Bronson  found  that  such  a  constant  air  resistance,  suitable  for
measurement  over  a  large  range,  could  be  constructed  by  using  as
a  source  of  radiation  a  small  plate  on  which  a  trace  of  radium  salt
had  been  evaporated.  This  was  placed  at  the  bottom  of  an  ebonite
cylinder  with  the  upper  electrode  several  centimetres  above  it.
*  W.  Campbell  {Phil,  Mag.  21,  p.  301,  1911)  states  that  a  resistance  consisting
of  a  mixture  of  xybol  and  absolute  alcohol  in  thermometer  tubing  is  very  suitable
for  the  purpose.
t  Bronson,  Amer,  Journ,  Sci,  10,  p.  185,  1906.
Ill]  METHODS  OF  MEASUREMENT  103

The  apparatus  was  completely  sealed  to  avoid  the  escape  of
emanation.  This  air  resistance  obeyed  Ohm's  Law  over  a  con-
sidemble  range,  so  that  the  steady  deflection  of  the  electrometer
was  pmportional  to  the  current.  By  applying  a  suitable  voltage
to  the  radium  plate  in  order  to  balance  the  greater  part  of  the
ionisation  current  which  was  being  measured,  the  electrometer
can  be  employed  to  measure  currents  accurately  over  a  wide
range.  When  the  ionisation  current  was  due  to  a  rays,  Bronson
observed  that  the  electrometer  needle  never  came  to  rest  but
oscillated  irregularly  about  a  mean  position.  This  is  now  known
to  be  due  to  the  probability  variations  in  the  rate  of  emission  of
a  particles.     (Section  75.)

41    Balance  and  compeniation  methods.    The  increasing

importance  of  accurate  measurements  of  ionisation  currents  has

led  to  the  use  of  a  number  of  compensation  and  balance  methods.
