﻿490 
  

  

  Mr. 
  B. 
  S. 
  Cohen 
  on 
  Production 
  of 
  Small 
  

  

  the 
  point 
  just 
  makes 
  contact 
  on 
  the 
  mercury 
  surface, 
  a 
  spark 
  is 
  

   observed. 
  

  

  If 
  then 
  the 
  distance 
  between 
  the 
  mercury 
  and 
  the 
  contact 
  

   is 
  slowly 
  decreased, 
  a 
  series 
  o£ 
  sparkless 
  points 
  will 
  be 
  reached 
  

   which 
  are 
  very 
  sharply 
  defined, 
  as 
  when 
  such 
  a 
  point 
  is 
  

   reached, 
  a 
  variation 
  either 
  way 
  of 
  about 
  *07 
  mm. 
  gives 
  the 
  

   sparking 
  condition 
  again. 
  

  

  The 
  writer 
  has 
  not 
  so 
  far 
  made 
  any 
  investigation 
  into 
  the 
  

   reason 
  for 
  this 
  action. 
  

  

  It 
  is, 
  however, 
  most 
  probable 
  that 
  this 
  action 
  is 
  partly 
  

   mechanical 
  as 
  at 
  the 
  sparkless 
  points 
  the 
  vibrating 
  surface 
  

   of 
  the 
  mercury 
  seems 
  to 
  be 
  set 
  in 
  a 
  rigid 
  condition. 
  

  

  Use 
  of 
  continuous 
  Waves 
  for 
  Absolute 
  Measurements. 
  — 
  The 
  

   wave 
  forms 
  illustrated 
  in 
  fig. 
  13 
  (PL 
  XVIII.), 
  fig. 
  16 
  

   (PI. 
  XIX.), 
  and 
  fig. 
  18 
  (PI. 
  XVI.) 
  might 
  conceivably 
  be 
  

   accurate 
  enough 
  for 
  use 
  when 
  making 
  absolute, 
  as 
  opposed 
  to 
  

   merely 
  comparative, 
  measurements. 
  

  

  The 
  following 
  table 
  gives 
  some 
  results 
  arrived 
  at 
  by 
  

   measuring 
  the 
  current 
  and 
  p.d. 
  with 
  standard 
  condensers 
  

   subjected 
  to 
  these 
  waves. 
  

  

  Wave 
  form 
  used. 
  

  

  Frequency. 
  

  

  Standard 
  

   Capacity 
  

   tested. 
  

  

  Percentage 
  

   error. 
  

  

  Wave 
  similar 
  to 
  that 
  illustrated 
  

   in 
  fig. 
  13 
  ] 
  

  

  Wave 
  similar 
  to 
  that 
  illustrated 
  [ 
  

   in 
  fig. 
  16 
  (a) 
  1 
  

  

  Wave 
  similar 
  to 
  that 
  illustrated 
  

   in 
  fig. 
  18 
  (b) 
  1 
  

  

  1442 
  

  

  854 
  

   523 
  

  

  833 
  

   1442 
  

  

  833 
  

  

  833 
  

  

  1440 
  

  

  •167 
  

   •33 
  

  

  1-0 
  

  

  •167 
  

   •167 
  

  

  •33 
  

  

  •167 
  

  

  •167 
  

  

  -1-8 
  

   -1-7 
  

   -1-0 
  

  

  +3 
  

  

  +4-2 
  

  

  +2-4 
  

   + 
  1-8 
  

   + 
  -5 
  

  

  

  These 
  results 
  were 
  determined 
  by 
  inserting 
  a 
  non-inductive 
  

   resistance 
  of 
  known 
  value 
  in 
  series 
  with 
  the 
  condenser, 
  taking 
  

   the 
  potential-difference 
  across 
  both 
  by 
  means 
  of 
  a 
  sensitive 
  

   electrostatic 
  voltmeter, 
  and 
  then 
  calculating 
  the 
  capacity 
  by 
  the 
  

   formula 
  A 
  . 
  10 
  u 
  /pY, 
  where 
  A 
  and 
  V 
  are 
  the 
  effective 
  values 
  

   of 
  the 
  charging 
  current 
  and 
  potential-difference 
  respectively. 
  

   This 
  is 
  very 
  satisfactory 
  as 
  very 
  little 
  time 
  was 
  spent 
  in 
  

   adjusting 
  the 
  waves. 
  

  

  So 
  far 
  as 
  these 
  tests 
  go 
  it 
  would 
  appear 
  that 
  the 
  amount 
  of 
  

   the 
  departure 
  from 
  the 
  sine 
  shape 
  in 
  these 
  continuous 
  waves 
  

   is 
  comparatively 
  inconsiderable. 
  

  

  