﻿452 
  H. 
  D. 
  Day 
  — 
  Magnetic 
  Increment 
  

  

  in 
  turn 
  is 
  fastened 
  to 
  the 
  upright 
  standard 
  supporting 
  the 
  

   whole 
  apparatus. 
  The 
  shaft 
  h 
  is 
  surmounted 
  by 
  a 
  circular 
  

   plate, 
  u, 
  having 
  its 
  outer 
  edge 
  graduated 
  into 
  degrees. 
  This 
  

   plate 
  is 
  pierced 
  by 
  a 
  thumb-screw, 
  i, 
  which, 
  by 
  being 
  screwed 
  

   down 
  against 
  a 
  flange 
  upon 
  the 
  upper 
  end 
  of 
  the 
  sleeve, 
  k, 
  

   holds 
  the 
  shaft 
  h 
  at 
  any 
  position 
  of 
  its 
  revolution 
  within 
  the 
  

   sleeve. 
  By 
  this 
  means, 
  the 
  system 
  of 
  wires 
  is 
  supported 
  from 
  

   the 
  standard, 
  and 
  a 
  definite 
  amount 
  of 
  twist 
  may 
  be 
  imparted 
  

   to 
  the 
  system 
  by 
  revolving 
  the 
  shaft 
  A, 
  registered 
  by 
  its 
  grad- 
  

   uated 
  head 
  and 
  maintained 
  by 
  clamping 
  the 
  thumb-screw 
  i. 
  

  

  In 
  these 
  experiments, 
  iron 
  wires 
  25 
  cm 
  long 
  in 
  the 
  clear 
  were 
  

   used. 
  Each 
  wire 
  was 
  surrounded 
  by 
  a 
  water-jacket, 
  — 
  the 
  

   upper, 
  by 
  the 
  jacket 
  jj, 
  the 
  lower, 
  by 
  the 
  jacket 
  j' 
  . 
  These 
  

   water-jackets 
  were 
  made 
  by 
  arranging 
  concentrically 
  two 
  brass 
  

   tubes 
  differing 
  in 
  diameter 
  by 
  about 
  3 
  mm 
  . 
  At 
  the 
  ends 
  of 
  the 
  

   tubes 
  the 
  spaces 
  between 
  them 
  were 
  filled 
  by 
  little 
  collars. 
  An 
  

   inlet 
  tube 
  was 
  inserted 
  at 
  the 
  upper 
  end 
  of 
  the 
  outer 
  tube 
  and 
  

   an 
  outlet 
  tube 
  at 
  the 
  lower 
  end. 
  The 
  whole 
  was 
  then 
  made 
  

   water-tight 
  by 
  soldering. 
  The 
  water-jackets 
  were 
  then 
  con- 
  

   nected 
  in 
  series 
  with 
  a 
  source 
  and 
  a 
  sink 
  and 
  a 
  stream 
  of 
  water 
  

   under 
  pressure 
  passed 
  through, 
  and 
  thus 
  the 
  wire 
  within 
  was 
  

   immersed 
  in 
  its 
  field 
  and 
  could 
  not 
  have 
  its 
  temperature 
  affected 
  

   by 
  that 
  of 
  the 
  helix. 
  

  

  It 
  has 
  already 
  been 
  stated 
  that 
  the 
  tensile 
  strain 
  upon 
  the 
  

   iron 
  wires 
  was 
  regulated 
  by 
  suspending 
  from 
  the 
  lower 
  wire 
  

   a 
  bucket 
  of 
  mercury 
  varying 
  in 
  weight 
  proportionally 
  with 
  the 
  

   cross-sections 
  of 
  the 
  wires. 
  A 
  short 
  piece 
  of 
  small 
  brass 
  rod 
  t 
  

   to 
  which 
  the 
  lower 
  wire 
  was 
  soldered, 
  was 
  slipped 
  within 
  a 
  

   short 
  sleeve 
  of 
  thin 
  tubing, 
  d, 
  and 
  held 
  by 
  a 
  pin 
  piercing 
  both 
  

   rod 
  and 
  sleeve. 
  The 
  bucket 
  (an 
  old 
  metal 
  cartridge) 
  was 
  sus- 
  

   pended 
  from 
  a 
  slanting 
  slit 
  in 
  the 
  lower 
  end 
  of 
  this 
  sleeve. 
  

  

  All 
  increments 
  of 
  rigidity 
  in 
  the 
  magnetized 
  wire 
  were 
  regis- 
  

   tered 
  as 
  scale-deflections 
  at 
  the 
  eye-piece 
  of 
  an 
  observing 
  tele- 
  

   scope 
  focussed 
  upon 
  the 
  mirror 
  g. 
  In 
  order 
  to 
  render 
  unneces- 
  

   sary 
  constant 
  shifting 
  of 
  the 
  telescope 
  for 
  focussing 
  upon 
  the 
  

   mirror, 
  provision 
  was 
  made 
  that 
  one-half 
  of 
  the 
  twist 
  given 
  the 
  

   wires 
  might 
  be 
  imparted 
  at 
  the 
  upper 
  end 
  in 
  the 
  manner 
  

   already 
  indicated, 
  and 
  the 
  other 
  half 
  at 
  the 
  lower 
  end. 
  It 
  is 
  

   evident 
  that 
  the 
  scale-image 
  could 
  thus 
  be 
  kept 
  within 
  the 
  

   field 
  of 
  the 
  telescope. 
  This 
  was 
  accomplished 
  as 
  follows 
  : 
  A 
  

   circular 
  plate, 
  p, 
  having 
  its 
  outer 
  edge 
  graduated 
  into 
  degrees, 
  

   was 
  fitted 
  to 
  the 
  lower 
  end 
  of 
  the 
  jacket 
  j\ 
  and 
  made 
  free 
  to 
  

   revolve 
  upon 
  it. 
  This 
  plate 
  could 
  be 
  held 
  in 
  any 
  position 
  of 
  

   its 
  revolution 
  by 
  a 
  thumb-screw, 
  jS, 
  which 
  could 
  be 
  screwed 
  up 
  

   against 
  a 
  flange, 
  n, 
  soldered 
  to 
  the 
  water-jacket. 
  Two 
  short 
  

   pillars, 
  located 
  in 
  a 
  diameter 
  of 
  the 
  plate 
  near 
  its 
  circumfer- 
  

   ence, 
  and 
  each 
  having 
  a 
  longitudinal 
  slit 
  lying 
  along 
  this 
  diam- 
  

  

  