﻿164 
  Mr. 
  D. 
  E. 
  Roberts 
  on 
  the 
  Effect 
  of 
  Temperature 
  and 
  

  

  The 
  resistance 
  o£ 
  the 
  graphite 
  specimens, 
  both 
  in 
  and 
  out 
  

   of 
  the 
  fields, 
  was 
  determined 
  by 
  a 
  potentiometer 
  method*, 
  

   being 
  compared 
  directly 
  with 
  known 
  resistances 
  (0*1-1*0 
  

   ohm). 
  The 
  current 
  and 
  potentiometer 
  leads 
  were 
  soldered 
  

   to 
  the 
  ends 
  o£ 
  the 
  specimen 
  investigated, 
  which 
  were 
  pre- 
  

   viously 
  coppered 
  electrolytically. 
  In 
  the 
  rotation 
  experi- 
  

   ments 
  the 
  four 
  leads, 
  which 
  consisted 
  of 
  four 
  insulated 
  

   straight 
  copper 
  wires, 
  about 
  30 
  cm. 
  long 
  and 
  bound 
  together 
  

   by 
  silk 
  so 
  as 
  to 
  form 
  a 
  fairly 
  rigid 
  rod, 
  were 
  fixed 
  inside 
  a 
  

   metal 
  tube. 
  This 
  tube 
  could 
  be 
  turned 
  within 
  a 
  second 
  

   shorter 
  coaxial 
  tube 
  rigidly 
  attached 
  to 
  the 
  frame 
  of 
  the 
  

   electromagnet 
  used, 
  and 
  carried 
  a 
  coaxial 
  graduated 
  disk. 
  

   The 
  angle 
  of 
  rotation 
  was 
  read 
  off 
  directly 
  by 
  means 
  of 
  a 
  

   fixed 
  pointer. 
  The 
  copper 
  leads 
  lay 
  along 
  the 
  axis 
  of 
  rota- 
  

   tion 
  of 
  the 
  apparatus, 
  and 
  for 
  the 
  determination 
  of 
  the 
  varia- 
  

   tion 
  of 
  the 
  transverse 
  effect 
  with 
  orientation 
  of 
  the 
  graphite 
  

   in 
  the 
  field, 
  the 
  graphite 
  was 
  soldered 
  to 
  the 
  leads 
  with 
  its 
  

   length 
  along 
  that 
  of 
  the 
  leads 
  and, 
  for 
  the 
  determination 
  of 
  

   the 
  longitudinal 
  effect, 
  at 
  right 
  angles 
  to 
  this 
  position. 
  The 
  

   current 
  through 
  the 
  graphite 
  during 
  a 
  series 
  of 
  measurements 
  

   was 
  varied 
  between 
  2 
  and 
  0*5 
  milliamperes 
  according 
  to 
  its 
  

   resistance. 
  In 
  order 
  to 
  eliminate 
  thermoelectric 
  and 
  other 
  

   junction 
  effects 
  f 
  the 
  current 
  in 
  the 
  main 
  circuit, 
  as 
  well 
  as 
  

   the 
  potentiometer 
  connexions, 
  were 
  successively 
  reversed. 
  

   The 
  changes 
  of 
  resistance 
  involved 
  being 
  considerable, 
  it 
  was 
  

   found 
  necessary 
  to 
  adjust 
  the 
  sensitiveness 
  of 
  the 
  potentio- 
  

   meter 
  during 
  a 
  single 
  series 
  of 
  readings 
  ; 
  this 
  was 
  initially 
  

   sufficient 
  to 
  detect 
  differences 
  of 
  0*0001 
  ohm. 
  Small 
  irregular 
  

   variations 
  in 
  the 
  resistance 
  of 
  a 
  particular 
  specimen 
  were 
  

   observed 
  after 
  it 
  was 
  subjected 
  to 
  the 
  action 
  of 
  magnetic 
  

   fields 
  or 
  to 
  widely 
  different 
  temperatures. 
  This 
  change, 
  

   however, 
  amounted 
  in 
  general 
  to 
  less 
  than 
  1 
  per 
  cent.; 
  no 
  

   evidence 
  was 
  ever 
  found 
  of 
  any 
  regular 
  hysteresis 
  with 
  

   respect 
  to 
  the 
  magnetic 
  field 
  or 
  with 
  temperature. 
  Through 
  

   the 
  kindness 
  of 
  Prof. 
  Hagen 
  and 
  Dr. 
  Hoffmann, 
  the 
  resistance 
  

   of 
  specimen 
  Gr 
  1. 
  15 
  — 
  that 
  used 
  in 
  the 
  experiments 
  at 
  different 
  

   temperatures 
  — 
  was 
  redetermined 
  at 
  18° 
  C. 
  in 
  the 
  Phys. 
  Tech. 
  

   Reichsanstalt 
  by 
  means 
  of 
  Diesselhorst's 
  "'compensation 
  

   apparatus 
  5 
  ^ 
  and 
  a 
  differential 
  galvanometer: 
  good 
  agree- 
  

   ment 
  was 
  found. 
  Some 
  of 
  the 
  preliminary 
  measurements 
  

   had 
  been 
  made 
  with 
  Wheatstone's 
  bridge 
  method, 
  and 
  when 
  

   repeated 
  potentiometrically, 
  practically 
  the 
  same 
  results 
  were 
  

   obtained. 
  

  

  * 
  F. 
  Kohlrausch, 
  Prakt. 
  Physik, 
  ii. 
  Auflage, 
  p. 
  422 
  (1910). 
  

   t 
  These 
  were 
  discussed 
  in 
  detail 
  by 
  F. 
  0. 
  Blake, 
  loc. 
  cit. 
  p. 
  465. 
  

   t 
  H. 
  Diesselhorst, 
  Zeitschr 
  . 
  fur 
  Instr.kunde 
  ,xxvi. 
  pp. 
  173, 
  297 
  (1906) 
  ; 
  

   xxviii. 
  pp. 
  1,38(1908). 
  

  

  