﻿Voltameter 
  for 
  Small 
  Currents. 
  159 
  

  

  measured 
  by 
  any 
  convenient 
  scale, 
  preferably 
  one 
  etched 
  on 
  

   the 
  glass, 
  with 
  or 
  without 
  a 
  micrometer. 
  

  

  For 
  the 
  successful 
  construction 
  of 
  the 
  apparatus 
  the 
  follow- 
  

   ing 
  points 
  should 
  be 
  attended 
  to 
  :• 
  — 
  The 
  parts 
  of 
  the 
  tube 
  where 
  

   the 
  electrodes 
  are 
  sealed-in 
  should 
  should 
  not 
  be 
  widened, 
  

   else 
  difficulty 
  in 
  filling 
  is 
  likely 
  to 
  ensue. 
  The 
  filling 
  can 
  be 
  

   done 
  by 
  placing 
  the 
  tube 
  — 
  aslant 
  — 
  with 
  one 
  point 
  dipping 
  

   under 
  mercury, 
  in 
  a 
  dish. 
  The 
  mercury 
  is 
  covered 
  by 
  

   solution 
  : 
  when 
  enough 
  has 
  been 
  drawn 
  in 
  by 
  the 
  pump 
  to 
  

   form 
  one 
  electrode, 
  the 
  point 
  is 
  lifted 
  momentarily 
  so 
  as 
  to 
  

   allow 
  solution 
  to 
  flow 
  in 
  instead 
  ; 
  then 
  depressed, 
  when 
  more 
  

   mercury 
  enters, 
  to 
  form 
  the 
  other 
  electrode. 
  The 
  filling 
  is 
  regu- 
  

   lated 
  most 
  conveniently 
  by 
  a 
  tap 
  sealed 
  on 
  to 
  the 
  upper 
  end 
  

   of 
  the 
  tube, 
  which 
  is 
  subsequently 
  detached. 
  The 
  solution 
  

   is 
  made 
  by 
  weighing 
  out 
  about 
  26*3 
  milligrams 
  of 
  mercurons 
  

   nitrate 
  per 
  c.c. 
  of 
  water 
  (i. 
  e. 
  decinormal), 
  and 
  adding 
  enough 
  

   nitric 
  acid 
  to 
  dissolve 
  the 
  basic 
  salt 
  that 
  forms. 
  The 
  current- 
  

   density 
  may 
  be 
  anything 
  up 
  to 
  10 
  or 
  15 
  milliamperes 
  per 
  

   sq. 
  cm. 
  ; 
  more 
  than 
  that 
  causes 
  polarization. 
  Hence 
  a 
  tube 
  

   of 
  2 
  sq. 
  mm. 
  cross-section 
  (ordinary 
  Sprengel-pump 
  tubing) 
  

   will 
  take 
  0*3 
  milliampere. 
  

  

  The 
  electrochemical 
  equivalent 
  of 
  mercurous 
  mercury 
  is 
  

   7*470 
  grams 
  per 
  ampere 
  per 
  hour, 
  or 
  0*5509 
  c.c. 
  Hence 
  

   with 
  a 
  current-density 
  of 
  0*010 
  ampere/sq. 
  cm., 
  the 
  rate 
  

   of 
  movement 
  would 
  be 
  0*005509 
  cm. 
  per 
  hour, 
  or 
  about 
  

   1 
  mm. 
  a 
  day. 
  The 
  meter 
  is 
  therefore 
  eminently 
  adapted 
  to 
  

   measure 
  long-continued 
  currents 
  of 
  very 
  small 
  magnitude. 
  

   Moreover, 
  with 
  proper 
  precautions 
  it 
  may 
  be 
  shunted, 
  as, 
  

   apart 
  from 
  temperature 
  changes, 
  the 
  resistance 
  keeps 
  steady. 
  

  

  The 
  following 
  is 
  a 
  record 
  of 
  such 
  an 
  instrument 
  : 
  — 
  

  

  Cross-section 
  of 
  tube 
  0*624 
  ram. 
  : 
  length 
  of 
  thread 
  of 
  

   solution 
  about 
  6 
  ram. 
  Micrometer 
  used, 
  1 
  scale- 
  

   division 
  = 
  0*0785 
  mm. 
  

  

  Time. 
  

  

  Micrometer. 
  

  

  h 
  m 
  

  

  

  11 
  10 
  

  

  40*0 
  

  

  12 
  15 
  

  

  41*0 
  

  

  Oct. 
  18. 
  11 
  10 
  40*0 
  Started 
  current 
  of 
  1/12000 
  amp. 
  

  

  12 
  15 
  41*0 
  

  

  13 
  15 
  42;0 
  Stopped. 
  

   Oct. 
  21. 
  Reset 
  micrometer. 
  

  

  11 
  50 
  40*0 
  Started 
  current 
  of 
  1/12000 
  amp. 
  

  

  15 
  40 
  43*7 
  

   Oct. 
  22. 
  9 
  50 
  60-5 
  

  

  Calculated 
  rate 
  of 
  movement 
  1 
  micrometer-division 
  in 
  

   1*067 
  hours 
  ; 
  observed 
  rate 
  1*073. 
  The 
  small 
  discrepancy 
  

   is 
  no 
  doubt 
  due 
  to 
  errors 
  of 
  experiment, 
  and 
  could 
  be 
  

   diminished 
  by 
  greater 
  care 
  in 
  calibration, 
  measurement 
  ot 
  

   current, 
  and 
  of 
  distance 
  moved. 
  

  

  