﻿456 
  Dr. 
  J. 
  E. 
  Almy 
  on 
  

  

  quite 
  masked 
  over 
  any. 
  effect 
  due 
  to 
  elongation 
  of 
  the 
  wire 
  

   by 
  expansion 
  of 
  the 
  shellac. 
  

  

  It 
  will 
  be 
  seen 
  that 
  our 
  observations 
  do 
  on 
  the 
  whole 
  

   support 
  the 
  conclusions 
  arrived 
  at 
  by 
  Drs. 
  Rosa 
  and 
  Babcock, 
  

   and 
  that 
  coils 
  hermetically 
  sealed, 
  or 
  coated 
  with 
  a 
  varnish 
  

   which 
  does 
  not 
  absorb 
  moisture, 
  must 
  have 
  an 
  advantage 
  

   over 
  manganin 
  coils 
  coated 
  with 
  shellac. 
  The 
  fact 
  that 
  some 
  

   high-resistance 
  coils 
  exhibit 
  the 
  " 
  humidity 
  effect 
  " 
  to 
  a 
  

   negligible 
  extent 
  only, 
  points 
  to 
  the 
  possibility 
  that 
  a 
  shellac 
  

   varnish 
  maybe 
  prepared, 
  which, 
  after 
  baking, 
  is 
  not 
  appreciably 
  

   hygroscopic. 
  

  

  XL. 
  Minimum 
  Spark 
  Potentials. 
  

   By 
  John 
  E. 
  Almy, 
  Ph.D.* 
  

  

  RECENT 
  work 
  of 
  Earhartf, 
  Shaw 
  J, 
  Hobbs§, 
  and 
  

   Kinsley 
  || 
  dealing 
  with 
  spark 
  potentials 
  for 
  very 
  short 
  

   spark-gaps 
  seem 
  to 
  show 
  that 
  the 
  spark-discharge 
  may 
  take 
  

   place 
  at 
  potentials 
  far 
  below 
  the 
  so-called 
  " 
  minimum 
  spark- 
  

   potential." 
  These 
  results, 
  so 
  signally 
  at 
  variance 
  with 
  the 
  

   results 
  of 
  CarrH 
  and 
  others, 
  who 
  have 
  found 
  every 
  evidence 
  

   of 
  a 
  minimum 
  value 
  of 
  potential, 
  below 
  which 
  it 
  was 
  

   impossible 
  to 
  obtain 
  a 
  discharge 
  with 
  any 
  spark-gap 
  or 
  any 
  

   gas 
  pressure 
  whatever, 
  seem 
  to 
  require 
  further 
  careful 
  

   investigation. 
  

  

  At 
  first, 
  two 
  spark 
  electrodes 
  were 
  mounted 
  upon 
  an 
  

   interferometer 
  of 
  the 
  Fabry-Perot 
  type, 
  one 
  electrode 
  being- 
  

   carried 
  upon 
  the 
  table 
  that 
  carries 
  the 
  movable 
  plate 
  of 
  the 
  

   interferometer, 
  the 
  other 
  being 
  fixed 
  adjacent 
  to 
  the 
  second 
  

   plate 
  of 
  the 
  interferometer. 
  The 
  supports 
  were 
  fairly 
  rigid, 
  

   being 
  of 
  massive 
  brass 
  plates, 
  reinforced. 
  In 
  attempting 
  to 
  

   use 
  electrodes 
  which 
  were 
  segments 
  of 
  a 
  sphere 
  of 
  5 
  cm. 
  

   radius, 
  and 
  later, 
  using 
  spheres 
  of 
  1 
  cm. 
  radius, 
  it 
  was 
  noted 
  

   that 
  a 
  discharge, 
  or 
  at 
  any 
  rate 
  a 
  short-circuit 
  of 
  the 
  dis- 
  

   charge 
  circuit, 
  was 
  obtained 
  with 
  potentials 
  considerably 
  

   below 
  the 
  " 
  minimum 
  potential 
  " 
  (about 
  350 
  volts 
  in 
  air 
  at 
  

   atmospheric 
  pressure), 
  but 
  that 
  when 
  this 
  occurred 
  the 
  

   electrodes 
  were 
  invariably 
  drawn 
  together 
  and 
  remained 
  

   fused 
  together 
  ; 
  while 
  in 
  case 
  the 
  potential 
  was 
  raised 
  above 
  

   the 
  minimum 
  potential 
  this 
  fusing 
  together 
  of 
  the 
  electrodes 
  

   rarely 
  if 
  ever 
  results. 
  It 
  sometimes 
  happened 
  that 
  with 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

   t 
  Earhart, 
  Phil. 
  Mag. 
  [6] 
  i. 
  p. 
  147 
  (1901). 
  

   X 
  Shaw, 
  Proc. 
  Rov. 
  Soc. 
  Ixxiii. 
  p. 
  337 
  (1903). 
  

   § 
  Hobbs, 
  Phil. 
  Mag. 
  [6] 
  x. 
  p. 
  617 
  (1905). 
  

   || 
  Kinsley, 
  Phil. 
  Mag. 
  [6] 
  ix. 
  p. 
  692 
  (1905). 
  

   11 
  Carr, 
  Proc. 
  Roy. 
  Soc. 
  lxxi. 
  p. 
  374 
  (1903). 
  

  

  