Objects of the experiment
Principles
Apparatus
Setup
Note:
Perform measurements in a dark chamber.
Helmholtz coils may be charged with more than 2 A for short
time only.
The experimental setup to determine the specific electron
charge is shown in Fig. 2, the electric connections in Fig. 3.
- Disconnect the tube power supply and turn all rotary potentiometers to left catch position.
- Connect the 6.3
-V input end of the fine beam tube to the
6.3-V outlet of the tube power supply.
- Short-circuit the positive pole of the 50-V outlet of the tube
power supply with the negative pole of the 500-V outlet
and connect with the socket “-” of the fine beam tube
(cathode).
- Connect the socket “+” of the fine beam tube (anode) with
the positive pole of the 500-V outlet, the socket W
(Wehnelt-cylinder) with the negative pole of the 50-V outlet.
- In order to measure the acceleration potential U connect
the voltmeter (measuring range 300 V–) to the 500-V outlet.
- Short the deflection plates of the fine beam tube to the
anode.
- Connect the DC power supply and ammeter (measuring
range 3 A–) in series with the Helmholtz coils.
Fig. 2 Experiment setup for determining the specific electron charge
a Helmholtz coils
b Fine beam tubes
c Measuring device
Fig. 3 Electric connection
Carrying out the experiment
- Move the left slide of the measuring device so that its inner
edge, mirror image and escape aperture of the electron
beam come to lay on one line of sight.
- Set the right slide for both inside edges to have a distance
of 8 cm.
- Sight the inside edge of the right slide, align it with its
mirror image and adjust the coil current I until the electron
beam runs tangentially along the slide edge covering the
mirror image (see Fig. 4).
- Reduce the acceleration potential U in steps of 10 V to
200 V and choose the coil current I so that the orbit of the
electron beam has a diameter of 8 cm.
- Record acceleration potential U and coil current I
- Power up the tube power supply and set acceleration
potential U = 300 V.
Thermionic emission starts after warming up for a few minutes.
- Optimize focussing of the electron beam by varying the
voltage at the Wehnelt-cylinder from 0...10 V until it leads
to a narrow, well defined beam with clear edge definition.
- Connect the DC power supply of the Helmholtz coils and
look for current I, at which the electron beam is deflected
into a closed orbit.
If the electron beam after leaving the anode is deflected to
the wrong (left) side:
- disconnect both power supplies.
- exchange the connections at the DC power supply in order
to change the polarization of the magnetic field.
If the electrons do not move on a closed orbit but on a helical
curve line:
- Loosen the mounting bolts of both holding brackets (read
the information manual for the fine beam tube).
- Carefully rotate the fine beam tube around its longitudinal
axis, until the electron beam runs on a closed circular orbit.
- Fasten mounting bolts.
Calibration of the Helmholtz magnetic field (optional):
The setup for calibrating the magnetic field is shown in Fig. 5.
The additionally recommended devices mentioned above are
required for making measurements.
- If applicable disconnect all power supply units.
- Remove the measuring device and the Helmholtz coil at
the front side, loosen the connection to the fine beam tube
and the mounting bolts of the two holding brackets (read
the instructions for the fine beam tube).
- Carefully remove the fine beam tube and place it e.g. in its
original case.
- Re-assemble the Helmholtz at the front side coil and connect.
- Connect the axial B-probe to the Teslameter (measuring
range 20 mT) and calibrate the zero-point (see Instruction
Manual for Teslameter).
- Move the axial B-probe parallel to the magnetic field of the
Helmholtz coils into the center of the pair of coils.
- Raise the coil current I from 0 to 3 A in steps of 0.5 A,
measure the magnetic field B, and record the measured
values.
After conclusion of the calibration:
- Reassemble the fine beam tube according to the instructions.
Measuring example
Evaluation and results
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