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3B SCIENTIFIC ® PHYSICS
Instruction sheet
12/12 ALF
1
2
3 4 5
1. Safety instructions
Hot cathode tubes are thin-walled, highly evacu-
ated glass tubes. Treat them carefully as there
is a risk of implosion.
•
Do not subject the tube to mechanical
stresses.
•
Do not subject the connection leads to any
tension.
•
The tube only may be used with tube holder
S (1014525).
If voltage or current is too high or the cathode is
at the wrong temperature, it can lead to the tube
becoming destroyed.
•
Do not exceed the stated operating parameters.
When the tube is in operation, the terminals of
the tube may be at high voltages with which it is
dangerous to come into contact.
•
Only use safety experiment leads for con-
necting circuits.
•
Only change circuit with power supply
equipment switched off.
•
Only exchange tubes with power supply
equipment switched off.
When the tube is in operation, the stock of the
tube may get hot.
•
If necessary, allow the tube to cool before
dismantling.
Thomson Tube S 1000617
80x10 m
-3
8x10 m
-3
6
7
8
The compliance with the EC directive on elec-
tromagnetic compatibility is only guaranteed
when using the recommended power supplies.
2. Description
The Thomson tube is intended for investigating
the deflection of electron beams in electrical and
magnetic fields. It can be used to estimate the
specific charge of an electron e/m and to deter-
mine the electron velocity v.
The Thomson tube comprises an electron gun
which emits a narrow, focussed ribbon of cath-
ode rays within an evacuated, clear glass bulb.
A tungsten 'hairpin' filament hot cathode is
heated directly and the anode takes the form of
a cylinder. The deflection of rays can be
achieved electrostatically by means of a built-in
plate capacitor formed by the pair of deflection
plates or magnetically with the help of the Helm-
holtz coils S (1000611) magnetically. The cath-
ode rays are intercepted by a flat mica sheet,
one side of which is coated with a fluorescent
screen and the other side of which is printed
with a milimetre graticule so that the path of the
electrons can be easily traced. The mica sheet
is held at 10° to the axis of the tube by the two
deflecting plates.
1
1 Guide pin
2 Connection pins
3 Cathode
4 Heater filament
5 Anode
6 Fluorescent screen
7 Lower deflection plate
8 Upper deflection plate
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