Preparation for Millikan’s experiment

Demonstration

This demonstration shows that charged particles will be attracted up or down when placed between parallel charged plates. It also shows that the electric field can support the weight of the charged particle between the plates. It might already have been shown when discussing how capacitors behave.

Apparatus and Materials

• Large metal plates, 2
• Power supply, 0 – 5 kV (Extra High Tension, EHT)
• Aluminium leaf
• Scissors
• Pillars of insulator, 3

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 accidently touch the plates.

Read our standard health & safety guidance

In effect, you are building a parallel plate capacitor. The top plate becomes positively charged and the bottom one negatively charged. There is a uniform electric field between the plates.

When one of the pieces of metal foil touches a plate then it becomes charged by sharing charges with the plate. It is then repelled by the charge on that plate and attracted to the opposite charge on the other plate. The charging process repeats and the pieces of foil bounce up and down.

The EHT has a floating earth so you can make the top plate positive or negative. However, we have assumed here that it will be positive. The experiment will work the other way but making the top plate positive is more like the Millikan experiment because it will exert an upwards force on negative charges (like Millikan’s charged oil drops).

Procedure

Setting up...
1. Place one metal plate on the table and connect it to earth.
2. Use the insulating pillars to support the other plate about 10 cm above the earthed plate, horizontal and insulated.
3. Cut or tear from the book some small scraps (1 cm) of metal leaf. (Or cut them in the shape of small men, say 2.5 cm high.)
Carrying out...
4. Connect the two plates to the EHT supply to establish a vertical electric field in the space between them. With the EHT switched off, connect the top plate to the positive terminal of the EHT and the bottom (earthed) plate to the negative.
5. Place the scraps of leaf in the space between the plates. Then the scraps acquire charges when they touch either plate and are driven to the other plate; so they dance up and down in the space between the plates.
6. To show that the electrostatic charges will maintain a similar dance, replace the upper plate by a sheet of plastic. Charge the plastic by rubbing.

Teaching Notes

• The Millikan experiment is very fiddly and so not generally done at school level. However, it is an experiment of great historic value because it produced a value for the charge on the electron and showed that electrons are discrete particles, each carrying the same charge.
• In Millikan’s experiment the apparatus is much smaller and the ‘dancing men’ are replaced by a tiny drop of oil (or in new models a tiny bead of plastic) which is allowed to gain a few extra electron charges. That tiny drop falls slowly, delayed by air resistance, and it rises slowly when the plates are suitably charged. The plate charges can be adjusted by changing the potential difference across the plates to hold the tiny drop at rest, floating in the air. Then the charge on the drop can be calculated.

This experiment was safety-tested in March 2008