﻿50 
  

  

  Mr. 
  It. 
  F. 
  Earhart 
  on 
  the 
  Discharge 
  

  

  a 
  Michelson 
  interferometer. 
  This 
  carriage 
  supports 
  the 
  

   mirror 
  M. 
  

  

  Fisr. 
  1. 
  

  

  The 
  interferometer 
  was 
  enclosed 
  in 
  a 
  heavy 
  iron 
  box, 
  and 
  

   was 
  securely 
  fastened 
  to 
  it 
  so 
  as 
  mechanically 
  to 
  form 
  part 
  

   of 
  the 
  same. 
  The 
  rod 
  in 
  which 
  the 
  needle 
  was 
  secured 
  by 
  

   means 
  of 
  a 
  set 
  screw 
  was 
  ^ 
  of 
  an 
  inch 
  in 
  diameter. 
  This 
  

   rod 
  passed 
  through 
  a 
  cylinder 
  of 
  hard 
  rubber 
  (R). 
  The 
  

   metal 
  rod 
  passed 
  through 
  the 
  insulating 
  cylinder 
  § 
  of 
  an 
  inch 
  

   from 
  the 
  centre. 
  The 
  manner 
  of 
  attaching 
  the 
  rubber 
  

   insulator 
  and 
  rod 
  to 
  the 
  box 
  is 
  indicated 
  in 
  the 
  figure. 
  It 
  

   was 
  necessary 
  to 
  withdraw 
  this 
  system 
  after 
  a 
  spark- 
  

   discharge 
  in 
  order 
  to 
  introduce 
  a 
  new 
  needle. 
  By 
  orienting 
  

   the 
  insulating 
  cylinder 
  a 
  new 
  needle 
  could 
  thus 
  be 
  brought 
  

   to 
  bear 
  on 
  a 
  portion 
  of 
  the 
  plane 
  which 
  had 
  been 
  undisturbed 
  

   by 
  the 
  previous 
  discharge. 
  The 
  motion 
  of 
  the 
  interferometer 
  

   carriage 
  and 
  plane 
  was 
  controlled 
  by 
  the 
  usual 
  screw 
  motion. 
  

   This 
  was 
  operated 
  through 
  a 
  stuffing-box 
  not 
  shown 
  in 
  the 
  

   figure. 
  Suitable 
  openings 
  for 
  admitting 
  dry 
  and 
  dust-free 
  

   air 
  were 
  provided. 
  The 
  electrical 
  arrangement 
  is 
  shown 
  in 
  

   fig. 
  2. 
  Here 
  P 
  and 
  P' 
  represent 
  the 
  electrodes. 
  A 
  double- 
  

   pole 
  double-throw 
  switch 
  made 
  it 
  possible 
  to 
  connect 
  these 
  

   in 
  a 
  circuit 
  containing 
  a 
  galvanometer 
  (G) 
  and 
  a 
  small 
  

   e.m.f. 
  The 
  usual 
  procedure 
  was 
  to 
  move 
  the 
  plane 
  against 
  

   the 
  point 
  and 
  determine 
  the 
  position 
  of 
  contact 
  by 
  testing 
  

   with 
  this 
  circuit 
  as 
  the 
  plane 
  was 
  pulled 
  off. 
  No 
  difficulty 
  

   was 
  experienced 
  in 
  securing 
  a 
  definite 
  and 
  positive 
  zero 
  

   setting. 
  The 
  plane 
  was 
  then 
  moved 
  forward 
  the 
  desired 
  

   distance, 
  this 
  distance 
  being 
  measured 
  in 
  terms 
  of 
  wave- 
  

   lengths 
  of 
  sodium 
  light. 
  The 
  double-throw 
  switch 
  was 
  then 
  

   thrown 
  over, 
  and 
  the 
  surfaces 
  made 
  part 
  of 
  the 
  circuit 
  con- 
  

   taining 
  the 
  storage-cells 
  and 
  liquid 
  resistance 
  R. 
  A 
  Weston 
  

   voltmeter, 
  together 
  with 
  a 
  suitable 
  multiplier 
  placed 
  in 
  

  

  