﻿154 
  Dr. 
  J, 
  G. 
  Gray 
  and 
  Mr. 
  A. 
  D. 
  Ross 
  on 
  an 
  

  

  This 
  consists 
  of 
  a 
  vertical 
  wire 
  placed 
  in 
  front 
  of 
  a 
  window 
  

   of 
  obscured 
  glass 
  fitted 
  in 
  a 
  metal 
  box 
  containing 
  an 
  electric 
  

   lamp. 
  By 
  altering 
  the 
  position 
  of 
  this 
  stand, 
  the 
  image 
  

   of 
  the 
  cross-wire 
  formed 
  by 
  the 
  mirror 
  of 
  the 
  magnetometer 
  

   can 
  be 
  produced 
  at 
  any 
  distance 
  from 
  110 
  cm. 
  upwards. 
  

   From 
  150 
  cm. 
  to 
  200 
  cm. 
  is 
  in 
  most 
  cases 
  a 
  suitable 
  value. 
  

   At 
  this 
  distance 
  it 
  is 
  received 
  on 
  an 
  engine-divided 
  glass 
  

   scale 
  of 
  the 
  usual 
  type. 
  

  

  E 
  is 
  a 
  deflector 
  stand 
  on 
  which 
  a 
  small 
  permanent 
  magnet 
  

   may 
  be 
  mounted 
  in 
  the 
  " 
  B 
  " 
  position 
  of 
  Gauss. 
  The 
  

   construction 
  of 
  the 
  stand 
  is 
  similar 
  to 
  that 
  of 
  the 
  stand 
  

   which 
  carries 
  the 
  magnetometer 
  proper. 
  On 
  the 
  top 
  of 
  it 
  is 
  

   fixed 
  a 
  rectangular 
  block 
  of 
  wood 
  provided 
  with 
  a 
  groove 
  for 
  

   receiving 
  the 
  magnet. 
  

  

  The 
  bed-plate 
  of 
  the 
  magnetometer 
  is 
  mounted 
  on 
  six 
  pairs 
  

   of 
  mahogany 
  feet, 
  which 
  are 
  fastened 
  to 
  a 
  rigid 
  table 
  by 
  

   means 
  of 
  brass 
  screws. 
  

  

  The 
  process 
  of 
  setting 
  up 
  the 
  apparatus 
  is 
  as 
  follows. 
  

   The 
  centre 
  of 
  the 
  magnetometer-needle 
  has 
  first 
  to 
  be 
  placed 
  

   on 
  the 
  axis 
  of 
  the 
  solenoid. 
  To 
  accomplish 
  this, 
  coil 
  Ci 
  

   (fig. 
  3) 
  is 
  removed, 
  and 
  the 
  solenoid 
  H 
  is 
  moved 
  along 
  the 
  

   bed-plate 
  towards 
  A 
  until 
  its 
  inner 
  end 
  is 
  almost 
  in 
  contact 
  

   with 
  the 
  back 
  of 
  the 
  magnetometer 
  casing. 
  The 
  stand 
  A 
  is 
  

   then 
  moved 
  in 
  its 
  channel 
  until 
  the 
  needle 
  is 
  brought 
  exactly 
  

   on 
  to 
  the 
  axis 
  of 
  the 
  helix, 
  and 
  is 
  then 
  permanently 
  fastened 
  

   in 
  this 
  position 
  by 
  means 
  of 
  the 
  clamping 
  screw 
  already 
  

   mentioned. 
  The 
  table 
  carrying 
  the 
  magnetometer 
  is 
  now 
  

   placed 
  so 
  that 
  the 
  long 
  channel 
  of 
  the 
  bed-plate 
  lies 
  due 
  east 
  

   and 
  west, 
  the 
  adjustments 
  being 
  carried 
  out 
  and 
  tested 
  by 
  

   means 
  of 
  the 
  well-known 
  method 
  described 
  in 
  Gray's 
  

   ' 
  Absolute 
  Measurements 
  in 
  Electricity 
  and 
  Magnetism.' 
  

   The 
  method 
  is 
  as 
  follows: 
  — 
  A 
  wire 
  is 
  stretched 
  out 
  vertically 
  

   beneath 
  the 
  needle, 
  and 
  accurately 
  parallel 
  to 
  the 
  short 
  

   channel 
  of 
  the 
  bench. 
  On 
  passing 
  a 
  current 
  through 
  this 
  

   wire 
  a 
  deflexion 
  of 
  the 
  needle 
  is 
  produced. 
  If 
  the 
  current 
  is 
  

   reversed 
  in 
  direction 
  the 
  deflexion 
  will 
  have 
  the 
  same 
  

   numerical 
  value 
  as 
  before, 
  provided 
  that 
  the 
  wire 
  lies 
  exactly 
  

   magnetic 
  north 
  and 
  south. 
  The 
  table 
  is 
  so 
  placed 
  that 
  this 
  

   condition 
  is 
  fulfilled, 
  and 
  its 
  feet 
  are 
  then 
  clamped 
  to 
  the 
  

   floor 
  by 
  means 
  of 
  L-shaped 
  brass 
  brackets. 
  The 
  scale 
  is 
  

   erected 
  on 
  a 
  separate 
  table 
  in 
  order 
  that 
  the 
  movements 
  of 
  

   the 
  observer 
  may 
  not 
  set 
  up 
  oscillations 
  of 
  the 
  needle. 
  The 
  

   coils 
  Ci, 
  H_, 
  and 
  Cg 
  are 
  now 
  connected 
  up 
  in 
  series 
  with 
  the 
  

   storage-battery, 
  ammeter, 
  and 
  variable 
  resistances, 
  &c., 
  care 
  

   being 
  taken 
  that 
  the 
  direction 
  of 
  the 
  current 
  in 
  Ci 
  and 
  G2 
  is 
  

   opposite 
  to 
  that 
  in 
  H. 
  The 
  perman^ent 
  adjustments 
  of 
  the 
  

   instrument 
  are 
  now 
  complete. 
  

  

  