3B SCIENTIFIC PHYSICS U18575 Instrucciones De Uso página 7

7. Operation
7.1 Mounting the tube onto the socket
•
Use gentle pressure to push the tube into the
socket on the base until the pin contacts are fully
inside the socket. Check that the guide pin is in its
proper, unique position (see Fig. 2).
•
Turn the tube to align it such that the electron gun
is parallel to the lengthways edge of the socket.
•
Use gentle pressure to shut the clip. The clip is
shut when you hear a click.
Fig. 2 Position of guide pin
7.2 Set up
•
Place the fine beam tube between the Helmholtz
coils.
•
Set up the tube as in fig. 3.
•
Connect the coils in series to the power supply
unit, so that equal current passes through both
coils.
•
To get a clearer view of the electron beam, con-
duct the experiment in a darkened room.
7.3 Adjusting the electron beam
•
Apply a heater voltage of say 10 V.
•
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
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.
8. 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
the radius remains constant. Take down these val-
ues.
•
Record other series of measured values for radii of
4 cm and 3 cm.
•
For further evaluation, plot the measured values in
2 2
a graph of r B against 2U.
The slope of the line through the origin corresponds to
e/m.
3
passing through the Helm-
H
so that
H