[     113     ] 
XL  The  Effect  of  High  Temperatures  on  the  Rate  of  Decay 
of  the  Active  Deposit  from  Radium.  By  Howard  L. 
Bronson,  Ph.D.* 
A  PRELIMINARY  account  o£  the  method  and  some  of 
the  results  obtained  have  already  been  given  in  the 
American  Journal  of  Science  for  February  and  July  1905. 
As  the  method  is  new  and  has  proved  very  satisfactory  in 
obtaining  the  decay  curves  of  radioactive  substances,  it  is 
here  given  in  some  detail. 
Apparatus  and  Method. 
In  the  ordinary  method  of  comparing  ionization  currents 
by  an  electrometer,  the  rate  of  movement  of  the  needle  is 
taken  as  a  measure  of  the  current.  In  doing  this,  it  is 
assumed  that  the  capacity  of  the  system  and  the  lag  of  the 
needle  behind  the  potential  is  the  same  for  different  rates. 
These  assumptions  may,  in  some  cases,  be  warranted,  but 
they  certainly  are  not  when  the  currents  compared  differ 
greatly  in  magnitude,  or  when  the  needle  is  moving  rapidly. 
In  the  latter  case  capacities  may  be  added  in  parallel,  which 
reduces  the  rate  of  movement,  but  creates  the  difficulty  of 
comparing  capacities.  This  takes  time  and  is  never  entirely 
satisfactory.  A  still  greater  difficulty  is  experienced  with 
the  "  rate  "  method,  when  rapidly  changing  ionization  cur- 
rents are  to  be  measured,  because,  from  the  nature  of  the 
method,  the  observations  can  be  neither  instantaneous  nor 
taken  in  rapid  succession.  The  desirability,  therefore,  of  a 
more  direct  and  rapid  method  of  measurement  is  evident. 
The  following  method,  suggested  by  Professor  Rutherford, 
has  been  found  by  the  writer  to  be  very  convenient  and 
satisfactory  for  all  radioactive  measurements  thus  far 
attempted.     The  theory  of  the  method  is  as  follows  : — 
The  arrangement  of  the  apparatus,  as  seen  in  fig.  1  (p.  144), 
differs  only  from  that  ordinarily  used  in  having  one  pair  of 
quadrants  of  the  electrometer  connected  not  only  to  the  testing 
vessel,  but  also  to  earth  through  a  very  high  resistance.  It  is 
evident  in  this  case  that,  when  the  key  is  closed,  the  ioniza- 
tion current  from  the  testing  vessel  will  continue  to  increase 
the  potential-difference  of  the  quadrants  until  the  conduction 
current  through  the  high  resistance  becomes  equal  to  the 
ionization  current.  Since  this  conduction  current  is  propor- 
tional to  the  potential-difference,  it  follows  that,  when  equi- 
librium is  reached,  the  ionization  current  also  will  be  pro- 
portional  to   it.     If  the   electrometer  is  well  adjusted,   the 
*  Communicated  by  Prof.  E.  Rutherford,  F.R.S. 
