308         Prof.  H.  A.  Bumstead  on  the  Heating  Effects 
and  zinc  are  separately  illuminated,  are  equal.  The  contrast 
with  the  effects  of  the  Rontgen  rays  is  sufficiently  marked. 
I  have  exposed  both  strips  at  once  to  weak  Rontgen  rays 
(giving  a  deflexion  of  about  2  cm.)  for  30  minutes  without 
observing  any  tendency  o£  the  radiometer  to  return  to  zero. 
2.  At  first  sight  there  appears  to  be  a  possibility  that  (on 
account  of  different  heat-conductivities,  &c.)  one  of  the  metals 
might  lose  more  heat  through  the  front  surface  and  the  other 
through  the  rear  surface,  thus  giving  rise  to  different  surface 
temperatures,  even  though  the  quantities  of  heat  generated 
were  the  same  in  both  cases.  A  number  of  considerations 
show  that  this  cannot  account  for  the  effects  observed.  It 
is  easy  to  show  that,  when  all  the  heat  enters  through  the 
front  surface,  the  ratio  of  the  temperature  of  the  front  face  to 
that  of  the  rear  face  will  be  (when  the  steady  state  is  attained) 
T,     "•"*' 
and  the  difference  will  be  less  than  this  when  the  heat  is 
generated  throughout  the  interior  of  the  metals.  Now  a 
superior  limit  to  the  value  of  k  may  be  readily  obtained,  from 
the  rates  of  cooling  of  the  strips,  by  assuming  infinite  con- 
ductivity in  the  metals  so  that  the  whole  strip  has  at  any 
instant  the  same  temperature  as  the  surface.  Under  these 
circumstances,  we  have 
where  C  is  the  specific  heat,  p  the  density,  and  I  the  thick- 
ness of  the  metal.     Also 
so  that 
h  =  j>yCpl. 
With  a  given  curve  of  cooling  of  the  surface  and  finite  con- 
ductivity, the  value  of  h  will  be  less  than  this  ;  because 
when  the  surface  has  cooled  to  any  temperature  the  interior 
will  have  a  higher  temperature,  and  thus  not  so  much  heat 
need  be  emitted  in  a  given  time  as  when  the  conductivity  is 
infinite.     In  this  way  we  find  that  for  the  lead  strip 
/k-007, 
for  the  zinc  strip 
A<-002, 
and  taking  h  =  '01  we  find,  in  the  worst  case, 
^  <  1-004. 
