804  Dr.  0.  Halm  on  some  Properties  of 
Magnetic  and  Electric  Deflexion  of  the  a  Rays. 
Some  experiments  were  begun  with  the  assistance  of 
Professor  Kutherford  to  see  if  the  a  rays  from  the  active 
deposit  of  thorium  were  deflected  in  a  magnetic  and  electric 
field.  The  experiments  showed  that  the  a  rays  were  deflected 
both  by  a  magnetic  and  electric  field  and  to  about  the  same 
extent  as  the  rays  from  radium  C.  A  brief  account  will  be 
given  here  of  some  preliminary  experiments  on  the  magnetic 
deflexion  of  the  a  rays. 
The  same  arrangement  was  employed  as  that  used  by 
Rutherford  for  the  determination  of  the  magnetic  deflexion 
of  the  a  rays  from  radium  C.  A  tbin  wire,  coated  with  the 
active  deposit  of  thorium,  served  as  the  source  of  rays.  Half 
of  the  active  wire  was  covered  with  a  thin  mica  sheet  while 
the  other  half  was  bare  ;  and  by  means  of  mica  screens  it  was 
arranged  that  one  half  of  tbe  small  photographic  plate  was 
acted  on  by  the  rays  from  the  bare  wire,  and  the  other  half 
by  the  rays  after  traversing  the  mica.  The  apparatus  was 
exhausted  of  air,  and  a  constant  magnetic  field  applied  for 
10  hours.  By  reversing  the  field  at  intervals,  two  sets  of 
bands  were  obtained  on  the  plate.  The  amount  of  deflexion 
of  the  traces  of  the  pencils  of  rays  on  the  two  halves  of  the 
plate  were  directly  compared  with  the  amount  of  deflexion 
observed  under  identical  conditions  of  the  rays  from  radium 
C.  The  photographic  effect  of  the  rays  from  the  active 
deposit  of  radiothorium  was  too  weak  for  reproduction.  It 
was  noted  that  the  edges  of  the  trace  of  rays  on  the  plate 
from  tbe  uncovered  half  of  the  wire  were  not  so  clearly 
defined  as  the  traces  obtained  for  the  rays  which  had  traversed 
the  mica  screen.  The  reason  of  this  difference  was  not  clear 
at  the  time  the  experiments  were  made,  as  the  rays  from  the 
active  deposit  had  not  then  been  shown  to  be  complex.  The 
rays  of  range  4*7  cms.  are  more  deflected  in  a  magnetic  field 
than  those  of  range  8*6  cms.,  and  also  have  a  weaker  photo- 
graphic effect.  The  outside  edge  of  the  trace  obtained  on 
the  plate  is  consequently  due  to  rays  which  have  a  com- 
paratively feeble  photographic  effect  and  the  edge  is, 
consequently,  not  clearly  defined.  The  mica  sheet  was  of 
sufficient  thickness  to  cut  off  most  of  the  rays  of  range  4*6  cms., 
and  the  trace  on  the  other  half  of  the  plate  was  due  to 
homogeneous  rays  and  was,  consequently,  better  defined. 
The  amount  of  deflexion  of  the  thorium  rays  after  passing 
through  the  mica  screen  was  about  20  per  cent,  less  than  for 
the    rays   from  radium  C  after   passing  through  the   same 
