 
  zero 
  was 
  determined. 
  

   Then, 
  to 
  measure 
  the 
  temp 
  -rature, 
  the 
  graphite 
  being 
  in 
  

   position, 
  this 
  reverse 
  current 
  was 
  set 
  up 
  and 
  the 
  resistance 
  

   of 
  the 
  graphite 
  found. 
  The 
  temperature 
  of 
  the 
  graphite 
  

  

  