﻿186 
  Dr. 
  T. 
  J. 
  I'a. 
  Brorawich 
  

  

  on 
  

  

  atomic 
  radii 
  as 
  given 
  by 
  Rankine*: 
  the 
  absolute 
  values 
  are- 
  

   given 
  in 
  the 
  third 
  column 
  using 
  Jeans's 
  corrected 
  value 
  for 
  

   helium. 
  The 
  values 
  of 
  E,the 
  ionization 
  potential, 
  calculated 
  

   from 
  (1), 
  are 
  given 
  in 
  the 
  fourth 
  column. 
  

  

  A 
  check 
  upon 
  these 
  results 
  is 
  afforded 
  by 
  some 
  recent- 
  

   experiments 
  made 
  by 
  Franck 
  and 
  Hertz 
  f 
  upon 
  the 
  ionization 
  

   potential 
  in 
  various 
  gases. 
  

  

  If 
  a 
  corpuscle 
  fall 
  freely 
  through 
  a 
  difference 
  of 
  potential 
  

   E 
  it 
  will 
  acquire 
  energy 
  He, 
  which 
  may 
  be 
  transferred 
  to- 
  

   one 
  of 
  the 
  electrons 
  of 
  an 
  atom 
  when 
  a 
  collision 
  takes 
  place 
  

   between 
  atom 
  and 
  corpuscle. 
  If 
  the 
  energy 
  be 
  just 
  sufficient 
  

   to 
  detach 
  this 
  electron 
  from 
  the 
  atom, 
  then 
  E 
  may 
  be 
  called 
  

   the 
  ionizing 
  potential. 
  The 
  source 
  of 
  the 
  corpuscles 
  in 
  the 
  

   experiments 
  of 
  Franck 
  and 
  Hertz 
  was 
  a 
  glowing 
  platinum 
  

   wire. 
  They 
  were 
  accelerated 
  by 
  an 
  electric 
  field 
  which 
  was 
  

   increased 
  till 
  positive 
  ions 
  were 
  produced 
  from 
  the 
  surrounding 
  

   gas, 
  showing 
  that 
  ionization 
  had 
  taken 
  place. 
  

  

  The 
  value 
  of 
  E 
  found 
  by 
  Franck 
  and 
  Hertz 
  for 
  helium,, 
  

   neon, 
  and 
  argon 
  are 
  given 
  in 
  the 
  fifth 
  column 
  of 
  the 
  table. 
  

   The 
  agreement 
  with 
  the 
  calculated 
  values 
  is 
  remarkably 
  

   close. 
  

  

  Observations 
  of 
  the 
  ionization 
  potential 
  for 
  krypton 
  and 
  

   xenon 
  would 
  be 
  of 
  interest 
  in 
  this 
  connexion, 
  and 
  it 
  is 
  to 
  be- 
  

   hoped 
  that 
  such 
  will 
  be 
  forthcoming. 
  

  

  Cavendish 
  Laboratory, 
  Cambridge. 
  

  

  X. 
  A 
  Note 
  on 
  the 
  Ballistic 
  Galvanometer. 
  By 
  T. 
  J. 
  

   Pa. 
  Beomwich, 
  Sc.jD., 
  F.B.S., 
  St. 
  Johns 
  College, 
  Cambridge 
  %.. 
  

  

  IN 
  discussing 
  the 
  mathematical 
  theory 
  of 
  the 
  oscillations 
  of 
  

   the 
  needle 
  in 
  a 
  ballistic 
  galvanometer, 
  Maxwell 
  treated 
  

   the 
  problem 
  in 
  two 
  parts 
  : 
  he 
  first 
  § 
  neglected 
  the 
  effect 
  of 
  

   damping 
  and 
  then 
  obtained 
  a 
  formula 
  which 
  may 
  be 
  written 
  

  

  Q 
  = 
  (2H/G/>)sinJ, 
  a 
  , 
  (A) 
  

  

  where 
  27r/p 
  is 
  the 
  complete 
  period 
  of 
  oscillation 
  of 
  the 
  

   needle, 
  and 
  a 
  is 
  the 
  angle 
  of 
  swing 
  produced 
  by 
  passing 
  a 
  

   quantity 
  of 
  electricity 
  Q 
  round 
  the 
  coil 
  of 
  the 
  galvanometer. 
  

   Secondly 
  ||, 
  Maxwell 
  treated 
  the 
  case 
  in 
  which 
  the 
  angle 
  

   of 
  swing 
  is 
  small 
  enough 
  for 
  sin# 
  to 
  be 
  replaced 
  by 
  6; 
  and 
  

  

  * 
  Rankine, 
  loc. 
  cit. 
  

  

  t 
  Franck 
  and 
  Hertz, 
  Berichte 
  d. 
  D. 
  P. 
  Gesellschaft, 
  ii. 
  pp. 
  34-45 
  (1913).. 
  

  

  X 
  Communicated 
  by 
  Sir 
  J. 
  J. 
  Thomson, 
  O.M., 
  F.R.8. 
  

  

  § 
  ' 
  Electricity 
  and 
  Magnetism/ 
  vol. 
  ii. 
  Art. 
  748. 
  

   II 
  L. 
  c. 
  Art. 
  749. 
  

  

  