﻿290 
  Messrs. 
  Spiers, 
  Twyman, 
  and 
  Waters 
  on 
  Variations 
  in 
  

  

  circulated 
  by 
  means 
  of 
  two 
  sets 
  of 
  vanes 
  ; 
  the 
  upper 
  set, 
  A, 
  in 
  

   the 
  form 
  of 
  a 
  screw-thread, 
  was 
  arranged 
  so 
  as 
  to 
  send 
  the 
  oil 
  

   downwards 
  on 
  rotation 
  ; 
  the 
  lower 
  set, 
  B, 
  were 
  radial 
  near 
  the 
  

   centre, 
  and 
  at 
  the 
  outer 
  ends 
  curved 
  so 
  as 
  to 
  force 
  the 
  oil 
  

   upwards. 
  Thus, 
  as 
  the 
  oil 
  was 
  forced 
  downwards 
  from 
  the 
  

   first 
  set 
  A, 
  it 
  w 
  T 
  as 
  caught, 
  and 
  first 
  sent 
  outwards 
  and 
  then 
  

   upwards 
  by 
  the 
  second 
  set 
  B, 
  thus 
  producing 
  a 
  constant 
  

   circulation. 
  During 
  our 
  experiments 
  the 
  temperature 
  in 
  this 
  

   bath 
  was 
  always 
  uniform 
  to 
  at 
  least 
  o, 
  02 
  C. 
  

  

  The 
  stretched 
  wire 
  (fig. 
  4), 
  three 
  metres 
  in 
  length, 
  was 
  

  

  Fisr. 
  4. 
  

  

  C 
  

  

  ^.iT 
  

  

  ^p£e£ 
  

  

  employed 
  as 
  the 
  potentiometer-wire. 
  The 
  resistance 
  of 
  this 
  

   being 
  known, 
  the 
  strength 
  of 
  current 
  necessary 
  to 
  give 
  a 
  

   drop 
  of 
  potential 
  of 
  one 
  ten-thousandth 
  of 
  a 
  volt 
  per 
  centi- 
  

   metre 
  length 
  was 
  calculated, 
  and 
  also 
  the 
  resistance 
  required 
  

   to 
  be 
  traversed 
  by 
  this 
  current 
  to 
  produce 
  a 
  potential-differ- 
  

   ence 
  between 
  its 
  extremities 
  equal 
  to 
  the 
  E.M.F. 
  of 
  our 
  

   standard. 
  The 
  resistance 
  R 
  x 
  was 
  adjusted 
  so 
  as 
  to 
  be 
  as 
  

   nearly 
  as 
  possible 
  equal 
  to 
  that 
  value, 
  and 
  the 
  deficit 
  was 
  

   found 
  in 
  terms 
  of 
  a 
  length 
  AP 
  of 
  the 
  stretched 
  wire. 
  For 
  

   example, 
  suppose 
  the 
  resistance 
  necessary 
  to 
  produce 
  a 
  

   potential- 
  difference 
  equal 
  to 
  the 
  E.M.F. 
  of 
  our 
  standard 
  to 
  be 
  

   found 
  by 
  calculation 
  to 
  be 
  214*30 
  ohms. 
  Plugs 
  would 
  be 
  

   taken 
  out 
  of 
  B.J 
  to 
  the 
  value 
  of 
  214 
  ohms, 
  and 
  the 
  point 
  P 
  on 
  

   the 
  wire 
  would 
  be 
  selected 
  such 
  that 
  the 
  resistance 
  of 
  AP 
  

   equalled 
  0*30 
  ohm. 
  Then 
  the 
  mercury 
  cups 
  a, 
  b 
  and 
  a?, 
  z 
  

   being 
  joined, 
  the 
  current 
  sent 
  through 
  the 
  wire 
  by 
  the 
  storage- 
  

   cell 
  B 
  was 
  adjusted 
  by 
  means 
  of 
  the 
  auxiliary 
  variable 
  resist- 
  

   ance 
  B 
  2 
  until 
  the 
  E.M.F. 
  of 
  the 
  standard 
  equalled 
  the 
  

   potential-difference 
  between 
  the 
  points 
  H 
  and 
  P. 
  In 
  this 
  

   way 
  the 
  wire 
  was 
  rendered 
  direct 
  reading 
  for 
  any 
  subsequent 
  

   comparisons, 
  one 
  centimetre 
  on 
  the 
  scale 
  corresponding 
  with 
  

   one 
  ten-thousandth 
  of 
  a 
  volt. 
  By 
  joining 
  the 
  mercury 
  cups 
  

   a, 
  b 
  and 
  #, 
  z, 
  the 
  value 
  of 
  the 
  current 
  could 
  be 
  readily 
  checked 
  

   at 
  any 
  time 
  during 
  the 
  experiment 
  without 
  interrupting 
  the 
  

   rest 
  of 
  the 
  circuit. 
  As 
  a 
  rule, 
  very 
  little 
  variation 
  was 
  found 
  

  

  