produced  by  Rotitgen  Rays  in  Different  Metals.        315 
aluminium  was  interposed.  The  last  experiment  in  the 
table  is  the  one  made  with  the  radiometer  instead  of  the 
electroscope. 
Absorptions. 
Experiment. 
Al. 
Pb. 
Zn. 
Pb 
zTr 
I 
1 
:::::: 
0-57 
0-45 
0-47 
0-45 
031 
0-34 
0-32 

0-925 
0926 
0944 
0-862 
0-790 
0-830 
0-809 
0-684 
0-676 
0-075 
0907 
0-924 
0-920 
0947 
0-856 
0-780 
0-803 
0-805 
0-684 
0-648 
0655 
0-880 
1-001 
1-007 
0-997 
1-007 
1-013 
1-034 
1-005 
1-000 
1043 
1-031 
1-030 
II 
Ill 
IV 
V 
VI 
VII 
VIII.     , 
IX.     .. 
X 
XI 
Average  
1-016 
The  average  value  of  the  ratio  is  applied  as  a  correction  in 
the  energy  measurements  below. 
The  energy  measurements  are  given  in  the  following  table. 
Column  I.  gives  the  time  of  exposure  of  the  strips  in  minutes. 
Columns  II.  and  III.  give  the  deflexions  in  centimetres, 
produced  by  exposure  of  the  lead  and  zinc  respectively. 
These  are  the  observed  deflexions,  corrected  for  the  effect 
of  the  induction-coil  upon  the  magnetic  "  torsion-head "  ; 
this  was  always  down-scale  and  varied  from  0*1  to  0'3  cm. ; 
it  was  determined  each  time  by  allowing  the  coil  to  run  with 
the  lead  screen  in  front  of  the  window  of  the  radiometer, 
The  plus  and  minus  signs  indicate  the  direction  of  the  de- 
flexion ;  thus  a  change  of  sign  in  either  column  represents  a 
shift  of  the  wheel,  so  that  the  positions  of  the  strips  are 
interchanged.  Columns  IV.  and  V.  contain  the  deflexions 
corrected  for  lack  of  balance  of  the  radiometer.  In  experi- 
ments 1,  2,  3,  and.  9,  the  radiometer  was  sensibly  balanced 
when  tested  by  the  two  lead  strips  ;  in  4,  5,  and  Q,  there  was 
a  lack  of  balance  of  15  per  cent,  in  the  positive  direction  ; 
in  7  and  8  the  lack  of  balance  was  10  per  cent,  in  the  same 
direction.  Accordingly,  in  the  first  case  the  positive  de- 
deflexions  (zinc)  are  reduced  15  per  cent.,  in  the  second  case 
the  positive  deflexions  (lead)  are  reduced  10  per  cent.  The 
changes  in  the  conditions  are  explained  by  the  fact  that 
between  the    groups   of   experiments  mentioned   the   radio- 
