﻿148 
  Dr. 
  J. 
  G, 
  Gray 
  and 
  Mr. 
  A. 
  D. 
  Ross 
  on 
  an 
  

  

  family 
  products 
  contained 
  in 
  the 
  igneous 
  rocks 
  tested 
  by 
  

   Strutt. 
  

  

  The 
  research 
  described 
  above 
  is 
  being 
  now 
  extended 
  to 
  a 
  

   large 
  number 
  of 
  specimens 
  of 
  surface 
  materials 
  of 
  the 
  earth's 
  

   crust, 
  but 
  the 
  results 
  so 
  far 
  obtained, 
  as 
  well 
  as 
  those 
  now 
  

   published 
  by 
  Professor 
  J. 
  Joly, 
  allow 
  us 
  to 
  conclude 
  that 
  

   thorium 
  has 
  certainly 
  a 
  considerable 
  importance 
  as 
  regards 
  

   terrestrial 
  radioactivity. 
  

  

  Rome, 
  Istituto 
  Fisico 
  della 
  R. 
  Universita, 
  

   May 
  1909. 
  

  

  XVI. 
  On 
  an 
  Improved 
  Form 
  of 
  Magnetometer 
  and 
  Accessories 
  

   for 
  the 
  Testing 
  of 
  Magnetic 
  Materials 
  at 
  Different 
  Tempera- 
  

   tures. 
  By 
  James 
  G. 
  Gray, 
  D.Sc, 
  F.R.S.E., 
  Lecturer 
  on 
  

   Physics 
  in 
  the 
  University 
  of 
  Glasgoio, 
  and 
  Alexander 
  D. 
  

   Ross, 
  M.A., 
  B.Sc, 
  F.R.S.F., 
  Assistant 
  to 
  the 
  Professor 
  of 
  

   Natural 
  Philosophy 
  in 
  the 
  University 
  of 
  Glasgow*, 
  

  

  [Plate 
  II.] 
  

  

  IN 
  the 
  usual 
  form 
  of 
  magnetometer 
  the 
  magnetizing 
  

   • 
  solenoid 
  is 
  placed 
  with 
  its 
  axis 
  in 
  the 
  magnetic 
  east 
  and 
  

   west 
  line 
  passing 
  through 
  the 
  magnetometer-needle. 
  The 
  

   effect 
  of 
  the 
  current 
  is 
  balanced 
  at 
  the 
  needle 
  by 
  means 
  

   of 
  a 
  compensating 
  coil 
  connected 
  up 
  in 
  the 
  circuit. 
  This 
  

   latter 
  coil 
  has 
  its 
  axis 
  coincident, 
  or 
  nearly 
  so, 
  with 
  that 
  

   of 
  the 
  solenoid. 
  When 
  a 
  feebly 
  magnetic 
  specimen 
  is 
  

   under 
  examination 
  the 
  solenoid, 
  and 
  consequently 
  the 
  com- 
  

   pensating 
  coil, 
  must 
  of 
  necessity 
  be 
  brought 
  up 
  close 
  to 
  the 
  

   needle. 
  If 
  large 
  magnetizing 
  currents 
  are 
  employed, 
  any 
  

   small 
  shift 
  of 
  the 
  coils 
  from 
  their 
  correct 
  positions 
  may 
  be 
  

   sufficient 
  to 
  seriously 
  impair 
  the 
  balance. 
  In 
  consequence 
  

   of 
  this 
  the 
  operation 
  of 
  adjusting 
  the 
  position 
  of 
  the 
  com- 
  

   pensating 
  coil 
  (the 
  solenoid 
  is 
  usually 
  clamped 
  once 
  for 
  all 
  

   in 
  a 
  convenient 
  position) 
  is 
  a 
  difficult 
  one, 
  especially 
  as 
  the 
  

   slight 
  inevitable 
  movement 
  o£ 
  the 
  coil 
  which 
  results 
  from 
  

   clamping 
  it 
  in 
  position 
  generally 
  results 
  in 
  the 
  balance 
  being 
  

   interfered 
  with. 
  

  

  Even 
  if 
  this 
  adjustment 
  be 
  accomplished 
  with 
  the 
  requisite 
  

   accuracy 
  for 
  the 
  undisturbed 
  position 
  of 
  the 
  magnetometer- 
  

   needle, 
  it 
  does 
  not 
  necessarily 
  follow 
  that 
  the 
  compensation 
  

   is 
  complete 
  for 
  the 
  needle 
  in 
  its 
  deflected 
  position. 
  In 
  

  

  * 
  Communicated 
  by 
  Professor 
  A. 
  Gray, 
  F.R.S. 
  : 
  read 
  before 
  the 
  Royal 
  

   Society 
  of 
  Edinburgh 
  on 
  February 
  1, 
  1909, 
  Reprinted 
  from 
  the 
  Pro- 
  

   ceediiigs 
  of 
  the 
  Royal 
  Society 
  of 
  Edinburgh. 
  

  

  