﻿Device 
  for 
  evaluating 
  Formulce 
  and 
  solving 
  Equations, 
  295 
  

  

  scale 
  at 
  Q 
  equals 
  A'l.rs. 
  This 
  follows 
  because 
  by 
  the 
  con- 
  

   struction 
  of 
  the 
  scales^ 
  

  

  AP 
  = 
  h 
  log 
  A',, 
  PQ 
  = 
  h 
  log 
  X2^ 
  

  

  and 
  thus 
  AQ 
  = 
  AP 
  + 
  PQ 
  

  

  = 
  h 
  log 
  Xi 
  + 
  h 
  log 
  X2 
  = 
  A 
  log 
  XiX2- 
  

  

  In 
  the 
  Arthur 
  Wright 
  device 
  the 
  slide 
  resistances 
  are 
  

   movable. 
  If 
  we 
  wish 
  to 
  form 
  the 
  product 
  pq^ 
  we 
  move 
  the 
  

   slide 
  so 
  as 
  to 
  make 
  the 
  index-line 
  II' 
  (fig. 
  3) 
  lie 
  over 
  

  

  Fig. 
  3. 
  

   I 
  

  

  /\j_ 
  

  

  AIL 
  

  

  ^WW 
  

  

  B 
  

  

  i^o 
  

  

  I' 
  

  

  the 
  reading 
  j9, 
  and 
  we 
  then 
  move 
  the 
  contact-finger 
  until 
  it 
  

   makes 
  contact 
  at 
  N, 
  where 
  TN, 
  on 
  the 
  logarithmic 
  scale, 
  

   is 
  q. 
  The 
  resistance 
  NB 
  will 
  then 
  equal 
  ^Ipq* 
  

  

  Y. 
  The 
  Contact 
  Fingers, 
  

   Fig. 
  4. 
  

   I 
  

  

  AAAA/ 
  

  

  B 
  

  

  The 
  ordinary 
  contact 
  finger 
  is 
  a 
  straight 
  piece 
  of 
  metal 
  

   wire 
  ONP 
  (fig. 
  4), 
  which 
  is 
  capable 
  of 
  rotation 
  about 
  a 
  

  

  X2 
  

  

  