Description
We offer e/m experiment usefull in physics and material science
labs. Our arrangement for measuring e/m, the charge to mass ratio
of the electron is a very simple set-up. It is based on Thomson’s
method. The e/m-tube is bulb-like and contains a filament, a
cathode, a grid, a pair of deflection plates and an anode. The tube
is filled with helium at a very low pressure. Some of the electrons
emitted by the cathode collide with helium atoms which get excited
and radiate visible light. The electron beam thus leaves a visible
track in the tube and all manipulations on it can be seen. The tube
is placed between a pair of fixed Helmholtz coils which produce a
uniform and known magnetic field. The socket of the tube can be
rotated so that the electron beam is at right angles to the
magnetic field. The beam is deflected in a circular path of radius
r depending on the accelerating potential V, the magnetic field B
and the charge to mass ratio e/m. This circular path is visible and
the diameter d can be measured and e/m obtained from the
relation
e/m = 8V/ B2d2
This set-up can also be used to study the electron beam deflection
for different directions of the magnetic field by varying the
orientation of the e/m-tube.
DESCRIPTION OF THE EXPERIMENTAL SET-UP
The central part of the set-up is the e/m-tube. This is
energized by
- Filament current supply,
- Deflection plates voltage supply,
- Continuously variable accelerating voltage supply to the
anode.
The tube is mounted on a rotatable socket and is placed between a
pair of Helmholtz coils. The tube can be rotated about a vertical
axis, varying the orientation of the electron beam with respect to
the Helmholtz coils.This allows magnetic deflection of the
beam to be demonstrated. Circular, helical or undeflected paths can
be seen. The direction of the current can be changed. The
magnetizing current I and the accelerating voltage V are
respectively measured by an ammeter and a voltmeter mounted on the
front of the panel. The diameter of the electron beam path is
measured by a detachable scale mounted in front of the bulb of the
tube. This scale has a slider with a hollow tube (fitted with cross
wires at its both ends) to fix the line of sight while making the
measurements of the beam path diameter. Base of the unit
contains the power supply that provides all the required potentials
and the current to the Helmholtz coils. The entire apparatus
is contained in a wooden case for convenient storage.
Specifications
Helmholtz coils of radii: *4 cm
Number of turns: **0 on each coil
Accelerating Voltage: 0 – **0V
Deflection plates voltage: *0V – **0V
Operating Voltage: **0V AC/ *0Hz
Typical results obtained with the above set-up for variation of the
diameter of the electron beam path with the accelerating voltage
for a current of 1A to the Helmholtz coils are shown in the
following graph. They lead to e/m equal to 1.*7x***1coul /
Kg.
