﻿Device 
  for 
  evaluating 
  Formidce 
  and 
  solving 
  Equations, 
  293 
  

  

  The 
  method 
  of 
  constructing 
  the 
  slide 
  resistances 
  (fig. 
  1) 
  is 
  

   as 
  follows 
  : 
  — 
  A 
  template 
  of 
  thin 
  insulating 
  material 
  is 
  made 
  

  

  Fig. 
  1. 
  

  

  I—J 
  I 
  L 
  

  

  I 
  

  

  (a) 
  

  

  te 
  9 
  e 
  X 
  ^ 
  5 
  

  

  in 
  the 
  shape 
  OSCY, 
  shown 
  in 
  fig. 
  1. 
  The 
  equation 
  to 
  the 
  

   curve 
  YPC 
  is 
  

  

  y 
  = 
  k 
  10-^''^ 
  

  

  where 
  OY=y^, 
  OS=/i, 
  and 
  SC 
  = 
  /:/10. 
  

  

  This 
  plate 
  is 
  wound 
  uniformly 
  with 
  a 
  hundred 
  turns 
  of 
  

   No. 
  36 
  insulated 
  manganin 
  wire, 
  the 
  temperature 
  coefficient 
  

   of 
  which 
  is 
  negligible, 
  and 
  the 
  wires 
  are 
  practically 
  parallel 
  

   to 
  OY. 
  At 
  points 
  along 
  OS 
  the 
  insulation 
  of 
  this 
  wire 
  

   is 
  removed, 
  and 
  contact 
  with 
  the 
  wires 
  is 
  made 
  by 
  the 
  

   contact-finger 
  described 
  below. 
  The 
  scale 
  OS 
  is 
  logarithmic. 
  

   If 
  />, 
  for 
  instance, 
  be 
  the 
  reading 
  at 
  the 
  point 
  N, 
  we 
  have 
  

  

  ON 
  = 
  Mogp, 
  

  

  and 
  thus 
  y 
  = 
  FN 
  = 
  kjp. 
  

  

  The 
  resistance 
  of 
  the 
  wire 
  between 
  and 
  N 
  will 
  be 
  

   approximately 
  proportional 
  to 
  the 
  area 
  ONPY. 
  This 
  area 
  is 
  

   given 
  by 
  

  

  -x:h 
  

  

  )>^-'i- 
  

  

  10-"'" 
  -d^v 
  

  

  Jik 
  

  

  {-i> 
  

  

  log. 
  10' 
  

  

  If, 
  then, 
  the 
  wire 
  on 
  the 
  frame 
  have 
  a 
  resistance 
  9R/10, 
  

   Phd. 
  Mag. 
  S. 
  6. 
  Vol. 
  1«. 
  No. 
  104. 
  Aug. 
  1909. 
  X 
  

  

  