3B Scientific Teltron Maltese Cross Tube D User Manual

3B SCIENTIFIC

Maltese Cross Tube D 1000649

Instruction sheet

09/13 ALF

PHYSICS

1 4-mm sockets for filament and

cathode 2 Heater filament 3 4-mm plug for connecting anode 4 Boss 5 Maltese cross 6 Fluorescent screen 7 Boss with 4-mm plug connected to

the Maltese cross

23 5 6

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 D

(1008507).

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 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.

2. Description

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 luminescent 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.

3. Technical data

Filament voltage: ≤ 7.5 V AC/DC Anode voltage: 2000 V to 5000 V

Anode current: 0.18 mA at U

= 4000 V

Voltage at cross: 2000 V to 5000 V

Current at cross: typ. 75 μA at U

4500 V

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 D 1008507 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

Additionally recommended: Protective Adapter, 2-Pole 1009961

4.1 Setting up the tube in the tube holder

• The tube should not be mounted or removed

unless all power supplies are disconnected.

• Push the jaw clamp sliders on the stanchion

of the tube holder right back so that the jaws open.

• Push the bosses of the tube into the jaws.

• Push the jaw clamps forward on the stan-

chions to secure the tube within the jaws.

• If necessary plug the protective adapter onto

the connector sockets for the tube.

4.2 Removing the tube from the tube holder

• To remove the tube, push the jaw clamps

right back again and take the tube out of the jaws.

5. Example experiments

5.1 Linear propagation of cathode rays

• Set up the tube as in Fig 1. Connect the

minus-pole of the anode voltage to the 4mm socket marked with a minus.

• First apply only the filament voltage.

Observe that the Maltese cross casts a sharp shadow on the luminescent screen in the visible

• 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 produce 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. Connect the

minus-pole of the anode voltage to the 4mm socket marked with a minus.

• 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 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

• Place the tube in the stand at 90° to its nor-

mal alignment.

• Insert the coil in the slot of the tube holder

so that the fluorescent screen is enclosed by a single Helmholtz coil. Alternatively it can be mounted on stand equipment (refer to Fig. 5).

• Set up the experiment as in Fig. 4. Connect

the minus-pole of the anode voltage to the 4-mm socket marked with a minus.

• 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.

light emitted by the glowing cathode.

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