3B Scientific Fine Beam Tube (Neon-filled) on Connector Base - Manual (Page 3)

3
5. Additionally required equipment
1 DC power supply 300 V (@230 V) 1001012
or
1 DC power supply 300 V (@2115 V)
1001011
and
1 DC power supply 20 V, 5 A (@230 V)
1003312
or
1 DC power supply 20 V, 5 A (@115 V)
1003311
or
1 DC power supply 500 V (@230 V)
1003308
or
1 DC power supply 500 V (@115 V)
1003307
1 Pair of Helmholtz coils 1000906
1 resp. 2 Analogue multimeter AM50 1003073
Safety leads
6. Operation
6.1 Set up
• Place the fine beam tube between the Helm-
holtz coils.
• To get a clearer view of the electron beam,
conduct the experiment in a darkened room.
6.1.1 Set up with the DC power supply unit 300 V
• Set up the tube as in fig. 1.
• Connect the voltmeter in parallel to the 300-V
output.
• Connect the coils in series to the DC power
supply 20 V, as shown in Fig. 2, so that equal
current passes through both coils.
6.1.2 Set up with the DC power supply unit 500 V
• 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 volt-
age).
• Wait about 1 minute for the heater tempera-
ture 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
Helmholtz 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.