produced  by  Rontgen  Rays  in  Different  Metals.        295 
of  the  suspended  system.  The  inner  edges  of  the  vanes 
were  4  mm.  apart.  The  various  joints  were  made  by  very 
small  drops  of  alcoholic  shellac  which  were  baked  on  with  a 
hot  glass  rod.  The  system  was  put  together  on  a  flat  brass 
table  and,  after  a  few  trials,  it  was  not  difficult  to  get  one  in 
which  the  aluminium  vanes  were  very  fairly  smooth,  plane, 
and  parallel  to  the  central  rod.  The  mirror,  for  use  with 
telescope  and  scale,  was  attached  to  the  central  rod  3  cm. 
above  the  middle  of  the  vanes  ;  it  was  usually  about  5  mm. 
square  and  was  made  of  specially  selected  microscope  cover- 
glass.  All  the  mirrors  used  gave  very  satisfactory  definition. 
In  the  final  system,  the  weight  of  the  mirror  was  13  milligrams, 
that  of  the  rest  of  the  suspended  system  5  miligrams  ;  the 
moment  of  inertia  was  approximately  0*002  grm.-cm.2  The 
two  aluminium  vanes  were  electrically  connected  by  means 
of  a  bit  of  light  phosphor-bronze  strip.  The  other  details 
of  the  suspension  are  shown  in  figs.  1  and  2.  The  working 
quartz-fibre,  F,  is  3  cm.  long,  and  is  suspended  from  a  glass 
rod  carrying  a  small  magnetic  needle,  N  :  as  this,  in  turn, 
is  suspended  by  a  second  fibre,  F',  it  may  be  caused  to  rotate 
by  means  of  the  bent  magnet  outside  the  case  ;  it  thus  serves 
as  a  sort  of  torsion  head  for  controlling  the  zero  of  the 
radiometer.  This  very  elegant  device  is  borrowed  from  the 
work  of  Nichols  and  Hull  on  the  pressure  of  radiation. 
The  metals  whose  heating  effects  were  to  be  compared 
were  mounted  upon  an  ebonite  disk  (W,  figs.  1  and  2)  6  mm. 
thick  and  7*8  cm.  in  diameter.  Three  rectangular  holes, 
3'6  x  1*4  cm.,  were  cut  in  this  disk,  at  an  angular  distance 
from  each  other  of  120°.  Across  these  holes  strips  of  the 
metals  under  investigation  were  fastened  with  a  little  soft 
wax ;  the  strips  were  2  cm.  long  and  1  cm.  wide  and  their 
inner  edges  were  4  mm.  apart.  Up  to  the  present,  1  have 
had  time  to  compare  only  two  metals,  lead  and  zinc.  The 
lead  strips  were  0'30  mm.  thick  and  the  zinc  strips  0'82  mm., 
these  thicknesses  being  chosen  because  they  gave  nearly  the 
same  absorption  of  the  liontgen  rays  used,  as  will  appear 
later.  The  thinner  lead  strips  were  blocked  up  from  the 
wheel  on  small  bits  of  card,  so  that  the  surfaces  of  lead  and 
zinc  towards  the  radiometer  vanes  were  in  the  same  plane. 
The  arrangement  of  the  metal  strips  on  the  ebonite  disk  was 
as  follows  :  across  one  of  the  rectangular  openings,  two  lead 
strips  were  placed ;  this  was  for  testing  the  balance  of  the 
radiometer  when  both  vanes  were  influenced  by  the  same 
metal ;  across  the  second  opening  a  lead  and  a  zinc  strip  were 
placed  side  by  side,  while  the  third  opening  also  contained  a 
lead  and  a  zinc  strip  but  in  reversed  order.     The  shaft  which 
