﻿Horizontal 
  Intensity 
  of 
  the 
  Earth 
  9 
  s 
  Magnetic 
  Field. 
  755 
  

  

  The 
  tube 
  was 
  then 
  moved 
  parallel 
  to 
  its 
  length, 
  and 
  at 
  every 
  

   part 
  it 
  was 
  found 
  that 
  when 
  the 
  intersection 
  of 
  the 
  cross- 
  

   threads 
  of 
  one 
  microscope 
  was 
  focussed 
  exactly 
  on 
  the 
  bottom 
  

   of 
  a 
  thread, 
  then 
  the 
  intersection 
  of 
  the 
  cross-wires 
  of 
  the 
  

   other 
  microscope 
  was 
  likewise 
  focussed, 
  and 
  everywhere 
  there 
  

   were 
  -il 
  threads 
  between. 
  Hence 
  the 
  screw-thread 
  had 
  been 
  

   very 
  accurately 
  cut. 
  

  

  To 
  determine 
  the 
  number 
  of 
  threads 
  per 
  cm. 
  the 
  following 
  

   experiment 
  was 
  performed 
  : 
  — 
  Two 
  travelling 
  microscopes, 
  

   placed 
  at 
  about 
  25 
  cm. 
  apart, 
  were 
  focussed 
  on 
  the 
  divisions 
  

   of 
  a 
  standard 
  metre-scale. 
  The 
  microscopes 
  were 
  so 
  arranged 
  

   that 
  their 
  direction 
  of 
  motion 
  was 
  parallel 
  to 
  the 
  length 
  of 
  

   the 
  metre- 
  bar. 
  This 
  bar 
  was 
  then 
  taken 
  away 
  and 
  the 
  helix, 
  

   on 
  a 
  support 
  resting 
  on 
  levelling-screws, 
  was 
  inserted 
  in 
  its 
  

   place. 
  The 
  levelling-screws 
  were 
  adjusted 
  until 
  the 
  surface 
  

   of 
  the 
  helix 
  was 
  in 
  focus 
  in 
  the 
  microscopes. 
  The 
  microscopes 
  

   were 
  now 
  moved 
  slightly 
  until 
  the 
  intersections 
  of 
  the 
  cross- 
  

   wires 
  were 
  focussed 
  exactly 
  between 
  two 
  particular 
  turns 
  of 
  

   wire. 
  The 
  number 
  of 
  tarns, 
  between 
  the 
  two 
  particular 
  

   points 
  on 
  which 
  the 
  cross-wires 
  were 
  focussed, 
  was 
  counted. 
  

   The 
  solenoid 
  was 
  now 
  removed 
  and 
  the 
  standard 
  metre-bar 
  

   again 
  inserted. 
  A 
  very 
  slight 
  longitudinal 
  motion 
  could 
  be 
  

   given 
  to 
  the 
  length 
  of 
  the 
  bar 
  by 
  means 
  of 
  a 
  screw, 
  turning 
  

   in 
  a 
  fixed 
  support, 
  and 
  pressing 
  against 
  the 
  end 
  of 
  the 
  bar. 
  

  

  The 
  bar 
  was 
  adjusted 
  by 
  means 
  of 
  this 
  screw 
  until 
  the 
  

   cross-wires 
  of 
  one 
  microscope 
  were 
  in 
  focus 
  on 
  a 
  division 
  of 
  

   the 
  metre-scale. 
  

  

  The 
  intersection 
  of 
  the 
  cross-wires 
  of 
  the 
  other 
  microscope 
  

   was 
  not 
  now 
  in 
  focus 
  on 
  a 
  division 
  of 
  the 
  scale, 
  but, 
  after 
  

   reading 
  its 
  position 
  with 
  the 
  vernier 
  eyepiece, 
  it 
  was 
  moved 
  

   slightly 
  until 
  the 
  intersection 
  of 
  the 
  cross-wires 
  was 
  focussed 
  

   on 
  a 
  division 
  of 
  the 
  scale. 
  The 
  reading 
  given 
  by 
  the 
  metre- 
  

   scale, 
  plus 
  the 
  distance 
  through 
  which 
  the 
  microscope 
  has 
  

   moved, 
  oives 
  the 
  distance 
  containing- 
  the 
  number 
  of 
  threads 
  

   counted. 
  

  

  The 
  25 
  cm. 
  between 
  which 
  the 
  number 
  of 
  threads 
  was 
  

   counted 
  was 
  in 
  the 
  central 
  portion 
  of 
  the 
  solenoid, 
  because 
  of 
  

   its 
  being 
  much 
  more 
  important 
  to 
  know 
  the 
  exact 
  number 
  of 
  

   turns 
  per 
  cm. 
  near 
  the 
  centre 
  than 
  at 
  either 
  end. 
  The 
  result 
  

   was 
  18'8739 
  turns 
  per 
  cm. 
  

  

  Bare 
  copper 
  wire 
  was 
  now 
  wound 
  in 
  the 
  screw-threads 
  and 
  

   the 
  wire 
  was 
  warmed 
  slightly 
  as 
  it 
  was 
  wound 
  on 
  in 
  order 
  to 
  

   make 
  it 
  grip 
  the 
  tube 
  more 
  efficiently 
  and 
  thus 
  ensure 
  

   ■accurate 
  winding. 
  The 
  tube 
  was 
  slowly 
  rotated 
  in 
  a 
  lathe, 
  

   and 
  the 
  wire, 
  directed 
  into 
  the 
  screw-threads 
  with 
  the 
  hand, 
  

   was 
  heated 
  by 
  sending 
  an 
  electric 
  current 
  through 
  it 
  just 
  

  

  