﻿258 
  

  

  Mr. 
  G. 
  B. 
  Bryan 
  on 
  the 
  Determination 
  of 
  

  

  They 
  were 
  turned 
  up 
  all 
  over 
  and 
  the 
  surfaces 
  were 
  then 
  

   ground 
  on 
  plate-glass 
  with 
  fine 
  emery 
  until, 
  after 
  washing, 
  

   one 
  plate 
  would 
  hold 
  the 
  other 
  up 
  when 
  they 
  were 
  pressed 
  

   together. 
  These 
  plates 
  were 
  then 
  sent 
  to 
  a 
  jeweller's 
  to 
  be 
  

   gilded, 
  and 
  unfortunately 
  the 
  gilder, 
  contrary 
  to 
  instructions, 
  

   cleaned 
  the 
  surfaces 
  in 
  the 
  usual 
  way 
  and 
  rather 
  spoilt 
  them 
  ; 
  

   they 
  were 
  found 
  to 
  be 
  slightly 
  irregular, 
  so 
  that 
  a 
  correction 
  

   could 
  not 
  be 
  estimated. 
  Thus 
  the 
  only 
  advantage 
  of 
  these 
  

   plates 
  over 
  the 
  others 
  was 
  the 
  gilding. 
  

  

  Methods 
  of 
  Determining 
  the 
  Resistance. 
  

   Three 
  methods 
  of 
  finding 
  the 
  resistance 
  were 
  tried 
  in 
  various 
  

   cases 
  : 
  — 
  

  

  (1) 
  Comparison 
  with 
  a 
  standard 
  resistance 
  by 
  direct 
  de- 
  

   flexion, 
  usiug 
  a 
  continuous 
  current 
  from 
  a 
  battery 
  of 
  secondary 
  

   cells. 
  The 
  galvanometer 
  used 
  had 
  a 
  resistance 
  of 
  17,380 
  ohms 
  

   and 
  gave 
  a 
  deflexion 
  of 
  one 
  division 
  with 
  a 
  current 
  of 
  about 
  

   10 
  _b 
  amperes. 
  

  

  Fig. 
  2. 
  Fig. 
  4. 
  

  

  H 
  

  

  AB. 
  Electrodes. 
  

   C. 
  Commutator. 
  

   B. 
  Battery. 
  

   G. 
  Galvanometer. 
  

   S. 
  Standard 
  resistance. 
  

   K. 
  Key. 
  

   PQR. 
  Slide 
  wire. 
  

  

  AB 
  

  

  -O 
  O 
  

  

  (2) 
  Comparison 
  with 
  a 
  standard 
  resistance 
  by 
  direct 
  de- 
  

   flexion, 
  using 
  an 
  alternating 
  current. 
  The 
  same 
  galvanometer 
  

   was 
  used 
  as 
  in 
  (1). 
  The 
  alternating 
  current 
  was 
  obtained 
  

   by 
  means 
  of 
  a 
  revolving 
  commutator. 
  This 
  commutator 
  

   had 
  two 
  brass 
  segments 
  on 
  an 
  ebonite 
  drum, 
  and 
  is 
  shown, 
  

   together 
  with 
  the 
  connexions, 
  in 
  fig. 
  2. 
  The 
  commutator 
  was 
  

  

  