﻿The 
  Porous 
  Plug 
  Experiment, 
  159 
  

  

  The 
  dimensions 
  given 
  above 
  for 
  the 
  internal 
  diameter 
  oli 
  

   the 
  solenoid 
  will 
  be 
  found 
  sufficient 
  for 
  receiving 
  a 
  double 
  

   vacuum 
  Devvar 
  tube 
  for 
  tests 
  at 
  —252° 
  C. 
  on 
  specimens 
  

   immersed 
  in 
  liquid 
  hydrogen. 
  

  

  A 
  slightly 
  modified 
  form 
  of 
  the 
  stand 
  supporting 
  the 
  

   solenoid 
  permits 
  of 
  the 
  latter 
  being 
  carried 
  in 
  an 
  east 
  and 
  

   west 
  position 
  on 
  one 
  of 
  the 
  arms 
  of 
  the 
  cross-piece 
  of 
  the 
  

   magnetometer. 
  The 
  apparatus 
  is 
  therefore 
  available 
  for 
  use 
  

   with 
  specimens 
  in 
  either 
  the 
  " 
  A 
  " 
  or 
  " 
  B 
  " 
  position 
  of 
  

   Gauss 
  ; 
  the 
  methods 
  described 
  in 
  Gray's 
  ' 
  Absolute 
  Measure- 
  

   ments 
  in 
  Electricity 
  and 
  Magnetism 
  ' 
  for 
  the 
  determination 
  

   of 
  the 
  effective 
  lengths 
  of 
  the 
  specimens 
  thus 
  become 
  

   available. 
  

  

  The 
  considerable 
  height 
  of 
  the 
  magnetometer-needle 
  above 
  

   the 
  level 
  of 
  the 
  magnetometer 
  base-board 
  (18 
  cm.) 
  would 
  

   also 
  permit 
  the 
  apparatus 
  to 
  be 
  readily 
  adapted 
  for 
  testing 
  

   by 
  the 
  "one-pole''' 
  method*. 
  

  

  Several 
  instruments 
  of 
  the 
  above 
  type 
  have 
  been 
  built 
  in 
  

   the 
  Physical 
  Institute 
  of 
  the 
  University 
  of 
  Glasgow, 
  and 
  are 
  

   giving 
  every 
  satisfaction. 
  

  

  XVII. 
  71ie 
  Porous 
  Plug 
  Experiment. 
  By 
  W. 
  A. 
  Douglas 
  

   KuDGE, 
  M.A.^ 
  Professor 
  of 
  Phi/sics, 
  Grey 
  University 
  

   College^ 
  Ploemfonteinf. 
  

  

  WHEN 
  a 
  gas 
  escapes 
  through 
  a 
  narrow 
  orifice 
  so 
  that 
  

   its 
  kinetic 
  energy 
  is 
  nearly 
  destroyed, 
  it 
  either 
  falls 
  

   or 
  rises 
  in 
  temperature. 
  This 
  alteration 
  in 
  temperature 
  is 
  

   Jknown 
  as 
  the 
  Joule 
  -Thomson 
  effect. 
  Few 
  experimental 
  

   determinations 
  have 
  been 
  made 
  of 
  this 
  effect 
  since 
  the 
  original 
  

   experiment 
  of 
  Joule 
  and 
  Thomson, 
  but 
  the 
  matter 
  has 
  been 
  

   so 
  fully 
  discussed 
  that 
  any 
  reference 
  to 
  the 
  previous 
  work 
  is 
  

   unnecessary. 
  

  

  The 
  author 
  has 
  made 
  many 
  experiments 
  on 
  the 
  thermal 
  

   change 
  occurring 
  during 
  the 
  expansion 
  of 
  a 
  liquid 
  gas, 
  using 
  

   for 
  the 
  purpose 
  liquid 
  carbonic 
  acid 
  contained 
  in 
  the 
  small 
  

   bulbs 
  used 
  for 
  aerating 
  water. 
  If 
  one 
  of 
  these 
  is 
  punctured 
  

   below 
  the 
  surface 
  of 
  water 
  c 
  ntained 
  in 
  a 
  calorimeter, 
  and 
  

   the 
  rate 
  of 
  evolution 
  of 
  the 
  gas 
  controlled, 
  a 
  reduction 
  in 
  

   temperature 
  will 
  take 
  place. 
  

  

  If 
  we 
  suppose 
  that 
  the 
  gas 
  remains 
  liquid 
  during 
  the 
  

   expansion, 
  which 
  might 
  be 
  the 
  case 
  if 
  the 
  rate 
  of 
  escape 
  was 
  

   slo.v, 
  then 
  the 
  pressure 
  would 
  remain 
  fairly 
  constant, 
  and 
  

   the 
  gas 
  would 
  thus 
  be 
  driven 
  through 
  the 
  plug 
  under 
  a 
  

  

  * 
  See 
  EwiDg's 
  ' 
  Magnetic 
  Induction 
  in 
  Iron 
  and 
  other 
  Metals,' 
  p. 
  39. 
  

   t 
  Communicated 
  by 
  the 
  Author. 
  

  

  