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7. Operation
7.1 Installation of fine beam tube
•
Screw on the cap nut by turning it anti-
clockwise.
•
Check the fine beam tube to ensure that
none of its contacts are bent.
•
Insert the tube vertically downward, making
sure that the contact pins and the coding pin
are correctly aligned (see Fig 1).
Fig. 1: Insertion of tube
•
Press the tube down with gentle pressure
until it sits firmly on the socket.
•
Note: Measure the height of the socket up to
the top of the cap nut and compare this with
the height of the tube. This will enable you to
see whether the tube is sitting correctly in
the socket.
•
Tighten the cap nut manually by turning it to
the right, making sure that the tube remains
vertical.
Caution: as long as the knurled screw is not
tightened, the tube is not secured and could
fall out when being transported.
Fig. 2: Inserted tube
7.2 Adjusting the electron beam
•
Set up the fine beam apparatus in a dark-
ened room.
•
Align the tube as illustrated above (with the
cathode ray gun perpendicular to the magnetic
field of the Helmholtz coils). For instructions on
how to rotate the tube, see section 7.3.
•
Set the adjustment knob for the heater voltage to
a position in the middle
•
Turn the knob for the coil current all the way
to the left, i.e. 0 A.
•
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
Helmholtz coils and check that the electron
beam curves upwards.
•
If the beam is deflected downwards, the tube
should be rotated by 180°.
•
Turn the coil current back up and check
whether the electron beam follows a closed
circular path. You may need to rotate the
tube slightly.
•
Carry out the experiment as described below.
7.3 Rotating the tube
The tube is mounted on a socket which can ro-
tate from -10° to 270°.
•
In order to rotate the tube, you must loosen
the knurled screw, but do not screw it all
the way out
•
Do not turn the tube itself, instead rotate the
turntable or the cap nut.
•
Tighten up the knurled screw again.
Caution: if the knurled screw is screwed out
all the way, the tube is not secured and could
fall out when being transported.
7.4 Changing the fuse
•
Turn off the power switch and unplug the
mains plug.
•
Pull out the fuse holder using a flat end
screwdriver (see Fig. 3).
•
Use the screwdriver as a lever from the side
of the mains socket.
4
(6 V approx.).
passing through the
H
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