﻿96 
  Dr. 
  Eussell 
  and 
  Mr. 
  Wright 
  : 
  The 
  Wright 
  Electrical 
  

  

  point 
  fixed 
  on 
  a 
  sliding 
  bar 
  parallel 
  to 
  and 
  vertically 
  over 
  

   •che 
  index-line 
  II'. 
  This 
  bar 
  can 
  move 
  in 
  the 
  direction 
  o£ 
  

   its 
  length. 
  A 
  pointer 
  L 
  attached 
  to 
  it 
  moves 
  along 
  a 
  fixed 
  

   logarithmic 
  scale 
  KQ. 
  The 
  length 
  of 
  this 
  scale 
  is 
  made 
  

   equal 
  to 
  A, 
  and 
  so 
  

  

  KL 
  = 
  7ilog^ 
  = 
  OT. 
  

  

  The 
  contact 
  finger 
  OP 
  can 
  be 
  adjusted 
  so 
  as 
  to 
  make 
  any 
  

   desired 
  angle 
  6 
  with 
  the 
  index-line 
  11^ 
  I£ 
  the 
  contact 
  

   be 
  made 
  at 
  N, 
  we 
  have 
  

  

  TN 
  = 
  OT 
  tan 
  6* 
  = 
  A 
  tan 
  6 
  log 
  x 
  = 
  h 
  log 
  x^^""^ 
  ; 
  

   and 
  thus, 
  i£ 
  AT=p, 
  the 
  resistance 
  from 
  N 
  to 
  B 
  will 
  be 
  

   Bj/{px^^^^). 
  By 
  giving 
  various 
  values 
  to 
  6 
  we 
  can, 
  with 
  the 
  

   help 
  of 
  a 
  table 
  of 
  natural 
  tangents, 
  obtain 
  all 
  positive 
  integral 
  

   or 
  fractional 
  powers 
  of 
  x. 
  If 
  N 
  lie 
  between 
  A 
  and 
  T, 
  6 
  is 
  

   negative, 
  and 
  so 
  we 
  obtain 
  all 
  the 
  negative 
  integral 
  or 
  

   fractional 
  powers 
  of 
  x. 
  

  

  YI. 
  Additio7i 
  and 
  Subtraction, 
  

  

  In 
  performing 
  addition 
  and 
  subtraction, 
  the 
  action 
  of 
  the 
  

   apparatus 
  can 
  be 
  understood 
  from 
  the 
  following 
  diagram. 
  

   P 
  and 
  Q 
  are 
  the 
  terminals 
  of 
  a 
  wire 
  bridge, 
  of 
  which 
  is 
  the 
  

   middle 
  point 
  (fig. 
  5). 
  One 
  pole 
  of 
  a 
  battery 
  of 
  dry 
  cells 
  is 
  

  

  A/WVi 
  

  

  A/WV 
  

  

  B, 
  

  

  B3 
  

  

  connected 
  with 
  and 
  the 
  other 
  pole 
  is 
  connected 
  with 
  the 
  

   B 
  ends 
  of 
  the 
  slide 
  resistances, 
  AjB^, 
  A2B2, 
  and 
  A3B3. 
  

   A 
  galvanometer 
  connects 
  P 
  with 
  Q. 
  Let 
  us 
  suppose 
  that 
  

   we 
  have 
  to 
  find 
  the 
  value 
  of 
  a2 
  + 
  a^. 
  We 
  connect 
  Q 
  with 
  the 
  

   contact-fingers 
  of 
  A2B2 
  and 
  A3B3 
  respectively, 
  and 
  set 
  the 
  

   fingers 
  at 
  the 
  marks 
  a2 
  ^-^d 
  a^ 
  on 
  their 
  respective 
  scales. 
  We 
  

   then 
  join 
  P 
  with 
  the 
  contact-finger 
  of 
  AiB^, 
  and 
  closing 
  the 
  

   keys 
  we 
  adjust 
  the 
  position 
  of 
  this 
  finger 
  until 
  there 
  is 
  no 
  

  

  