300         Prof.  H.  A.  Bumstead  on  the  Beating  Effects 
of  either  surface  in  the  steady  state,  and  hence  to  the  repulsion 
of  the  radiometer  vane  exposed  to  that  surface. 
The  first  step  in  a  series  of  experiments  was  to  test  the 
balance  of  the  radiometer  by  exposing  the  two  lead  strips 
simultaneously  to  the  Rontgon  rays.  The  balance  was  usually 
found  to  be  fairly  good,  and  could  be  to  some  extent  adjusted 
by  moving  the  bulb  horizontally  before  the  window;  the 
stand  carrying  the  bulb  could  be  moved  by  a  horizontal  screw^ 
for  this  purpose.  In  two  of  the  series  of  experiments  (quoted 
below)  it  was  found  impossible  to  get  a  good  balance  by- 
moving  the  bulb  ;  in  these  cases  the  lack  of  balance  was 
determined  and  applied  as  a  correction.  The  wheel  was  then 
clicked  round  to  the  second  position,  in  which  one  vane  of  the 
radiometer  was  opposite  to  a  lead  strip  and  the  other  opposite 
to  a  zinc  strip.  This  was  a  somewhat  delicate  operation  as 
each  click  caused  a  violent  disturbance  of  the  radiometer 
(partly  magnetic  and  partly  thermal),  and  it  was  necessary  to 
get  it  under  control  by  means  of  a  subsidiary  magnet  before 
making  another  click  ;  otherwise  the  vanes  might  have 
become  entangled  in  the  moving  wheel  and  the  suspension 
broken.  After  the  shift  from  one  position  to  another  several 
hours  had  to  elapse  before  the  radiometer  was  again  fit  for 
use.  Several  exposures  to  Rbntgen  rays  were  then  made  in 
which  the  zinc  and  lead  strips  were  exposed  separately  and 
both  together.  The  wheel  was  then  moved  to  the  third 
position,  in  which  the  zinc  and  lead  strips  were  in  reversed 
order,  and  similar  observations  taken.  It  may  be  said  at 
once  that  the  reversed  position  of  the  lead  and  zinc  did  not 
alter  the  character  of  the  results. 
The  general  nature  of  the  results  may  be  most  readily  set 
forth  by  considering  a  particular  experiment  as  an  example. 
In  fig.  3  the  results  of  this  experiment  are  shown  graphically  ; 
the  abscissae  represent  time  in  minutes,  the  ordinates  deflexions 
of  the  radiometer  in  centimetres  on  the  scale  ;  the  position  of 
the  wdieel  was  such  that  a  positive  deflexion  means  a  repulsion 
by  the  zinc  of  its  vane,  a  negative  deflexion  a  repulsion  by  the 
lead  of  its  vane.  Between  2m  and  5m,  the  zinc  strip  was 
exposed  to  the  rays ;  the  rays  were  then  cut  off  and  the  zinc 
allowed  to  cool  for  5  minutes  (5m-10m),  the  lead  was  then 
exposed  for  3  minutes  (10m-13m)  ;  it,  in  turn,  was  allowed 
to  cool  for  5  minutes  (13m-18m)  and  then  both  strips  exposed 
simultaneously  for  3  minutes  (18m-21m),  after  which  the  rays 
were  again  cut  off.  It  is  plain  from  the  figure  that  neither 
metal  was  exposed  long  enough  to  the  rays  for  the  steady 
state  to  be  attained;  but  it  is  also  plain  that  the  great 
preponderance  of  the  lead  over  the  zinc  is  not  due  to  this 
