﻿320 
  Prof. 
  Lyle 
  on 
  Circular 
  Filaments 
  or 
  Circular 
  

  

  by 
  means 
  of 
  a 
  clamp 
  as 
  in 
  the 
  diagram. 
  A 
  pair 
  of 
  uprights 
  F 
  

   and 
  another 
  pair 
  F' 
  rise 
  from 
  the 
  base, 
  each 
  pairbeing 
  joined 
  

   near 
  the 
  top 
  by 
  a 
  cross-piece. 
  In 
  the 
  upper 
  sides 
  o£ 
  these 
  

   •cross-pieces 
  rectangular 
  slots 
  are 
  cut 
  in 
  which 
  a 
  strong 
  bar 
  D 
  

   can 
  slide 
  parallel 
  to 
  the 
  other 
  slide 
  B. 
  D 
  can 
  be 
  clamped 
  in 
  

   any 
  position 
  to 
  the 
  slow-motion 
  screw 
  b 
  which 
  is 
  of 
  fine 
  pitch 
  

   and 
  has 
  a 
  short 
  range. 
  To 
  the 
  lower 
  side 
  of 
  D 
  is 
  fixed 
  a 
  

   piece 
  E, 
  one 
  of 
  whose 
  faces 
  is 
  carefully 
  planed 
  and 
  set 
  per- 
  

   pendicular 
  to 
  D. 
  To 
  this 
  piece 
  the 
  smaller 
  coil 
  is 
  to 
  be 
  

   attached. 
  Resting 
  on 
  the 
  upper 
  ends 
  of 
  the 
  four 
  pillars 
  F 
  

   and 
  F' 
  is 
  a 
  brass 
  table 
  fitted 
  with 
  short 
  levelling-screws 
  for 
  

   ■carrying 
  the 
  magnetometer. 
  The 
  telescope 
  and 
  scale 
  are 
  set 
  

   up 
  on 
  the 
  left 
  side, 
  and 
  to 
  the 
  opposite 
  ends 
  of 
  the 
  beams 
  

   B 
  and 
  D 
  are 
  attached 
  (as 
  in 
  the 
  diagram) 
  carefully 
  ruled 
  

   glass 
  millimetre-scales 
  S 
  and 
  S' 
  by 
  means 
  of 
  which, 
  with 
  the 
  

   assistance 
  of 
  two 
  micrometer-microscopes, 
  the 
  motion 
  of 
  B 
  

   and 
  of 
  D 
  and 
  so 
  of 
  either 
  coil 
  in 
  the 
  axial 
  direction 
  can 
  be 
  

   accurately 
  measured. 
  

  

  There 
  is 
  little 
  difficulty 
  in 
  setting 
  up 
  and 
  centering 
  the 
  

   coils, 
  but 
  I 
  have 
  found 
  the 
  following 
  method 
  convenient 
  : 
  — 
  The 
  

   magnetometer 
  being 
  replaceable 
  in 
  a 
  given 
  position, 
  the 
  axial 
  

   line 
  through 
  the 
  centre 
  of 
  the 
  magnet 
  is 
  determined 
  by 
  

   directing 
  a 
  horizontal 
  telescope 
  whose 
  line 
  of 
  sight 
  is 
  parallel 
  

   to.D 
  to 
  the 
  centre 
  of 
  the 
  magnet. 
  A 
  small 
  scriber 
  which 
  

   takes 
  the 
  place 
  of 
  the 
  magnetometer 
  is 
  now 
  adjusted 
  so 
  that 
  

   the 
  imao-e 
  of 
  its 
  point 
  coincides 
  with 
  the 
  cross 
  wires 
  of 
  the 
  

   telescope. 
  Across 
  one 
  face 
  of 
  each 
  coil 
  a 
  fine 
  wire 
  is 
  stretched 
  

   along 
  a 
  diameter 
  by 
  means 
  of 
  a 
  centre 
  square 
  and 
  the 
  middle 
  

   points 
  of 
  the 
  diameters 
  marked. 
  The 
  coils 
  are 
  now 
  clamped 
  

   in 
  position 
  so 
  that 
  the 
  middle 
  points 
  of 
  these 
  diametral 
  wires 
  

   coincide 
  with 
  the 
  point 
  of 
  thescriber. 
  

  

  17. 
  In 
  measuring 
  the 
  equivalent 
  radii 
  of 
  the 
  coils 
  we 
  must 
  

   first 
  determine 
  the 
  ratio 
  of 
  the 
  resistances 
  that 
  are 
  included 
  

   in 
  the 
  parallel 
  arms 
  containing 
  the 
  coils 
  when 
  their 
  actions 
  

   on 
  the 
  needle 
  balance 
  each 
  other, 
  the 
  needle 
  being 
  at 
  the 
  

   common 
  centre 
  of 
  the 
  coils. 
  The 
  adjustment 
  of 
  the 
  coils 
  so 
  

   that 
  the 
  needle 
  is 
  at 
  their 
  common 
  electrical 
  centre 
  is 
  of 
  

   importance, 
  and 
  is 
  managed 
  as 
  follows. 
  First 
  adjust 
  by 
  eye, 
  

   then 
  arrange 
  the 
  resistances 
  so 
  that 
  with 
  currents 
  flowing 
  the 
  

   needle 
  is 
  not 
  affected 
  ; 
  now, 
  by 
  means 
  of 
  the 
  slow-motion 
  

   screws 
  attached 
  to 
  each 
  slide, 
  the 
  coils 
  can 
  in 
  turn 
  be 
  moved 
  

   axially 
  until 
  they 
  both 
  have 
  maximum 
  magnetic 
  effect 
  on 
  the 
  

   needle. 
  When 
  this 
  has 
  been 
  effected 
  in 
  the 
  usual 
  way 
  for 
  

   adjusting 
  for 
  a 
  maximum 
  or 
  minimum, 
  we 
  know 
  that 
  the 
  

   centre 
  of 
  the 
  needle 
  lies 
  at 
  the 
  common 
  electrical 
  centre 
  of 
  

   the 
  two 
  coils. 
  

  

  