﻿84 
  Prof. 
  L. 
  R. 
  lugersoll 
  on 
  Magnetic 
  

  

  less 
  than 
  '003° 
  have 
  frequently 
  been 
  measured, 
  and 
  it 
  is 
  

   believed 
  that, 
  with 
  care, 
  rotations 
  of 
  three 
  or 
  four, 
  perhaps 
  

   even 
  of 
  single, 
  seconds 
  could 
  be 
  detected. 
  

  

  Pkeliminary 
  Measurements. 
  

  

  Calibration 
  of 
  Magnetic 
  Field. 
  — 
  It 
  was 
  desired 
  to 
  test 
  the 
  

   transmission 
  and 
  reflexion 
  rotations 
  of 
  nlms 
  under 
  exactly 
  

   the 
  same 
  conditions 
  of 
  field 
  strength. 
  Since 
  they 
  could 
  not 
  

   be 
  located 
  in 
  the 
  same 
  part 
  of 
  the 
  field 
  for 
  the 
  two 
  tests, 
  this 
  

   necessitated 
  a 
  calibration 
  of 
  the 
  field. 
  To 
  do 
  this 
  a 
  small 
  

   flip-coil 
  was 
  used 
  having 
  the 
  same 
  area 
  as 
  that 
  of 
  the 
  cross- 
  

   section 
  of 
  the 
  beam 
  as 
  it 
  passed 
  through, 
  or 
  was 
  reflected 
  

   from, 
  a 
  film. 
  From 
  a 
  large 
  number 
  of 
  measurements 
  curves 
  

   were 
  plotted 
  showing 
  the 
  variation 
  of 
  field 
  strength 
  with 
  

   position 
  and 
  with 
  magnet 
  current 
  ; 
  and 
  the 
  current 
  was 
  

   accordingly 
  adjusted 
  in 
  practice 
  to 
  give 
  the 
  same 
  field 
  strength 
  

   whatever 
  the 
  position 
  of 
  the 
  film. 
  

  

  Sensibility/ 
  Reduction 
  Facto7's. 
  — 
  As 
  a 
  very 
  exact 
  knowledge 
  

   of 
  the 
  reduction 
  effect 
  of 
  the 
  series 
  resistances 
  on 
  the 
  galvano- 
  

   meter-bolometer 
  sensibility 
  is 
  required, 
  this 
  was 
  measured 
  in 
  

   three 
  ways: 
  — 
  by 
  a 
  step 
  by 
  step 
  process 
  ; 
  by 
  the 
  use 
  of 
  rotating 
  

   sectors 
  ; 
  and 
  by 
  computations 
  from 
  the 
  results 
  of 
  mechanical 
  

   rotations 
  of 
  the 
  polarizer. 
  Four 
  resistances 
  giving 
  reduction 
  

   factors 
  styled 
  A, 
  B, 
  C, 
  and 
  D 
  respectively, 
  were 
  used 
  ; 
  and 
  in 
  

   the 
  step 
  by 
  step 
  way 
  these 
  factors 
  were 
  measured 
  by 
  comparing 
  

   the 
  effect 
  on 
  a 
  certain 
  galvanometer 
  deflexion 
  obtained, 
  with 
  

   full 
  sensibility 
  of 
  the 
  introduction 
  of 
  the 
  resistance 
  A. 
  This 
  

   was 
  then 
  similarly 
  compared 
  with 
  B 
  and 
  the 
  process 
  repeated 
  

   for 
  C 
  and 
  D. 
  Very 
  careful 
  galvanometer 
  proportionality 
  cor- 
  

   rections 
  were 
  required 
  for 
  the 
  success 
  of 
  this 
  method, 
  and 
  at 
  

   best 
  there 
  are 
  certain 
  theoretical 
  objections 
  to 
  it. 
  

  

  In 
  the 
  second 
  way, 
  by 
  the 
  aid 
  of 
  a 
  rotating 
  sector 
  which 
  

   transmitted 
  only 
  ^^ 
  of 
  the 
  incident 
  light 
  the 
  reduction 
  factor 
  

   was 
  measured 
  by 
  the 
  comparison 
  of 
  two 
  nearly 
  equal 
  

   galvanometer 
  deflexions. 
  For 
  the 
  energy 
  transmitted 
  by 
  

   the 
  rotating 
  sector 
  and 
  measured 
  with 
  the 
  highest 
  sensibility 
  

   gave 
  almost 
  the 
  same 
  throw 
  as 
  the 
  total 
  energy 
  measured 
  

   with 
  the 
  resistance 
  C 
  in 
  series. 
  The 
  values 
  of 
  A 
  and 
  B 
  were 
  

   likewise 
  determined 
  with 
  the 
  aid 
  of 
  suitable 
  sectors. 
  

  

  By 
  rotating 
  the 
  polarizer 
  through 
  small 
  and 
  accurately 
  

   known 
  angles, 
  a 
  third 
  series 
  of 
  values 
  was 
  obtained, 
  for 
  these 
  

   rotations 
  were 
  measured 
  just 
  as 
  if 
  they 
  were 
  of 
  electromag- 
  

   netic 
  origin 
  ; 
  and, 
  by 
  computing 
  back, 
  the 
  necessary 
  values 
  

   to 
  be 
  assigned 
  to 
  the 
  reduction 
  factors 
  were 
  easily 
  found. 
  

   These 
  values 
  agreed 
  very 
  well 
  with 
  those 
  determined 
  by 
  the 
  

   last 
  method, 
  as 
  may 
  be 
  seen 
  from 
  the 
  following 
  table. 
  

  

  