﻿80 
  Prof. 
  L. 
  E/. 
  Ingersoll 
  on 
  Magnetic 
  

  

  coils 
  were 
  most 
  effective, 
  and 
  when 
  excited 
  with 
  16 
  amps, 
  at 
  

   60 
  volts, 
  gave 
  a 
  field 
  of 
  about 
  6000 
  units, 
  which, 
  while 
  not 
  

   as 
  high 
  as 
  might 
  be 
  desired 
  — 
  on 
  account 
  of 
  the 
  large 
  air-gap 
  

   necessary 
  — 
  was 
  very 
  satisfactory 
  for 
  present 
  purposes. 
  Now 
  

  

  Fig. 
  1. 
  — 
  General 
  arrangement 
  of 
  apparatus. 
  

  

  it 
  was 
  very 
  desirable 
  to 
  have 
  as 
  small 
  an 
  external 
  field 
  as 
  

   possible, 
  particularly 
  at 
  the 
  polarizer 
  which 
  had 
  to 
  be 
  very 
  

   near 
  the 
  magnet, 
  and 
  this 
  was 
  secured 
  by 
  a 
  combination 
  of 
  

   series 
  and 
  parallel 
  connexions 
  of 
  the 
  remaining 
  four 
  coils, 
  

   with 
  suitable 
  resistances, 
  until 
  a 
  compass-needle 
  remained 
  

   unaffected 
  at 
  the 
  position 
  of 
  the 
  polarizer. 
  This 
  could 
  be 
  

   done 
  without 
  material 
  sacrifice 
  of 
  the 
  field 
  due 
  to 
  the 
  pole- 
  

   coils. 
  

  

  Optical 
  System. 
  — 
  Light 
  from 
  the 
  D.C. 
  Nernst-glower 
  Gi 
  

   in 
  its 
  firebrick 
  enclosure 
  was 
  converged 
  by 
  the 
  mirror 
  Mj 
  

   to 
  an 
  image 
  between 
  the 
  pole-pieces, 
  and 
  to 
  another 
  image 
  on 
  

   the 
  slit 
  of 
  the 
  spectrometer 
  by 
  the 
  mirror 
  M2. 
  A 
  double 
  

   image 
  prism 
  P 
  of 
  4*5 
  cms. 
  aperture 
  served 
  as 
  polarizer, 
  only 
  

  

  