﻿Bancs 
  — 
  Trial 
  of 
  Interferential 
  Induction 
  balance. 
  109 
  

  

  provided 
  with 
  cross-hairs, 
  must 
  be 
  so 
  mounted 
  as 
  to 
  be 
  easily 
  

   focussed, 
  moved 
  about 
  and 
  inclined. 
  

  

  Whenever 
  a 
  current 
  passes 
  through 
  either 
  of 
  the 
  helices 
  A 
  

   and 
  B 
  the 
  cores 
  a, 
  h 
  will 
  in 
  general 
  expand, 
  unless 
  the 
  field 
  be 
  

   very 
  intense. 
  Current 
  for 
  this 
  purpose 
  is 
  furnished 
  by 
  one 
  or 
  

   more 
  storage 
  cells 
  e, 
  passes 
  through 
  a 
  commutator 
  C 
  and 
  a 
  

   switch 
  board 
  D, 
  by 
  which 
  either 
  one 
  or 
  both 
  the 
  helices 
  may 
  

   be 
  put 
  into 
  the 
  circuit. 
  The 
  circuit 
  also 
  contains 
  inductive 
  

   resistance 
  H, 
  non-inductive 
  resistance 
  R 
  and 
  an 
  interrupter 
  L 
  

   for 
  intermittent 
  currents. 
  J 
  is 
  a 
  key. 
  As 
  the 
  magnetic 
  

   expansion 
  due 
  to 
  complete 
  saturation 
  is 
  only 
  a 
  few 
  mil- 
  

   lionths, 
  the 
  effects 
  of 
  thermal 
  expansion 
  must 
  be 
  excluded. 
  

   Hence 
  the 
  frame 
  of 
  each 
  helix 
  is 
  a 
  thin 
  shell-like 
  water- 
  

   jacket. 
  Water 
  arrives 
  at 
  w,» 
  passes 
  in 
  a 
  thin 
  sheet 
  between 
  

   the 
  core 
  and 
  the 
  wire 
  of 
  helix 
  A, 
  then 
  similarly 
  through 
  helix 
  

   B 
  and 
  leaves 
  at 
  w' 
  . 
  The 
  same 
  temperature 
  is 
  thus 
  obtained 
  

   for 
  both 
  helices 
  and 
  strong 
  electrical 
  currents 
  passed 
  through 
  

   them 
  are 
  without 
  thermal 
  effect 
  on 
  the 
  core. 
  

  

  The 
  two 
  helices 
  were 
  wound 
  identically 
  and 
  chiefly 
  for 
  con- 
  

   tinuous 
  currents. 
  They 
  each 
  consisted 
  of 
  eight 
  layers 
  of 
  ~No. 
  

   21 
  copper 
  wire 
  with 
  410 
  turns 
  in 
  each 
  layer, 
  and 
  they 
  were 
  

   3T 
  cm 
  long. 
  The 
  diameter 
  of 
  the 
  internal 
  canal 
  was 
  about 
  # 
  8 
  cm 
  , 
  

   and 
  the 
  outer 
  wall 
  of 
  the 
  water-jacket 
  on 
  which 
  the 
  wire 
  was 
  

   wrapped 
  about 
  1 
  *5 
  cm 
  . 
  In 
  view 
  of 
  the 
  long 
  helices 
  the 
  enclosed 
  

   iron 
  cores 
  (about 
  28 
  cm 
  long, 
  0*6 
  cm 
  in 
  diameter) 
  were 
  in 
  a 
  fairly 
  

   constant 
  field. 
  * 
  Their 
  rear 
  ends 
  were 
  soldered 
  into 
  a 
  rod 
  of 
  

   brass 
  snugly 
  fitting 
  the 
  box 
  of 
  the 
  helix 
  and 
  rigidly 
  fastened 
  

   by 
  screws. 
  The 
  front 
  ends 
  were 
  soldered 
  to 
  very 
  thin 
  copper 
  

   tubes 
  which 
  protruded 
  beyond 
  the 
  helix 
  and 
  served 
  as 
  holders 
  

   for 
  the 
  mirrors 
  m, 
  n. 
  The 
  cores 
  were 
  made 
  as 
  straight 
  

   as 
  possible, 
  adjusted 
  symmetrically 
  and 
  coaxially 
  with 
  the 
  

   helix, 
  leaving 
  a 
  clear 
  space 
  of 
  about 
  l 
  mm 
  between 
  core 
  and 
  

   helix. 
  The 
  fastening 
  of 
  the 
  cores 
  is 
  merely 
  suggested 
  in 
  the 
  

   figure. 
  

  

  When 
  mounted 
  the 
  apparatus 
  must 
  necessarily 
  be 
  placed 
  on 
  

   a 
  pier 
  or 
  other 
  firm 
  support 
  ; 
  but 
  even 
  this 
  is 
  insufficient 
  unless 
  

   the 
  vibrations 
  are 
  damped 
  at 
  different 
  parts 
  of 
  the 
  apparatus. 
  

   Thus 
  it 
  is 
  necessary 
  to 
  support 
  the 
  helices 
  from 
  two 
  points 
  

   near 
  their 
  ends, 
  and 
  independently 
  to 
  prevent 
  transverse 
  vibra- 
  

   tions 
  of 
  the 
  core. 
  After 
  many 
  trials 
  I 
  found 
  that 
  a 
  short 
  plug 
  

   of 
  vasilene 
  in 
  the 
  tubes 
  near 
  the 
  ends 
  m, 
  n 
  was 
  preferable 
  to 
  

   mechanical 
  devices. 
  Prof. 
  Mayer's 
  important 
  paper 
  should 
  be 
  

   consulted 
  on 
  all 
  these 
  points. 
  

  

  § 
  3. 
  If 
  both 
  mirrors 
  move 
  towards 
  the 
  point 
  of 
  convergence 
  

   symmetrically 
  and 
  equally, 
  the 
  same 
  number 
  of 
  wave 
  lengths 
  

  

  * 
  Mayer: 
  Phil. 
  Mag. 
  [4], 
  xlvi, 
  p. 
  177, 
  1873. 
  

  

  