3B Scientific Teltron Maltese Cross Tube S User Manual
3B SCIENTIFIC
Maltese Cross Tube S 1000011
Instruction sheet
12/12 ALF
®
PHYSICS
1 Guide pin
2 Connection pins
3 Cathode
4 Heater filament
5 Anode
6 Maltese cross
7 Fluorescent screen
8 4-mm plug connected to
Maltese cross
12 34 5 6 7 8
1. Safety instructions
Hot cathode tubes are thin-walled, highly
evacuated 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 may only 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.
• Only use safety experiment leads for con-
necting circuits.
• Only change circuits 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.
The compliance with the EC directive on electromagnetic compatibility is only guaranteed
when using the recommended power supplies.
The Maltese cross tube is used to demonstrate
how cathode rays propagate in straight lines in
the absence of any electric or magnetic field.
This is shown by placing a Maltese cross in the
path of the beam to throw a shadow on a fluorescent screen. The tube is also used to show
how beams can be focussed by magnetic fields.
This serves as an introduction to electron optics.
The Maltese cross tube is a highly evacuated
tube with an electron gun consisting of a pure
tungsten, 'hairpin' heater filament and a cylindrical anode all contained in a clear glass bulb.
The electron gun emits a diverging beam which
is detected by a fluorescent screen. An aluminium Maltese cross is suspended in the centre of
the bulb. The lower limb of the cross has a small
hole 3-mm in diameter so that the orientation of
the shadow can be distinguished when it is affected by a magnetic field.
2. Description
1
3. Technical data
Filament voltage: ≤ 7.5 V AC/DC
Anode voltage: 2000 V to 5000 V
Anode current: typ. 20 μA at U
4500 V
A
Voltage at cross: 2000 V to 5000 V
Current at cross: typ. 75 μA at U
4500 V
A
Glass bulb: 130 mm diam. approx.
Total length: 260 mm approx.
4. Operation
To perform experiments using the Maltese cross
tube, the following equipment is also required:
1 Tube holder S 1014525
1 High voltage power supply 5 kV (115 V, 50/60 Hz)
1003309
or
1 High voltage power supply 5 kV (230 V, 50/60 Hz)
1003310
1 Coil from Helmholtz pair of coils S 1000611
1 DC Power Supply 20 V, 5 A (115 V, 50/60 Hz)
1003311
or
1 DC Power Supply 20 V, 5 A (230 V, 50/60 Hz)
1003312
1 Bar magnet 1003112
4.1 Setting up the tube in the tube holder
The tube should not be mounted or removed
unless all power supplies are disconnected.
• Press tube gently into the stock of the holder
and push until the pins are fully inserted.
Take note of the unique position of the guide
pin.
4.2 Removing the tube from the tube holder
• To remove the tube, apply pressure with the
middle finger on the guide pin and the thumb
on the tail-stock until the pins loosen, then
pull out the tube.
5. Example experiments
5.1 Linear propagation of cathode rays
• Set up the tube as in Fig 1.
• First apply only the filament voltage.
Observe that the Maltese cross casts a sharp
shadow on the luminescent screen in the visible
light emitted by the glowing cathode.
• Turn on the high-tension supply to the an-
ode.
Observe that an equally sharp and exactly overlapping shadow is cast on the screen by the
charged particles.
The experiment demonstrates that the charges,
cathode rays, are propagated linearly and pro-
duce shadows in exactly the same manner as
visible light.
5.2 Electrostatic charging effects
• Set up the circuit as in Fig 1.
• Isolate the metal cross from the anode po-
tential.
Negative charges accumulate on the cross and
when equilibrium is reached, they oppose the
collection of any further charge. Cathode rays
passing close to this opposing field are deflected
and produce a distortion of the luminescent
shadow (refer to Fig. 3).
Connecting the cross to the cathode potential
results in such a distortion that the image is
magnified beyond the limits of the fluorescent
screen.
5.3 Deflection by a magnetic field
• Set up the circuit as in Fig 1.
• With the tube operating, bring a magnet
close to the tube.
Observe that the shadow moves. The amount of
deflection depends on both the strength of the
magnetic field and the accelerating voltage applied to the electron gun.
Relate the direction of deflection, the field and
the motion of the charges using Fleming's law of
motion (left-hand rule). Cathode rays under the
influence of magnetic fields appear to behave in
a similar manner to electric currents in conductors.
5.4 Introduction to electron optics
• Set up the experiment as in Fig. 2.
• Insert the base of the coil into the groove of
the tube holder from the front so that the
fluorescent screen is enclosed by the single
Helmholtz coil. Make sure that the connectors point forwards.
• Turn on the power supply for the tube and
observe the shadow.
• Turn on the coil current and slowly increase
it.
By increasing the magnetic field (raising the
voltage to the coil) the image is seen to rotate,
diminish to a spot and then enlarge again in
inverted form.
Anode voltage variations provide a further element of control.
Cathode rays and deflecting fields can thus be
used to magnify shadow images in a manner
analogous to an optical lens system.
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