The electroscope: a charge detector
An electroscope has a metal knob at the top joined by a metal rod to two thin, light metal leaves inside a glass jar. When the leaves hang together, the electroscope is not charged. If a charged rod touches the knob, charge flows down the metal to both leaves. Both leaves now have the same kind of charge, so they repel and spread apart. The more charge, the wider they spread. This shows that an object is charged and lets us compare amounts. To find the type of charge, first charge the electroscope. Then bring another charged object near the knob without touching. If the leaves spread more, that object has the same type of charge. If they come closer together, it has the opposite type. The French physicist Charles-Augustin de Coulomb studied the force between charges in the late 1700s, and the unit of charge, the coulomb, is named after him. Building an electroscope needs tools and glue for adults, so we only study how it works.
| Test | Leaves | What it tells us |
|---|---|---|
| charged rod touches the knob | spread apart | the electroscope is charged |
| a charged object near: leaves spread more | wider | same type of charge |
| a charged object near: leaves come closer | narrower | opposite type of charge |
Role-play an electroscope. Two children are the leaves, standing side by side at the top of a poster of an electroscope. A third child is the charged rod and gives each leaf the same colour card (the same charge). What do the leaves do? (They step apart.) Then try with a charged balloon held near two thin paper strips hanging side by side from tape: say what you see, and note that damp air can spoil it.