Rate  of  Decay  of  Active  Deposit  from  Radiu 
m. 
145 
The  advantages  of  this  "  constant  deflexion  "  method  are 
obvious :  deflexions  are  independent  of  the  capacity,  and 
testing  vessels  may  therefore  be  interchanged  without  error, 
measurements  can  be  made  over  a  wide  range  without  any 
difficulty,  and  observations  can  be  taken  instantaneously  and 
in  as  rapid  succession  as  desirable.  In  some  cases  they  were 
taken  at  five-second  intervals. 
The  precision  of  an  individual  measurement  is,  however, 
limited  by  a  slight  oscillation  of  the  needle.  Repeated 
attempts  have  been  made  to  eliminate  this  by  better  shielding 
from  external  electrostatic  action,  and  by  careful  attention 
to  contacts,  but  all  to  no  purpose.  The  most  probable  ex- 
planation seems  to  be  that  it  is  due  to  exceedingly  small  and 
rapid  changes  in  the  ionization  current  itself.  This  difficulty, 
however,  is  not  serious,  for  it  seldom  causes  an  error  in  a 
single  observation  of  as  much  as  one  per  cent. 
In  the  actual  carrying  out  of  the  experiments  the  following- 
arrangement  of  apparatus  was  found  to  be  the  most  convenient 
and  satisfactory  : — 
Fiff.  2. 
Testing  Vessel 
The  electrometer  was  of  the  ordinary  quadrant  type, 
except  that  it  was  fitted  with  a  very  light  needlo  of  silvered 
mica,  suspended  by  a  fine  phosphor-bronze  wire.  The 
needle  was  permanently  connected  to  a  battery  of  small 
accumulators,  which  kept  its  potential  constant.  As  ordinarily 
used  the  needle  was  kept  at  a  potential  of  85  volts,  and  gave 
a   deflexion  of   15   cm.   for  a  potential-difference  of   1  volt 
Phil.  Mag.  S.  6.  Vol.  11.  No.  61.  Jan.  1906.  L 
