﻿292 
  Dr. 
  Russell 
  a7id 
  Mr. 
  Wright 
  : 
  The 
  Wright 
  Electrical 
  

  

  by 
  Mr. 
  Arthur 
  "Wright, 
  enables 
  us 
  to 
  evaluate 
  almost 
  all 
  

   mathematical 
  expressions 
  by 
  simple 
  mechanical 
  and 
  electrical 
  

   operations. 
  In 
  the 
  present 
  model 
  the 
  accuracy 
  is 
  not 
  high, 
  

   being 
  o£ 
  the 
  order 
  of 
  one 
  per 
  cent. 
  ; 
  but 
  this 
  accuracy 
  suffices 
  

   in 
  most 
  engineering 
  calculations. 
  The 
  apparatus, 
  however, 
  

   can 
  be 
  easily 
  elaborated 
  to 
  give 
  a 
  far 
  higher 
  accuracy. 
  The 
  

   main 
  object 
  of 
  the 
  authors 
  in 
  this 
  paper 
  is 
  to 
  explain 
  the 
  

   principles 
  on 
  which 
  it 
  works, 
  and 
  to 
  point 
  out 
  a 
  few 
  of 
  its 
  

   many 
  applications 
  in 
  practical 
  calculations. 
  

  

  Although 
  some 
  of 
  the 
  operations 
  described 
  below 
  — 
  as, 
  for 
  

   example, 
  the 
  finding 
  of 
  the 
  roots 
  of 
  a 
  numerical 
  algebraical 
  

   equation 
  of 
  the 
  n\h 
  degree 
  — 
  can 
  easily 
  be 
  done 
  by 
  the 
  expert 
  

   mathematician, 
  yet 
  even 
  in 
  his 
  case 
  the 
  apparatus 
  can 
  be 
  a 
  

   help, 
  as 
  it 
  gives 
  him 
  at 
  once 
  the 
  approximate 
  values 
  of 
  the 
  

   roots. 
  In 
  general, 
  he 
  can 
  then 
  find 
  them 
  to 
  any 
  required 
  

   degree 
  of 
  accuracy 
  by 
  the 
  methods 
  given 
  in 
  the 
  Theory 
  of 
  

   Equations. 
  

  

  II. 
  Historical. 
  

  

  The 
  history 
  of 
  calculating 
  machines, 
  and 
  an 
  excellent 
  

   description 
  of 
  many 
  of 
  them, 
  is 
  given 
  by 
  M. 
  d'Ocagne 
  in 
  his 
  

   book 
  Le 
  Calcul 
  Simplifie 
  (1905). 
  Valuable 
  information 
  is 
  

   also 
  given 
  in 
  Professor 
  Henrici^s 
  article 
  on 
  " 
  Mathematical 
  

   Instruments 
  " 
  in 
  the 
  Encyclopcedia 
  Britannica 
  (10th 
  edition). 
  

   We 
  need 
  only 
  mention, 
  therefore, 
  the 
  statical 
  method 
  * 
  of 
  

   solving 
  equations 
  due 
  to 
  Professor 
  Vernon 
  Boys, 
  the 
  Torres 
  

   logarithmic 
  arithmophore 
  t, 
  which 
  is 
  a 
  very 
  elaborate 
  piece 
  

   of 
  mechanism, 
  and 
  the 
  extremely 
  ingenious 
  electrical 
  method 
  X 
  

   due 
  to 
  M. 
  F. 
  Lucas, 
  of 
  finding 
  the 
  real 
  and 
  imaginary 
  roots 
  of 
  

   an 
  equation 
  of 
  the 
  nth. 
  degree. 
  The 
  last 
  method, 
  however, 
  is 
  

   not 
  rigorously 
  accurate 
  and 
  would 
  be 
  very 
  laborious 
  to 
  apply 
  

   in 
  practice. 
  

  

  III. 
  TJie 
  Slide 
  Resistances, 
  

  

  In 
  the 
  electrical 
  device 
  described 
  below 
  the 
  principle 
  of 
  the 
  

   ordinary 
  logarithmic 
  slide-rule 
  is 
  combined 
  with 
  addition 
  and 
  

   subtraction, 
  by 
  utilising 
  the 
  laws 
  according 
  to 
  which 
  resist- 
  

   ances 
  combine 
  in 
  series 
  or 
  parallel. 
  The 
  products 
  found 
  by 
  

   the 
  slide-rule 
  method 
  are 
  represented 
  either 
  by 
  the 
  resistances 
  

   or 
  by 
  the 
  reciprocals 
  of 
  the 
  resistances 
  of 
  certain 
  wires. 
  In 
  

   the 
  latter 
  case 
  the 
  currents 
  through 
  them 
  can 
  easily 
  be 
  added 
  

   or 
  subtracted 
  by 
  a 
  Wheatstone 
  Bridge 
  Method. 
  Hence 
  the 
  

   sums 
  or 
  differences 
  of 
  the 
  products 
  can 
  be 
  found. 
  

  

  * 
  Phil. 
  Mag. 
  vol. 
  xxi. 
  p. 
  241 
  (1886). 
  

  

  t 
  M. 
  d'Ocagne, 
  /. 
  c. 
  ante^ 
  p. 
  123. 
  

  

  j- 
  Comptes 
  Rendus, 
  t. 
  cvi. 
  p. 
  1072 
  (1888). 
  

  

  