HO  Prof.  E.  Rutherfo 
I'd  on  some 
Retardation  of  the  Velocity  of  the  Particles  from  Radium  C. 
In  my  previous  paper,  I  showed  that  in  a  constant  mag- 
netic field  reversed  at  intervals,  the  width  between  the 
two  bands  obtained  on  the  photographic  plate  steadily  in- 
creased when  successive  layers  of  foil  were  placed  over  the 
active  wire  coated  with  radium  C.  A  clear  photographic 
impression  was  observed  with  twelve  layers  of  foil  each 
'00031  cm.  thick  placed  over  the  wire.  At  this  stage,  the 
distance  between  the  bands  indicated  that  the  velocity  of  the 
«  particles  had  been  reduced  to  62  per  cent,  of  their  maximum 
value.  Using  13  layers  of  foil,  I  was  unable  to  observe  any 
photographic  effect  of  the  rays. 
I  have  repeated  these  experiments,  and  have  obtained  sub- 
stantially the  same  results.  A  photographic  effect  was  observed 
at  12  layers,  but  disappeared  with  13  layers. 
These  experiments  have  clearly  brought  out  the  fact  that 
the  photographic  effect  produced  by  the  a  rays  decreases  with 
the  thickness  of  foil  traversed,  and  falls  off  very  rapidly 
between  11  and  13  layers  of  foil.  Experiments  are  at  present 
in  progress  to  determine  as  accurately  as  possible  the  variation 
of  the  velocity  of  the  a  particles  between  these  limits. 
For  the  purpose  of  illustration  of  the  increased  width  of  the 
bands  after  passing  aluminium,  I  have  used  the  arrangement 
employed  by  M.  Becquerel,  described  above. 
The  rays  from  the  active  wire  were  divided  into  two  parts  by 
mica  screens.  On  one  side  the  active  wire  was  bare,  and  on 
the  other  covered  with  8  layers  of  foil.  The  photograph 
obtained  in  a  vacuum  is  shown  in  fig.  1  (PI.  IV.). 
In  order  to  have  about  the  same  intensity  of  photographic 
effect  for  the  two  sets  of  rays,  the  screens  were  placed  so  as 
to  cover  about  |  of  the  length  of  active  wire  with  the 
aluminium-foil.  The  greater  divergence  of  the  rays  after 
passing  through  8  layers  of  foil  is  clearly  shown  in  the  figure. 
Both  sets  of  lines  are  well  defined  with  fairly  sharp  edges  *. 
*  In  the  figure,  the  width  of  the  lines  after  traversing  the  aluminium 
foil  is  seen  to  be  somewhat  broader  than  the  others.  This  difference  is 
much  more  marked  than  I  have  obtained  in  other  and  similar  experiments. 
This  broadening  resulted  from  an  accident  during  the  exposure  of  the 
wire  to  the  emanation.  In  order  to  obtain  a  very  active  wire,  too  high  a 
voltage  was  applied  to  the  wire,  and  there  was  a  discharge  across  the 
gas.  This  caused  a  black  deposit  near  one  end  of  the  copper  wire,  and 
this  portion  of  the  wire  was  afterwards  placed  under  the  aluminium. 
Some  of  the  rays,  before  escaping  from  the  wire,  had  to  pass  through  this 
deposit,  and  consequently  were  slightly  reduced  in  velocity.  The  rays 
consequently  were  not  homogeneous  in  the  beginning.  As  the  photograph, 
however,  very  clearly  brings  out  the  retardation  of  the  velocity  of  the 
ex.  particles,  it  was  not  thought  necessary  to  repeat  this  experiment  under 
more  ideal  conditions. 
