﻿Improi^ed 
  Foi'm 
  of 
  Magnetometer. 
  153 
  

  

  so 
  as 
  to 
  form 
  a 
  water-jacket 
  BB 
  measuring 
  4 
  cm. 
  in 
  internal 
  

   and 
  6 
  cm 
  in 
  external 
  diameter. 
  On 
  the 
  outside 
  of 
  this 
  is 
  

   wound 
  86S 
  turns 
  of 
  No. 
  15 
  s.W.G. 
  copper 
  wire 
  in 
  four 
  layers 
  

   (only 
  one 
  layer 
  is 
  shown 
  in 
  the 
  figure). 
  The 
  wire 
  is 
  double 
  

   silk-covered, 
  and 
  each 
  layer 
  is 
  varnished 
  over 
  after 
  winding. 
  

   The 
  terminals 
  of 
  the 
  coil 
  are 
  mounted 
  on 
  an 
  ebonite 
  block 
  at 
  

   one 
  end 
  of 
  the 
  solenoid. 
  D 
  and 
  C 
  are 
  the 
  inlet 
  and 
  outlet 
  

   tubes 
  of 
  the 
  water-jacket. 
  Although 
  the 
  water-jacket 
  is 
  

   somewhat 
  narrow 
  it 
  is 
  found 
  to 
  be 
  effective 
  in 
  keeping 
  the 
  

   helix 
  of 
  wire 
  cool, 
  even 
  though 
  the 
  interior 
  is 
  raised 
  to 
  a 
  

   temperature 
  of 
  over 
  1000° 
  C. 
  by 
  means 
  of 
  an 
  electric 
  furnace. 
  

   The 
  water-jacket 
  is 
  made 
  small 
  in 
  capacity 
  in 
  order 
  to 
  keep 
  

   down 
  the 
  mean 
  radius 
  of 
  the 
  solenoid, 
  and 
  hence 
  maintain 
  

   the 
  end 
  effect 
  of 
  the 
  solenoid 
  small. 
  The 
  field 
  at 
  the 
  centre 
  

   of 
  a 
  coil 
  of 
  length 
  21 
  and 
  radius 
  a 
  is 
  less 
  than 
  that 
  given 
  by 
  

   0'4:7rnC 
  in 
  the 
  ratio 
  (P 
  — 
  2a^)IP, 
  where 
  ?i 
  is 
  the 
  number 
  of 
  

   turns 
  in 
  the 
  coil 
  per 
  unit 
  length 
  and 
  C 
  is 
  the 
  magnetizing 
  

   current 
  in 
  amperes. 
  In 
  the 
  case 
  of 
  the 
  solenoid 
  now 
  de- 
  

   scribed 
  the 
  reduction 
  in 
  the 
  field 
  from 
  the 
  value 
  0'47rnC 
  

   due 
  to 
  the 
  finite 
  length 
  of 
  the 
  coil 
  is 
  only 
  1*14 
  per 
  cent. 
  

   The 
  solenoid 
  is 
  carried 
  on 
  a 
  mahogany 
  base-board 
  provided 
  

   with 
  two 
  vertical 
  supports 
  terminating 
  in 
  V-shaped 
  grooves 
  

   to 
  receive 
  the 
  coil. 
  The 
  position 
  of 
  the 
  solenoid 
  carrier 
  in 
  

   the 
  channel 
  of 
  the 
  magnetometer 
  board 
  may 
  be 
  fixed 
  by 
  

   means 
  of 
  a 
  brass 
  clamp 
  (shown 
  in 
  fig. 
  3). 
  This 
  friction 
  

   clamp 
  is 
  furnished 
  with 
  two 
  screws 
  which 
  press 
  mahogany 
  

   blocks 
  against 
  the 
  outer 
  surface 
  of 
  the 
  wooden 
  strips 
  forming 
  

   the 
  channel 
  of 
  the 
  magnetonieter 
  bed-plate. 
  

  

  Ci 
  and 
  (^ 
  (figs. 
  2 
  and 
  3) 
  are 
  circular 
  coils 
  of 
  15 
  cm. 
  radius 
  

   erected 
  on 
  wooden 
  stands 
  provided 
  with 
  brass 
  clamps 
  as 
  in 
  

   the 
  case 
  of 
  the 
  solenoid. 
  Each 
  coil 
  is 
  wound 
  in 
  three 
  

   sections, 
  the 
  terminals 
  of 
  which 
  are 
  screwed 
  into 
  the 
  base 
  of 
  

   the 
  stand. 
  The 
  sections 
  in 
  the 
  case 
  of 
  Ci 
  contain 
  5, 
  7, 
  and 
  

  

  9 
  turns 
  of 
  wire 
  respectively^ 
  and 
  in 
  the 
  case 
  of 
  C2 
  6, 
  8, 
  and 
  

  

  10 
  turns. 
  These 
  sections 
  may 
  be 
  used 
  singly 
  or 
  in 
  com- 
  

   bination, 
  and 
  accordingly 
  there 
  is 
  a 
  wide 
  range 
  of 
  variability 
  

   in 
  the 
  [)Owers 
  of 
  the 
  coils. 
  C3 
  is 
  a 
  coil 
  of 
  similar 
  construction, 
  

   but 
  has 
  a 
  radius 
  of 
  only 
  6 
  cm., 
  and 
  is 
  built 
  in 
  tw^o 
  sections 
  of 
  

   1 
  and 
  3 
  turns 
  of 
  wire 
  respectively. 
  

  

  D 
  is 
  a 
  coil 
  having 
  a 
  radius 
  of 
  1*2 
  cm., 
  and 
  its 
  function 
  is 
  

   to 
  prevent 
  loss 
  of 
  time 
  due 
  to 
  the 
  needle 
  vibrating 
  about 
  its 
  

   position 
  of 
  equilibrium. 
  It 
  is 
  connected 
  up 
  in 
  series 
  with 
  a 
  

   single 
  cell 
  and 
  a 
  reversing 
  key 
  ; 
  and 
  by 
  properly 
  tapping 
  the 
  

   key 
  a 
  series 
  of 
  impulses 
  is 
  communicated 
  to 
  the 
  needle, 
  which 
  

   is 
  thus 
  quickly 
  brought 
  to 
  rest. 
  

  

  L 
  is 
  a 
  further 
  sliding 
  stand 
  carrying 
  the 
  object 
  screen. 
  

  

  