A model of Millikan’s experiment

Demonstration

A qualitative experiment to show the principle of a force on a charged body between two parallel plates.

Apparatus and Materials

• Macro-Millikan apparatus
• Power supply, 0–5 kV (Extra High Tension, EHT)
• Proof plane
• Polythene tile
• Charging cloth

Health & Safety and Technical Notes

Use an EHT supply of no more than 5 kV, which is current-limited to less than 5 mA.. The power supply for the heater MUST have adequate insulation. Leads used MUST have shrouded connectors and insulation capable of withstanding 5 kV.

Make all connections with the power supply turned off. Do not adjust connections while the EHT is switched on.

Electron beam tubes are fragile. Because they are evacuated, they will implode if they break. The tubes are also expensive, so handle them with great care. Use the purpose-designed holders during practical work.

Note that when switching the EHT supply off, it can take a little while for the voltage output from the EHT to fall to zero. Allow sufficient time before disconnecting.

Be Careful not to touch the exposed plates.

Read our standard health & safety guidance

The illustrations show the ‘macro-Millikan’ apparatus. It consists of a pair of metal plates, one of which has a central hole through which a small polystyrene ball can pass. The ball has a length of nylon thread attached, and at the other end of the thread is a Pyrex glass spring.

The best arrangement for securing the upper end of the spring is to attach it to another loop in a nylon thread. This is taken up over a pulley connected to the ceiling, and then to an eyelet on a block of wood. Alternatively, a support from a long retort stand rod can be used, as shown below, and brought up over the plates, forming a hooked support.

Procedure

1. With the EHT switched off, set the plates up horizontally, 7 to 10 cm apart, one above the other.
2. Connect the earth terminal of the power supply to its negative terminal. Connect the lower plate to the earth terminal of the EHT power supply. Connect the upper plate to the positive terminal. To reverse the field, change the leads to the supply, but in each case the lower plate should be earthed.
3. Lower the small conducting sphere with its nylon suspension through the hole in the upper plate. The upper end of the nylon suspension is looped and connected to the Pyrex glass spring.
4. Rub the polythene tile and put the proof plane on it, touching to charge the proof plane by induction. Without making contact with the plates, bring the proof plane up to the conducting sphere to charge it by contact. Adjust the suspension so that the sphere is almost exactly half-way between the two plates. This is most conveniently done in the first arrangement described above. By moving the block of wood nearer to or away from the apparatus, the sphere can be lowered or raised.
5. Switch on the EHT supply, set at 2 - 4 kV. The sphere will move as the extra force stretches the spring. The sphere can be brought back to the central position by moving the block of wood. Students will see the movement quite clearly if their eyes are in line with the plates.

Teaching Notes

• There are no measurements to be made in this demonstration: it is a qualitative experiment to show the principle of a force on a charged body between two parallel plates. Quantitative experiments do not lead to very satisfactory results unless considerable trouble is taken. There is a tendency for the charge to leak away along the suspension.
• If the ball is near one plate, the charge on it induces an opposite charge on that plate, so there is attraction. You do not want that ‘image-force’ to appear in the demonstration. The ball must be almost exactly half-way between the two plates, so that the image forces cancel out.
• Students' eyes need to be in line with the plates. It is easier to see the movement of the ball if a plane mirror is placed behind the suspension with a horizontal line ruled across it, whilst a small cardboard disc is attached to the nylon suspension to act as a pointer. This pointer is aligned with the mark on the mirror before the field is switched on, but after the sphere has been positioned at the centre of the plate. However, this special arrangement may divert attention from the general idea and need not be used in this qualitative demonstration.

• You may like to experiment with a realistic model of the Millikan experiment. Place a very light metal-coated ball in the field between the plates. Adjust the field to make the ball float upwards, fall slowly downwards, or even remain poised at rest for a short time. The ball is charged by contact with one of the plates, which then repels it. A very light ball is needed, or perhaps a scrap of aluminium leaf. When the ball is poised, its equilibrium is made unstable by image forces. To minimize that disadvantage, the charge on the ball should be made as small as possible and the electric field as large as possible.

This experiment was safety-tested in March 2008