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Fine beam tube or Double beam tube – measurement of e/m for the electron
This tube is different from the deflection tube in that it is filled with low pressure helium gas. The
advantage of this is that you can see the path of the beam of electrons since they ionise the gas
which then glows as the electrons collide with helium atoms. The greater the potential difference
between the anode and the cathode the further the electrons will travel through the gas before
they lose their energy and are brought to rest. Put more exactly the range of the beam can be
shown to be proportional to the accelerating voltage and therefore to the electron's energy.
Tangential electron gun
Low pressure
helium gas
Radial electron gun
Figure 1
Magnetic field
perpendicular to the
Helmholtz coils
There are also two electron guns, one firing the beam tangentially and the other radially through
the tube (See Figure 1).
For the tangential gun
½ mv2 = eV and
Bev = mv2/R
where v is the electron velocity. So combining these two equations:
Double beam tube:
e/m = 2V/B2R2