3B Scientific Fine Beam Tube T - Manual (Page 9)

3
• Connect the voltmeter in parallel to the 300-V
output.
• Connect the coils in series to the DC power supply
20 V U33020, as shown in Fig. 2, so that equal cur-
rent passes through both coils.
6.1.2 Set up with the DC power supply unit 500 V
U33000
• Set up the tube as in fig. 4.
6.2 Adjusting the electron beam
• Apply a heater voltage of say 7.5 V. (the heater
voltage must be below the cut-off voltage).
• Wait about 1 minute for the heater temperature to
stabilise.
• Slowly increase the anode voltage to 300 V (the
electron beam is initially horizontal and is visible
as a weak, bluish ray).
• Select the Wehnelt voltage so that a very clear and
narrow electron beam is visible.
• Optimise the focus and brightness of the electron
beam by varying the heater voltage.
• Increase the current I
H
passing through the Helm-
holtz coils and check that the electron beam
curves upwards.
• If the electron beam is not deflected at all:
• Reverse the polarity of one of the coils so that
current passes in the same direction through both
coils.
If the electron beam does not curve upwards:
• Swap the connections on the power supply unit to
reverse the polarity of the magnetic field.
• Continue increasing the current passing through
the coils watch until the electron beam forms a
closed circle.
If the path does not form a closed circle:
• Slightly turn the fine beam tube, along with its
base, around its vertical axis.
7. Sample experiment
Determination of the specific charge of an electron
e/m
• Select the current passing through the coils so that
the radius of the circular path is for example 5 cm.
Note the set current value.
• Decrease the anode voltage in steps of 20 V to
200 V. In each case, set the coil current I
H
so that the
radius remains constant. Take down these values.
• Record other series of measured values for radii of
4 cm and 3 cm.
• For further evaluation, plot the measured values in
a graph of r
2 B 2 against 2U (see Fig. 3).
The slope of the line through the origin corresponds to e/m.
ı
O
On Off
PE 0...300 V 6...12 V 0...-50 V
U8521371
Fig. 1 Electrical connections from the fine beam tube to the DC power supply unit 300 V U8521371