Determination of specific charge (e/m) of an electron – Thomson’s method.

Determination  of  specific  charge  (e/m)  of  an  electron  –
Thomson’s method.

In  1887, J.J.  Thomson,  measured  the  specific  charge  (e/m)  of
the  cathode  ray  particles.  The  specific  charge  is  defined  as  the  charge
per  unit  mass  of  the  particle.  Thomson  discovered  that  the  value  of
(e/m)  was  independent  of  the  gas  used  and  also  independent  of  the
nature  of  the  electrodes.

Principle
The  fact  that  the  cathode  rays  (electrons)  are  deflected  by
electric  and  magnetic  fields  is  made  use  of  in  this  method.
Experimental arrangement

A  highly  evacuated  discharge  tube  used  in  this  experiment.
Cathode rays are produced by the discharge between the cathode and the anodes.
A thin pencil of cathode
ray comes out through fine pin
holes  in  the  anode  discs.  The
cathode   rays   then   pass
between  two  parallel  metal
plates as shown in figure
 and strike the flat  face  of  the  tube.  This  face
is   coated   with   suitable
fluorescent material. A spot of
light  is  produced   But  when  a  potential  difference  V  is  applied
between tow plates, the beam is deflected By the use of a pair
of  coils,  uniform  magnetic  field  is  produced  perpendicular  to  the  plane
of  the  paper  and  outwards  through  out  the  region  between vertical deflection plates.