## Millikan's method for money

Practical Activity for 14-16

**Demonstration**

In this demonstration, students find the mass of a single coin by measuring handfuls of coins. The mass of a single coin represents the basic unit of charge. The handfuls represent the charge on an oil drop.

Apparatus and Materials

- Box of coins or dice or plastic tokens (50 or more – the more, the better)
- Scales, preferably digital
- Beaker, large plastic, 2

Health & Safety and Technical Notes

Read our standard health & safety guidance

To avoid any problem of coins being pocketed by students, it is best to use coins that are not in circulation as money i.e. use foreign or old coins.

There are many alternatives to coins. Coins are good because they all have the same mass. And the mass is fairly big (so any variation in mass is insignificant). However, you can use dice, tokens, tiddlywinks or many other alternatives as long as their masses are similar.

Make sure all the coins are similar.

Make sure you know the mass of the whole collection of coins (to ensure you have them all back at the end).

Procedure

- Put an empty beaker on the balance and set the reading to zero.
- Take the beaker off and pour or drop in a handful of coins. It is best not to do this on the balance because you might damage the balance with the force from the falling coins.
- Measure the mass and record it so that all students can see it.
- Remove the beaker and remove all the coins. Put in another handful. Measure and record its mass.
- Take as many readings as you think are needed for students to be able to calculate the mass of a single coin by finding the highest common factor.
- Ask the students to find the highest common factor from the masses of handfuls that are on the board. A good place for them to start is to subtract values that look close together.

Teaching Notes

- One of the key ideas in Millikan’s oil drop experiment is the use of the ‘highest common factor’ to find the charge on a single electron. In 1910 Robert Millikan measured the charge on a number of oil drops. He reasoned that, each time, the charge on the drop was made up of whole number multiples of a basic unit charge. He found this basic unit of charge by looking for the highest common factor that could be multiplied up to give the charge on each oil drop that he measured.
- In this demonstration, students find the mass of a single coin by measuring handfuls of coins. The mass of a single coin represents the basic unit of charge. The handfuls represent the charge on an oil drop. The handfuls are always sufficiently big that you can’t see how many coins are there. After a number of measurements, students can find the highest common factor to deduce the mass of a single coin.
- It is worth the teacher knowing beforehand what the mass of a single coin is.
- It is best not to add or take away coins from the beaker. This is because it will be clear to perceptive students how many coins you added or took away. Nevertheless, it is worth helping the students by trying to get a few readings with fairly similar number of coins.
- You could discuss the way that Millikan rejected some numbers from his results because they didn’t fit with his ideas on what the charge should be. You could model this by privately using a calculator to see if any of the measurements are not exact multiples of the mass of a single coin. Any that aren’t, you could either change or delete. Discuss whether this is good scientific experimental technique.

*This experiment was safety-tested in March 2008*