﻿132 
  Prof. 
  Poynting 
  and 
  Mr. 
  Todd 
  on 
  a 
  Method 
  of 
  

  

  become 
  more 
  penetrating 
  than 
  the 
  radiation 
  characteristic 
  

   of 
  the 
  absorber, 
  this 
  increase 
  in 
  the 
  absorption 
  being 
  in- 
  

   timately 
  connected 
  with 
  the 
  emission 
  of 
  tertiary 
  radiation 
  

   by 
  the 
  absorber 
  in 
  these 
  circumstances 
  (see 
  paper 
  on 
  " 
  The 
  

   Absorption 
  of 
  X-Rays/' 
  Barkla 
  & 
  Sadler, 
  Phil. 
  Mag. 
  May 
  

   1909). 
  

  

  The 
  fraction 
  of 
  this 
  increase 
  in 
  the 
  absorption 
  of 
  the 
  

   energy 
  of 
  the 
  secondary 
  beam, 
  which 
  is 
  re-emitted 
  as 
  tertiary 
  

   radiation, 
  is 
  not 
  constant, 
  but 
  decreases 
  as 
  the 
  secondary 
  

   beam 
  becomes 
  more 
  penetrating, 
  slowly 
  at 
  first, 
  and 
  then 
  

   more 
  rapidly 
  when 
  a 
  very 
  penetrating 
  secondary 
  beam 
  is 
  

   used. 
  

  

  In 
  conclusion 
  I 
  wish 
  to 
  thank 
  Dr. 
  Barkla 
  for 
  the 
  interest 
  

   he 
  has 
  shown 
  throughout 
  this 
  research, 
  and 
  especially 
  for 
  

   his 
  kindly 
  criticism 
  and 
  advice 
  during 
  the 
  writing 
  of 
  this 
  

   paper. 
  

  

  George 
  Holt 
  Physics 
  Laboratory, 
  

  

  University 
  of 
  Liverpool, 
  

  

  24tli 
  March, 
  1909. 
  

  

  XII. 
  On 
  a 
  Method 
  of 
  Determining 
  the 
  Sensibility 
  of 
  a 
  Balance, 
  

   % 
  J. 
  H. 
  Poynting, 
  ScD., 
  F.R.S., 
  and 
  G. 
  W. 
  Todd, 
  

  

  M.Sc^ 
  

  

  IN 
  the 
  method, 
  as 
  we 
  have 
  arranged 
  it, 
  a 
  small 
  frame 
  

   (fig. 
  1, 
  end 
  view) 
  is 
  fixed 
  at 
  the 
  centre 
  of 
  the 
  beam 
  of 
  a 
  

   16-inch 
  Oertling 
  balance. 
  This 
  carries 
  two 
  Ys 
  about 
  2 
  cm. 
  

  

  Fig. 
  1. 
  

  

  '-ff" 
  

  

  \/-^S^ 
  

  

  End 
  view 
  of 
  V 
  frame 
  fixed 
  to 
  balance-beam. 
  

  

  apart, 
  and 
  in 
  the 
  Ys 
  lies 
  a 
  straight 
  wire 
  or 
  fibre 
  about 
  3^ 
  cm. 
  

   long, 
  parallel 
  to 
  the 
  beam 
  and 
  level 
  with 
  the 
  central 
  knife- 
  

   edge. 
  This 
  wire 
  takes 
  the 
  place 
  of 
  the 
  ordinary 
  rider, 
  and 
  

  

  * 
  Communicated 
  by 
  the 
  Physical 
  Society 
  : 
  read 
  June 
  25, 
  1909. 
  

  

  