produced  by  JRontgen  Rays  in  Different  Metals.       299 
very  hard  there  was  little  difficulty  with  the  insulation,  though 
there  was  often  a  good  deal  of  brush  discharge  from  the  leads. 
The  earthed  metallic  case  of  the  radiometer  protected  it  com- 
pletely from  any  electrostatic  disturbance.  A  lead  screen, 
2  mm.  thick,  which  hung  by  a  quadriiilar  suspension  from  the 
ceiling  about  2  cm.  in  front  of  the  aluminium  window, 
permitted  both  strips  to  be  exposed  to  the  Rontgen  rays,  or  both 
screened,  or  either  exposed  while  the  other  was  screened.  A 
large  cardboard  screen  was  kept  between  the  Rontgeri  bulb 
and  the  radiometer,  to  diminish  the  effects  upon  the  instrument 
due  to  the  heating  of  the  bulb. 
Measurements  of  the  absorption  of  the  lead  and  zinc  were 
carried  on  simultaneously  with  the  radiometer  observations. 
For  this  purpose  an  electroscope  was  set  up  270  cm.  from  the 
bulb  ;  it  had  an  aluminium  window  2  cm.  in  diameter  and 
the  remainder  was  covered  with  sheet  lead  2  mm.  thick.  As 
this  did  not  sufficiently  protect  it  from  the  rays,  a  lead  wall 
was  built  up  of  blocks  two  inches  thick  between  the  electro- 
scope and  bulb  with  a  hole  opposite  the  window ;  this  gave 
satisfactory  protection  when  the  window  was  screened. 
Observations  were  made  with  a  micrometer-microscope  and 
stop-watch  in  the  usual  manner. 
Experiments. 
The  experimental  method  was  based  upon  the  following- 
considerations  : — The  Rontgen  rays  absorbed  in  the  strips  will 
generate  heat  throughout  the  mass  of  the  metals  and  a  steady 
state  of  temperature  will  be  reached  when  the  heat  generated 
per  second  is  equal  to  the  heat  lost  per  second  through  the 
two  surfaces  by  radiation  and  convection.  (The  possibility  of 
any  appreciable  loss  by  conduction  through  the  ebonite  support 
of  the  strips  will  be  considered  later.)  The  heat  lost  through 
any  surface  is  proportional  to  its  emissivity  and,  for  such 
small  temperature  differences,  to  its  excess  of  temperature 
above  its  surroundings.  The  lead  and  zinc  strips  had  the 
same  surface — aluminium  leaf — and  it  is  natural  to  assume 
that  the  emissivity  is  the  same  in  both  cases  ;  this  assumption, 
however,  will  be  justified  experimentally.  Moreover,  it  will 
appear  from  theoretical  considerations  that,  in  the  steady 
state,  the  difference  in  temperature  between  the  front  and  back 
surfaces  of  either  metal  is  a  small  fraction  of  the  excess  of  its 
temperature  above  its  surroundings;  and  this  too  will  he 
confirmed  experimentally.  It  follows  therefore  that,  very 
approximately,  half  the  heat  generated  in  either  strip  is  lost 
through  each  of  its  two  surfaces,  and  that  the  total  heat 
generated  per  second  will  be  proportional  to  the  temperature 
