produced  by  Rontgen  Rays  in  Different  Metals.       297 
carried  the  ebonite  disk  was  provided  with  a  ratchet-wheel 
with  sixty  teeth;  and  the  disk  could  be  rotated  from  one 
position  to  another  by  an  electromagnet  inside  the  radio- 
meter-case. The  current  for  this  purpose  was  led  in  by  means 
of  wires  passing  through  small  glass  tubes  sealed  with  sealing- 
wax,  into  holes  in  the  base-plate  of  the  instrument.  Certain 
marks  on  the  edge  of  the  ebonite  disk  which  could  be  seen 
through  the  observing  window  enabled  one  to  be  sure  that 
the  disk  was  in  proper  position  in  any  given  case.  All  the 
metal  strips,  before  being  mounted  on  the  disk,  were  covered 
on  both  sides  with  leaf  aluminium  which  was  held  by  the 
thinnest  possible  layer  of  soft  wax,  put  on  when  the  metal 
was  hot.  This  was  to  give  both  metals  the  same  surface,  so 
that  the  loss  of  heat  from  the  surface  for  a  given  rise  in 
temperature  might  be  the  same  in  both  cases.  The  ebonite 
disk  was  also  covered  with  aluminium  leaf  to  avoid  electrostatic 
effects  ;  to  the  same  end  a  small  quantity  of  an  impure 
radium  salt  was  put  inside  the  case  in  a  small  open  dish. 
All  the  metal  strips  were  connected  to  the  shaft  of  the  wheel 
by  thin  copper  wires  ;  the  shaft  in  turn  was  connected  to  the 
case  and  to  earth. 
The  portions  of  the  apparatus  hitherto  described  stood 
upon  a  brass  base-plate  turned  flat  and  smooth,  which  rested 
on  three  levelling-screws.  Through  this  plate  passed  the 
tube  which  led  to  the  P205  bulb,  pump,  and  McLeod  gauge. 
The  whole  was  covered  with  a  heavy,  cylindrical,  brass 
casting,  12*5  cm.  in  internal  diameter,  29  cm.  high,  and  with 
walls  1*4  cm.  thick.  This  heavy  metal  case  was  found  to  be 
necessary  owing  to  the  sensitiveness  of  the  radiometer  to 
thermal  disturbances ;  with  a  blackened  glass  bell-jar  which 
was  first  tried,  the  zero  was  so  unsteady  that  nothing  could 
be  done  Avith  the  instrument.  As  a  further  protection 
against  thermal  disturbance,  the  cover  and  base  were 
surrounded  with  cotton-wool  and  a  felt  jacket  drawn  over 
the  whole.  The  case  was  provided  with  two  windows, 
2*9  cm.  in  diameter;  one  of  these  (A,  fig.  1)  was  covered 
with  sheet  aluminium  1*2  mm.  thick,  and  served  to  admit 
the  beam  of  liontgen  rays ;  the  other  was  of  glass  for 
observing  the  deflexions.  Both  were  put  on  with  sealing- 
wax  and  the  joints  covered  with  soft  wax.  The  inside  of  the 
cover  was  painted  with  lampblack  in  alcohol  with  a  little 
shellac  to  make  it  stick.  The  bottom  of  the  cover  was 
turned  flat,  and  the  joint  between  it  and  the  brass  plate 
was  surrounded  by  a  mixture  of  equal  parts  by  weight  of 
vaseline,  paraffin,  and  rubber  which,  after  being  put  on,  was 
glazed  over  with  a  small  gas-flame.      'When  all   the   joints 
