Demonstration

The main purpose of this experiment is to explain the principle of an electron gun. You can also use the apparatus to demonstrate a valve diode – a device that lets the current flow in just one direction.

Apparatus and Materials

• Hot filament diode tube and stand
• Power supply, HT
• Power supply, 6.3 V, AC, for the heater filament (this is often included on the HT supply)
• Demonstration meter with a centre zero dial, -2.5 mA. to +2.5 mA

Health & Safety and Technical Notes

HT (high tension) power supplies (generally supplying voltages up to 400 V) can cause fatal electric shock.

It is essential that all HT connectors and cables are rated at the voltage to be used. The HT connectors should be the shrouded type so that accidental contact is highly unlikely. Any meter used in the HT circuit should be a type rated for the voltage used, and with shrouded connectors. All HT connections should be made with the HT switched off, and no adjustments made to the HT connections or wires once the HT is switched on.

The practical work with HT supplies should only be undertaken by teachers with good knowledge of HT electricity and the dangers.

Students should observe well away from the apparatus when it is being used.

Post-16 students may undertake the practical with supervision. See Topics in Safety (ASE 2001), Chapter 17...

CLEAPSS

The tubes are fragile (and expensive!) and should be handled carefully. They will implode if broken. Use the stands specifically designed for holding them.

Read our standard health & safety guidance

Follow the manufacturer’s instructions for setting up the diode.

Ensure that you can identify the following:

• The 6.3 V supply to the cathode heater. (If you connect the wrong voltage to the heater you can easily damage the tube beyond repair.)
• The HT (High Tension) supply to the electrode. Set this to zero.
• The collection plate and its connection terminal in the diode tube.

With no potential difference (p.d.) across the tube, a small current of about 50 mA. flows, owing to the energy with which electrons are emitted from the filament (Edison effect). But this will probably not be noticed in the experiment described here.

Procedure

1. Set up the diode in its stand, and connect the heater filament to the 6.3 V supply.
2. Connect the plate in the tube, through the milliammeter, to the HT supply.

3. Connect the other terminal of the HT supply to earth and to one of the filament terminals as shown in the diagram below. The supply enables the plate to be at 400 volts either positive or negative relative to the filament.

4. With the filament heater switched off, try a big positive potential difference (p.d.) and then a big negative p.d. You could try a bit of drama here by building up the possibility of getting a big current to flow through the vacuum with a big enough p.d.; then feign concern when there is no current.
5. In reality, with the filament not glowing, there will be no current for any p.d. (positive or negative).
6. Now switch on the filament with a positive voltage on the collection plate. You will get a current.
7. Try a negative p.d. on the collection plate. There will be no current.

Teaching Notes

Electron gun

• This experiment shows that charges can flow through the vacuum - as long as one terminal is heated and that this heated terminal is a cathode.
• It is reasonable to infer that the charges originate from the heated element (because with the heater switched off, there is no current).
• Given that charges only flow through the vacuum when the heated electrode is a cathode, it is also reasonable to infer that the charges are negative. Positive charge would not flow from a cathode to an anode, whereas negative charges will (being attracted to the positive anode).
• You can explain the results of the experiment using the idea of electrons. These tiny negative particles are free to move in the metal. As the metal is heated up, some of them ‘evaporate’ from the surface. They form a ‘gas’ of electrons above the surface of the hot plate. If the heated plate is put in a circuit and made negative with respect to another plate, the electrons are pulled through the vacuum and so a current flows between the plates. If the heated electrode is positive, the negative electrons are pulled back to the electrode’s positive surface.
Valve diode
• The essential story is that the diode can carry a current only one way. So you should let the class take the measurements 'both ways' with the valve diode. With the milliameter in the circuit, you can measure the current for different voltages in each direction. You can point out that there is a current when the diode is connected one way round (with the heated element connected to the negative terminal of the supply) and not when it is connected the other way round. In other words, the ‘valve’ lets current through one way, but not the other.
• You could mention that this is the basis of early valve diodes. The birth of the diode marked the beginning of electronics. However, diodes have now been replaced with components made of semi-conducting materials like silicon. Semi-conductor devices are often called ‘solid state’ because they do not rely on the ‘gas’ of electrons passing between contacts in a vacuum tube.

This experiment was safety-tested in May 2007

Demonstration

In this simple demonstration with a fine beam tube you can show an electron beam. You can also bend it using an electric field and a magnetic field produced by some Magnadur magnets. These are interesting in their own right, and are good preparation for other experiments.

Apparatus and Materials

• Fine beam tube and stand
• Power supply, low voltage, variable, 0 - 24 V, smoothed
• Power supply, HT, 0-250 V, with special shrouded connecting leads
• Power supply, 6.3 V, AC, for the heater filament (this is often included on the HT supply)
• Magnadur magnets, 2
• Connecting leads

Health & Safety and Technical Notes

The HT supply can deliver a fatal current. Use 4 mm leads with plugs that have sprung shrouds for all high-voltage connections. Ensure that the member of staff supervising the dark room is aware of the hazards and their control.

The practical work with a HT supplies should only be undertaken by teachers with a good knowledge of HT electricity and the dangers.

Students should observe well away from the apparatus when it is being used.

Post-16 students may undertake the practical with supervision. See Topics in Safety (ASE 2001), Chapter 17...

CLEAPSS

Read our standard health & safety guidance

Setting up the fine beam tube: Follow the manufacturer’s instructions for setting up the fine beam tube. (This demonstration does not involve the Helmholtz coils, so remove these if this can be done simply.)

Ensure that you can identify the following:

• The 6.3 V supply to the cathode heater (if you connect the wrong voltage to the heater you can easily damage the tube beyond repair).
• The HT supply to the anode. Set this to zero. The negative terminal of the HT goes to a socket, which is often near to the heater terminals.
• The low-voltage supply to the deflecting plates. Set this to zero.

A tube which has not been used for a while may not emit electrons. It may be possible to encourage it to do so by increasing the heater voltage slightly, to around 1 V or so. Monitor it carefully. Ensure that the heater current is only slightly exceeded.

You can use the low voltage power supply or the batteries for deflecting the electron beam.

Power supply:

• A smoothing unit may be needed with the low voltage power supply.
• You will need to ensure that only one point is earthed. The low voltage supply will have the negative terminal earthed. As this is connected to the anode, ensure that the positive terminal of the HT supply is earthed.
• Do not connect the low voltage power supply to the heater. This will damage it. The heater needs a supply of about 6 V. This is usually included on the HT power supply.
• Some power supplies have moving coil voltmeters incorporated in them. This type is helpful in this experiment.

Batteries

• You can use batteries instead of the low voltage supply. You will need three 6 V battery packs connected together to get a decent deflection. You can change the deflection by increasing the number of cells being used.
• Deflect the beam one way by connecting the negative terminal to the earthed anode and the positive terminal to the deflection plate.
• Deflect it the other way by connecting the positive terminal to the earthed anode and the negative terminal to the deflection plate.

Procedure

1. Select the gun which gives a horizontal electron beam. (There may be a selection switch.)
2. Always switch the heater on first, and only when it is glowing turn up the accelerating voltage on the anode.

3. When the filament is glowing, carefully increase the anode potential difference (p.d.}. At a p.d. which may be as low as 50 volts, the fine beam should be seen. With some tubes it may take 3-4 minutes to be clearly visible. As the p.d. is slowly increased, the beam will lengthen and strike the glass envelope of the tube.
4. Reduce the p.d. and show this transition several times. Do not increase the p.d. beyond about 200 volts.
5. With the beam striking the wall, apply 10 to 20 volts (d.c.) to the deflecting plates and observe the movement of the beam. Reverse the connections to the deflecting plates and repeat. Increase the p.d. on the deflecting plates to the maximum available, and repeat the reversing procedure.

6. Now deflect the beam with a magnet with face polarity (a Magnadur magnet}. Bring the magnet near to the envelope of the tube and point out the deflection of the beam.

7. You can use two such magnets on opposite sides of the tube to produce a more uniform field. See diagram above. Take care not to bring a magnet into violent contact with the glass.

8. If there are field coils, then a variable low voltage connected to them (up to 6 V or so) will deflect the beam into a circle, whose radius decreases with increasing voltage.

Teaching Notes

• This may be students’ first glimpse of an electron beam. Allow them to enjoy it. This experiment is best demonstrated to the students in groups of four to five in a darkened room if full value is to be obtained.
• Always reduce the anode to zero volts when not actually observing the beam, because the tube has a finite lifetime.
• The electron beam is visible because there is a low-pressure gas in the tube. Electrons striking the gas molecules give them energy, which is then released as light. When they re-radiate this energy, hydrogen gas glows blue and helium gas glows green.
• Draw the parallel with old television tubes by changing the beam to a horizontal one if possible. This has an electron gun like the fine beam tube. The electron beam is usually deflected magnetically rather than electrostatically and a different method of making it visible is used.
• You could also draw the parallel with a cathode ray tube, as found in an oscilloscope. (See related experiments.)
• You could try connecting a low frequency alternating supply to the deflecting plates. The beam will move from side to side. NB remove any smoothing components first! This is extended in the experiment.
• The hot electron gun releases electrons, and a potential difference of about 180 V will accelerate electrons to about 8 x 10
⁶ m s ^-1 . In a TV tube with 25 kV it is about 3 x 10 ⁷ m s ^-1 .
• Catching up on the catapult field. It is worth reminding students of the catapult effect and Fleming’s left hand motor rule. The deflection of the beam is consistent with the electrons having a negative charge. That is, to explain the direction of the deflection, the current must be flowing into the electron gun. Therefore, the charge on the particles carrying it must be negative, because they are flowing out of the electron gun.

This experiment was safety-tested in May 2007

Demonstration

Applying a constant voltage to the deflection plates will move the electron spot. Applying an alternating voltage will produce a line on the screen as the beam moves up and down. You can relate this to the lines produced on a CRO or a cathode ray television screen.

Apparatus and Materials

• Fine beam tube and stand
• Power supply, HT, 0-250 V, with special shrouded connecting leads
• Power supply, 6.3 V, AC, for the heater filament (this is often included on the HT supply)
• Power supply, L.T. variable (with AC and DC option)
• Batteries, 12 V, 2 (optional)
• CRO for comparison

Health & Safety and Technical Notes

HT (high tension) power supplies (generally supplying voltages up to 400 V) can cause fatal electric shock.

It is essential that all HT connectors and cables are rated at the voltage to be used. The HT connectors should be the shrouded type so that accidental contact is highly unlikely. Any meter used in the HT circuit should be a type rated for the voltage used, and with shrouded connectors.

All HT connections should be made with the HT switched off, and no adjustments made to the HT connections or wires once the HT is switched on.

The Practical work with HT supplies should only be undertaken by teachers with a good knowledge of HT electricity and the dangers. Students should observe well away from the apparatus when it is being used. Post-16 students may undertake the practical with supervision. See Topics in Safety (ASE 2001), Chapter 17...

CLEAPSS

The tubes are fragile (and expensive!) and should be handled carefully. They will implode if broken. Use the stands specifically designed for holding them.

Read our standard health & safety guidance

Ensure that you can identify the following:

• The 6.3 V supply to the cathode heater. (If you connect the wrong voltage to the heater you can easily damage the tube beyond repair.)
• The High Tension (HT) supply to the anode. Set this to zero. The cathode is often one of the heater terminals.
• The low-voltage supply to the deflecting plates. Set this to zero.

If the tube has been unused for some time, the cathode might not emit electrons. Carefully increase the heater voltage by about 1 V, monitoring it. Do not allow the heater current to rise much above the recommended values.

The low voltage power supply should be used with a smoothing capacitor.

Some power supplies have moving-coil voltmeters incorporated in them. This type is helpful in this experiment.

You can use the 12-volt batteries as an alternative to the low voltage supply, especially if your DC supply is not very smooth. The batteries should be connected in series with each other and a centre tap joined to one of the deflecting plates. A lead from the other deflecting plate is connected successively to different tapping points on the batteries, to show the effect of a change in potential difference (p.d.).

Procedure

1. Select the gun which gives a horizontal electron beam. (There may be a selection switch.)
2. Heat the cathode, then turn on the HT gun potential difference (p.d.) and let students look at the tube closely in a half-dark room. Allow them time to enjoy looking at the beam.
3. Show the beam being deflected when a p.d. of 20 to 30 V is applied to the deflecting plates. If students have seen an oscilloscope in use, point out the comparison. (In this case, you can only deflect the spot on the screen up and down, not horizontally.) You could also point out the similarity to a cathode ray tube as used in many TV sets and computer monitors.
4. You can use the smoothed power supply or the batteries.
5. Now show the deflection with alternating voltages. The AC output of the L.T. variable-voltage supply should be connected to the deflecting plates and the p.d. slowly increased from 0 to 25 volts. (It is advisable to have one of the deflecting plates connected to the anode.) The spot should turn into a line.

Teaching Notes

• This may be students' first glimpse of a 'visible' electron beam. Give them time to enjoy looking at it and explaining what is happening. This experiment is best demonstrated to the students in groups of four to five in a darkened room if full value is to be obtained.
• Always reduce the anode to zero volts when not actually observing the beam, because the tube has a finite life time.
• The electron beam is visible because there is a low-pressure gas in the tube. Electrons striking the gas molecules give them energy, which is then released as light. Hydrogen gas glows blue and helium gas glows green.
• You could explain the operation of the fine beam tube using a script provided in the Guidance note Electron guns.
• The attraction of the electron beam by the positive plate indicates that electrons are negative. The gun muzzle is also connected to the positive terminal of the HT supply in order to attract the electrons out of the gun.
• Connect the deflecting plates, as shown in the diagram. The electron beam now passes through a transverse electric field produced by the low voltage supply, and is deflected towards the positive plate.
• The higher the potential difference (p.d.) between the plates, the stronger the electric field and the more the beam deflects.
• When the p.d. of the low voltage supply is reversed, the field reverses, and the beam deflects in the opposite direction.
• With an alternating potential difference connected between the deflecting plates, the beam will swing backwards and forwards.
• If the alternation of the potential is rapid, the eye will not be able to follow the beam’s movement. A fan of ‘green light’ will extend from the gun and through the plates, indicating the path of the electrons.
• If students have seen an oscilloscope in use, point out the comparison. (In this case, you can only deflect the spot on the screen up and down, not horizontally.) You could also point out the similarity to a cathode ray tube as used in older TV sets and computer monitors. However, they use magnetic fields to deflect the beam.
• The alternating voltage makes the electron scan out a line (in this case a vertical one). On other TV sets, the electron scans out a horizontal line; then it shifts down and scans another one, and so on 625 times. It looks like a line because of our persistence of vision and the persistence of the phosphorescent screen – i.e. the glow remains on the screen for a very short period after the electron has moved on.

This experiment was safety-tested in April 2007

Demonstration

This demonstration shows that a metal cross blocks off a beam of electrons. Using a magnet, you can show how the electrons can be deflected, by distorting the shadow they cast.

Apparatus and Materials

• Maltese cross tube and stand
• Power supply, EHT
• Power supply, 6.3 V, AC, for the heater filament (this is often included on the HT supply)
• Bar magnet (optional)
• Old cathode ray TV, if you have one

Health & Safety and Technical Notes

The tubes are fragile (and expensive!) and should be handled carefully. They will implode if broken. Use the stands specifically designed for holding them.

Read our standard health & safety guidance

Set the tube up according to the manufacturer’s instructions.

Ensure that you can identify the following:

• The 6.3 V supply to the cathode heater (if you connect the wrong voltage to the heater you can easily damage the tube beyond repair).
• The EHT supply for the anode. Set this to zero. The cathode is often one of the heater terminals.

If the tube has been unused for some time, the cathode might not emit electrons. Carefully increase the heater voltage by about 1 V, monitoring it. Do not allow the heater current to rise much above the recommended values.

There are sometimes problems with the shadow of the cross turning into a ‘clover leaf’ shape. This can be prevented by connecting the cross to the anode and turning down the high voltage supply a bit.

However, it is not essential to connect the Maltese cross to anything, and it may be more convincing if this is not done.

If the fluorescence disappears, this may be because the cross is getting negatively charged by electrons. The fluorescence will reappear if the cross is momentarily connected to the anode.

With some tubes, electrons hitting the cross cause other electrons to be emitted from the cross. These secondary electrons travel to the positive anode, and keep the cross from becoming appreciably negative with respect to the anode. If the cross is connected to the anode, the cross is at the same potential as the anode all the time. This arrangement may therefore be preferred.

It's worth hanging on to an old cathode ray (CR) TV for the demonstration described in step 6. You might even be able to get hold of cheap or free CR TVs from friends, as people upgrade to LCDs and plasma TVs. They must still be electrically safe and tested each year.

Procedure

1. Set the tube up in the stand. Connect the filament heater to the 6.3V supply. Connect the positive terminal of the EHT power supply to the perforated anode and also to earth. The negative terminal of the supply is connected to the filament.

2. Set the EHT voltage to zero and switch on the 6.3 V supply to the heater filament.
3. With no output from the EHT supply, the light from the filament can be seen on the fluorescent screen at the end of the tube, and there will be a sharp shadow of the Maltese cross.
4. Once the heater is glowing, increase the potential difference (p.d.) to the anode with the EHT. As the p.d. is raised to about 3 kV, the thermionic emission produces fluorescence on the screen.
5. Try bringing a magnet near the tube; the fluorescent shadow will move. The optical shadow will remain undeflected.
6. If you have an old cathode ray (CR) TV, it is entertaining to bring a powerful magnet near the screen or close to the tube. This will distort the picture. It’s advisable not to do this with your best TV. This can cause permanent distortion of some shadow mask colour TVs. It is best to show it on a monochrome TV or old colour TV.

Teaching Notes

• This experiment is best demonstrated to the students in groups of four to five in a darkened room if full value is to be obtained.
• Always reduce the anode to zero volts when not actually observing the beam, because the tube has a finite life time.
• Discuss the fact that the fluorescent shadow only appears when the heater filament is a cathode, showing that the electrons are negative.
• You may be able to reinforce this with the direction of the distortion produced by the shadow. Use Fleming’s left hand rule to show that the electric current is flowing towards the cathode i.e. that negative charges are flowing out of it.
• Point out that the beam does not pass through the metal cross. Therefore the electrons are absorbed by the metal.
• More advanced students may appreciate the fact that the magnet shifts the shadow coherently implies that all particles in the stream are being deflected by the same amount. This means they have the same ratio of charge to mass. All particles are also likely to have the same charge and same mass (though, from this experiment alone we cannot be sure).

This experiment was safety-tested in April 2007

Demonstration

This demonstration shows that electron streams travel in straight lines. It is useful preparation for measuring the deflection in an electric field. These two experiments are similar to...

Deflecting an electron beam

However, the advantage of the deflection tube over the fine beam tube is that you can take measurements from its scale if you wish.

Apparatus and Materials

• Power supply, 6.3 V AC, for the heater filament (often included on the EHT supply)
• Deflection tube
• Stand for tube
• Power supply, EHT

Health & Safety and Technical Notes

A school EHT supply is limited to a maximum current of 5 mA., which is regarded as safe.

Although the school EHT supply is safe, shocks can make the demonstrator jump. It is therefore wise to see that there are no bare high voltage conductors; use female 4 mm connectors where required.

Read our standard health & safety guidance

Set the tube up according to the manufacturer’s instructions.

Ensure that you can identify the following:

• The 6.3 V supply to the cathode heater. (If you connect the wrong voltage to the heater you can easily damage the tube beyond repair.)
• The EHT supply for the anode. Set this to zero. The cathode is often one of the heater terminals.
• The terminals for the deflecting plates.

The experiment works well without any point connected to earth. In this case it is likely that leakage between different points of the circuit and earth will cause the negative terminal to be at some negative potential (e.g. — 1 kV) and the positive terminal to be at some positive potential (e.g. + 2 kV).

However, it is better to earth some point of the circuit, so that all potentials are fixed with respect to earth. With tubes such as this one where the electron beam is used after it has passed through the anode, it is best to earth the anode.

With the tube anode at earth potential, the heater circuit will be 5 kV below earth potential, and therefore the heater circuit connectors should be made so that accidental contact with the circuit is highly unlikely. The connectors and cables should be rated at better than 5 kV. Use a 6.3 V AC supply designed for valve heater circuits. Ensure the transformer isolation is rated to withstand 6 kV across the secondary and primary winding, and secondary winding to earth.

Avoid the use of batteries or general power supplies for the heater circuit.

Some power supplies have moving coil voltmeters incorporated in them. This type is helpful in this experiment.

Make sure you connect the 6.3 V heater supply to the heating filament. Too big a voltage can cause irreparable damage.

The beam from the deflection tube is produced by a horizontal slit in the anode. The beam fans out to produce a ‘V’ of electrons in the horizontal plane. This is aimed at a vertical fluorescent screen inside the tube. The vertical screen is at an angle to the beam direction, so the fan of electrons cuts across the screen, producing a straight line along it.

Procedure

1. Set up the deflection tube in its special stand.

2. Connect the 6.3 V supply to the filament. Make sure you connect a 6.3 V supply to the filament. (See technical note 2 above.)

3. You won’t use the deflecting plates in this experiment. Connect them together and then to the anode on the tube.
4. Connect the negative terminal of the EHT supply to the filament and the positive terminal to the anode.
5. Set the EHT to zero volts and switch on the 6.3 V supply to the heater filament.
6. With no output from the EHT supply, the light from the filament produces a line on the inclined fluorescent screen.
7. Increase the potential difference (p.d.) to about 3 kV: a fluorescent line appears. This is the path of the electron beam.
8. Show that the electron beam travels in a straight horizontal line.

Teaching Notes

• This experiment is best demonstrated to the students in groups of four to five in a darkened room if full value is to be obtained.
• Always reduce the anode to zero volts when not actually observing the beam, because the tube has a finite life time.
• The line on the fluorescent screen is formed when the electrons strike the vertical fluorescent screen.
• The fact that the electrons seem to travel in a straight line suggests that they are moving extremely fast.

This experiment was safety-tested in August 2007

Demonstration

The deflection tube allows you to show the parabolic path of an electron beam passing through a uniform electric field. The graduated scale allows you to take measurements if you wish. This is the main advantage of the deflection tube over the fine beam tube.

Most of the qualitative ideas of this experiment can be shown using the experiment...

Deflecting an electron beam

Apparatus and Materials

• Power supply, EHT, 1 (or 2 if a second one is available)
• Power supply, 6.3 V, AC, for the heater filament (this is often included on the HT supply)
• Magnadur magnets, 2 (optional)
• Electron deflection tube and stand

Health & Safety and Technical Notes

The tubes are fragile (and expensive!) and should be handled carefully. They will implode if broken. Use the stands specifically designed for holding them.

Read our standard health & safety guidance

Set the tube up according to the manufacturer’s instructions.

Ensure that you can identify the following:

• The 6.3 V supply to the cathode heater, if you connect the wrong voltage to the heater you can easily damage the tube beyond repair.
• The EHT supply for the anode. Set this to zero. The cathode is often one of the heater terminals.
• The terminals for the deflecting plates.

The basic deflection is achieved by bending the beam with the same voltage as is used to accelerate it. With this simple arrangement, it is not possible to show the effect of varying the deflecting potential difference (p.d.) on its own, because the anode p.d. would also be changed. Changing the accelerating p.d. alters the speed of the electrons and so leaves the deflection unaltered.

If two EHT power supplies are available, you can use the following arrangement to produce a variable deflection.

It is very important to earth the anode in this case. If the cathode were earthed, for example, there could be 10 kV between the positive terminal of the second power supply, and the neutral side of its mains winding. This is likely to damage the insulation of the transformer.

With the tube anode at earth potential, the heater circuit will be 5 kV below earth potential, and therefore the heater circuit connectors should be made so that accidental contact with the circuit is highly unlikely. The connectors and cables should be rated at better than 5 kV. Use a 6.3 V AC supply designed for valve heater circuits, and ensure the transformer isolation is rated to withstand 6 kV across the secondary and primary winding, and secondary winding to earth. Avoid the use of batteries or general power supplies for the heater circuit.

The deflecting power supply can also be connected the other way round, to make the deflecting plate negative to the anode.

The beam from the deflection tube is produced by a horizontal slit in the anode. So the beam fans out to produce a ‘V’ of electrons in the horizontal plane. This is aimed at a vertical fluorescent screen inside the tube. The vertical screen is at an angle to the beam direction. So the fan of electrons cuts across the screen, producing a straight line along it.

Procedure

1. Set up the deflection tube in its special stand.

2. Connect the 6.3 V supply to the filament. Make sure you connect the 6.3 V supply to the filament. (See technical note 2 above.)

3. Start with the deflection plates connected together and also connected to the anode on the tube.
4. Connect the negative terminal of the EHT supply to the filament and the positive terminal to the anode.
5. Set the EHT to zero volts, and switch on the 6.3 V supply to the heater filament.
6. With no output from the EHT supply, the light from the filament produces a line on the inclined fluorescent screen where the light strikes it.
7. Increase the potential difference (p.d.) to about 3 kV: a fluorescent line appears. This is the path of the electron beam. Point out that the electron beam travels in a straight horizontal line.

8. Then, while one plate is left connected to the anode, connect the other plate to the negative terminal of the EHT supply. This produces a vertical electric field between the plates, deflecting the beam into a parabolic path.
9. If you have not shown an electron beam being deflected by magnets, you could do it here. See here...

   Deflecting an electron beam

Teaching Notes

• This experiment is best demonstrated to the students in groups of four to five in a darkened room if full value is to be obtained.
• Always reduce the anode to zero volts when not actually observing the beam, because the tube has a finite life time.
• The beam is deflected, which shows there is a force on it. The force is consistent with the beam being made of negatively charged particles.
• The beam is deflected by a finite amount. So it must be made of something with mass. This seems obvious now, but, it is an important piece of deduction. We can deduce that the beam is made of particles with some mass and a negative charge.
• The beam stays intact as it is deflected. At first glance, this suggests that all the particles are the same. However, the mathematics shows that the shape of the curve is independent of the charge and mass of the particles. This is because, if the charge increases, the acceleration will increase in both the electron gun and between the deflection plates.
Likewise, any changes in mass will produce the same proportional change in acceleration in both the electron gun and the deflecting field. See guidance note:

Deflection in electric fields

• The beam travels at a uniform horizontal velocity and so the horizontal displacement varies linearly with time. It also experiences a constant vertical force, so it has a constant vertical acceleration, a. The vertical displacement, s_v , varies as the square of time, t . (s
_v =0.5 _at_ ² ). Hence the path of the beam is a parabola. See guidance note:

Deflection in electric fields

• The fluorescent screen has a graticule on it, and the shape of the parabolic path for different accelerating voltages can be recorded.
• This is analogous to a ballistic experiment in a uniform gravitational field. Whenever you throw something on the surface of the Earth, it traces out a parabola because the vertical acceleration is constant and the horizontal velocity is constant.

This experiment was safety-tested in April 2007