produced  by  Rdntgen  Rays  in  Different  Metals. 
305 
is  shown  in  fig.  5  below.      The  following  values  of  y  are 
obtained  in  the  same  manner  from  these  curves  :  — 
Zinc.  Lead. 
V. 
4-0 
20 
1-0 
dV 
dt- 
245 
1-22 
0-57 
7- 
0o4 
061 
0  57 
Average    
057 
dV 
V. 
df 
7- 
40 
5-2 
1-3 
2-0 
26 
1-3 
1-0 
1-1 
1-1 
Average    
1-23 
It  will  be  seen  that  the  agreement  is  as  good  as  could  be 
expected  from  the  very  rough  nature  of  the  determinations. 
Returning  now  to  the  experiments  plotted  in  fig.  3,  it 
is  to  be  observed  that  the  steady  deflexion  for  lead  is  nearly 
twice  as  great  as  that  for  zinc.  The  fractions  of  the  incident 
rays  absorbed  by  the  two  strips  were  determined  by  means  of 
the  electroscope  with  the  pieces  of  metal  from  which  the  strips 
had  been  cut  ;  the  pieces  of  lead  and  zinc  were  placed  behind 
a  sheet  of  aluminium  of  the  same  thickness  as  the  window  of 
the  radiometer.  It  was  found  that  the  lead  absorbed  79  per 
cent,  and  the  zinc  78  per  cent,  of  the  rays  which  got  through 
the  aluminium  :  in  this  particular  case  the  spark-gap  in  the 
automatic  adjuster  of  the  Rontgen  bulb  was  7*5  cm.  long. 
Thus  (unless  there  has  been  some  error  in  the  experimental 
method  or  in  the  interpretation  of  the  result)  it  appears  that 
for  practically  equal  absorptions  of  Rontgen  rays  in  lead 
and  in  zinc,  about  twice  as  much  energy  is  generated  in  the 
lead  as  in  the  zinc.  It  may  be  said  at  once  that  essentially 
the  same  result  was  obtained  in  all  the  experiments,  with  rays 
of  different  hardness,  with  both  positions  of  the  strips  and  with 
the  radiometer  vanes  sometimes  between  the  window  and  the 
strips,  as  well  as  in  the  position  indicated  in  fig.  1.  Before 
giving,  however,  the  numerical  results  of  all  the  experiments 
it  will,  I  think,  be  conducive  to  clearness  to  consider  the 
possible  sources  of  error  (so  far  as  they  have  occurred  to  me) 
and  their  influence  upon  the  result. 
Possible  Sources  of  Error. 
1.  After  it  had  been  found  that  the  lead  strip  predominated, 
and  before  the  rates  of  heating  and  cooling  of  the  strips  had 
been  worked  out,  an  effect  was  observed  which  caused  con- 
siderable perplexity.  The  slight  drift  of  the  zero-point  due 
to  the'warming  up  of  the  room,  after  the  observer  entered, 
Phil  Maq,  S.  6.  Vol.  11.  No.  62.  Feb.  1906.  X 
