1
012-03471E
e/m Apparatus
!
Introduction
The PASCO Model SE-9638 e/m Apparatus provides a
simple method for measuring e/m, the charge to mass ratio
of the electron. The method is similar to that used by J.J.
Thomson in 1897. A beam of electrons is accelerated
through a known potential, so the velocity of the electrons is
known. A pair of Helmholtz coils produces a uniform and
measurable magnetic field at right angles to the electron
beam. This magnetic field deflects the electron beam in a
circular path. By measuring the accelerating potential (V),
the current to the Helholtz coils (I), and the radius of the
circular path of the electron beam (r), e/m is easily calcu-
lated: e/m = 2V/B
2
r
2
. (The calculations are explained in the
operation section of this manual.)
The e/m apparatus also has deflection plates that can be used
to demonstrate the effect of an electric field on the electron
beam. This can be used as a confirmation of the negative
charge of the electron, and also to demonstrate how an
oscilloscope works.
A unique feature of the e/m tube is that the socket rotates,
allowing the electron beam to be oriented at any angle (from
0-90 degrees) with respect to the magnetic field from the
Helmholtz coils. You can therefore rotate the tube and
examine the vector nature of the magnetic forces on moving
charged particles. Other experiments are also possible with
e/m tube
Helmholtz coils
Controls
Figure 1 The e/m Apparatus
Equipment
the e/m tube. For example, you can use a small permanent
magnet instead of the Helmholtz coils to investigate the
effect of a magnetic field on the electron beam.
Mirrored
scale
Electron Gun
Helium Filled
Vacuum tube
Figure 2 e/m Tube
Deflection Plates
The e/m Tube
⎛
The e/m tube (see Figure 2) is filled with
helium at a pressure of 10
-2
mm Hg, and contains an electron
gun and deflection plates. The electron beam leaves a visible
trail in the tube, because some of the electrons collide with
helium atoms, which are excited and then radiate visible
light.
The electron gun is shown in Figure 3. The heater heats the
cathode, which emits electrons. The electrons are accelerated
by a potential applied between the cathode and the anode.
The grid is held positive with respect to the cathode and
negative with respect to the anode. It helps to focus the
electron beam.
CAUTION:
The voltage to the heater of the
electron gun should NEVER exceed 6.3 volts,
unless noted otherwise on tube. Higher voltages
will burn out the filament and destroy the e/m tube.