﻿254 
  Pierce 
  — 
  Application 
  of 
  the 
  Radio- 
  Micrometer 
  

  

  and 
  the 
  distance 
  from 
  the 
  bottom 
  of 
  the 
  magnet 
  to 
  the 
  thermal- 
  

   junction 
  is 
  6'7 
  cm 
  . 
  The 
  pole 
  pieces 
  are 
  4 
  mm 
  apart, 
  and 
  are 
  

   covered 
  with 
  copper 
  foil 
  to 
  prevent 
  iron 
  from 
  getting 
  on 
  the 
  

   coil. 
  

  

  A 
  better 
  idea 
  of 
  the 
  suspension 
  can 
  be 
  had 
  from 
  fig. 
  3, 
  

   which 
  is 
  a 
  section 
  through 
  the 
  axis 
  of 
  the 
  field 
  magnet. 
  The 
  

   coil 
  consists 
  of 
  a 
  single 
  loop, 
  2 
  rara 
  wide, 
  of 
  number 
  34, 
  B 
  and 
  

   S, 
  silk-covered 
  copper 
  wire. 
  After 
  forming 
  the 
  loop, 
  the 
  ends 
  

   4 
  of 
  the 
  wire 
  are 
  twisted 
  together 
  for 
  a 
  

  

  distance 
  of 
  about 
  5'5 
  cm 
  downward 
  and 
  

   are 
  then 
  separated 
  to 
  form 
  the 
  lead-ter- 
  

   minals 
  a, 
  a. 
  The 
  resonating 
  system 
  con- 
  

   sists 
  of 
  two 
  copper 
  cylinders, 
  o, 
  o, 
  8 
  mm 
  

   long, 
  and 
  l 
  mm 
  in 
  diameter. 
  These 
  cylin- 
  

   ders 
  are 
  about 
  l 
  mm 
  apart, 
  and 
  to 
  their 
  

   nearer 
  ends 
  are 
  soldered 
  respectively 
  a 
  

   constantin 
  and 
  a 
  manganine 
  wire 
  reduced 
  

   by 
  aqua 
  regia 
  to 
  have 
  a 
  diameter 
  of 
  01 
  

   or 
  -02 
  mm 
  .* 
  These 
  small 
  wires 
  are 
  crossed 
  

   at 
  &, 
  conveyed 
  off 
  at 
  right 
  angles 
  and 
  

   soldered 
  to 
  the 
  leads, 
  a, 
  a, 
  thus 
  forming 
  

   a 
  closed 
  circuit 
  with 
  the 
  coil. 
  The 
  res- 
  

   onators, 
  o, 
  o, 
  are 
  sewed 
  by 
  silk 
  threads 
  to 
  a 
  mica 
  vane, 
  v, 
  and 
  

   the 
  lead 
  wires 
  are 
  fastened 
  to 
  the 
  vane 
  by 
  sealing 
  wax 
  at 
  w. 
  

   The 
  springiness 
  of 
  the 
  leads 
  keeps 
  the 
  finer 
  wires 
  in 
  contact 
  at 
  k. 
  

   The 
  clear 
  space 
  between 
  the 
  pole 
  pieces 
  of 
  the 
  field 
  magnet 
  

   is 
  about 
  3 
  mm 
  , 
  the 
  sides 
  of 
  the 
  brass 
  tube 
  being 
  cut 
  away 
  to 
  

   admit 
  them. 
  The 
  magnet, 
  M, 
  can 
  be 
  pulled 
  back, 
  withdraw- 
  

   ing 
  the 
  pole-pieces, 
  to 
  allow 
  the 
  suspension 
  to 
  be 
  lowered 
  into 
  

   the 
  tube. 
  When 
  pushed 
  forward 
  again, 
  it 
  closes 
  up 
  snugly 
  

   around 
  the 
  coil 
  and 
  prevents 
  air 
  currents 
  from 
  coming 
  into 
  the 
  

   cavity 
  in 
  which 
  is 
  the 
  suspension. 
  

  

  When 
  electric 
  waves 
  of 
  proper 
  length 
  enter 
  the 
  cavity, 
  R, 
  

   they 
  are 
  converged 
  by 
  the 
  reflector 
  and 
  cause 
  electric 
  surgings 
  

   up 
  and 
  down 
  between 
  the 
  cylinders 
  o, 
  o, 
  heating 
  the 
  thermal 
  

   junction 
  h 
  and 
  producing 
  a 
  steady 
  current 
  through 
  the 
  coil. 
  

   Being 
  in 
  a 
  strong 
  magnetic 
  field, 
  the 
  coil 
  turns 
  so 
  that 
  its 
  plane 
  

   makes 
  a 
  small 
  angle 
  with 
  the 
  axis 
  of 
  the 
  magnet. 
  These 
  small 
  

   deflections 
  are 
  read 
  by 
  an 
  Elliott 
  reading 
  telescope 
  of 
  such 
  

   good 
  definition 
  that 
  tenths 
  of 
  a 
  millimeter 
  can 
  be 
  estimated 
  

   with 
  considerable 
  accuracy. 
  

  

  The 
  thermal-junction 
  must 
  be 
  carefully 
  protected 
  from 
  

   radiant 
  heat. 
  To 
  effect 
  this 
  the 
  magnet, 
  the 
  back 
  of 
  the 
  

   reflector, 
  and 
  the 
  suspension 
  tube 
  are 
  covered 
  by 
  wood 
  3 
  cm 
  

   thick 
  ; 
  the 
  opening 
  of 
  the 
  reflector 
  is 
  closed 
  by 
  a 
  sheet 
  of 
  

  

  *For 
  an 
  explanation 
  of 
  the 
  method 
  of 
  making 
  these 
  small 
  wires, 
  see 
  Lebedew's 
  

   paper, 
  1. 
  c. 
  

  

  