[S/M | t | ★★★★] | keywords: electrometer, electroscope, charge and voltage measurement

What it shows:
The electroscope, or electrometer, is an instrument that measures electric charge or voltage by means of repulsive electrostatic force.

How it works:
We generally use two types of electroscope: a small "gold-leaf" type and a larger Braun type. The "gold-leaf" electroscope is very sensitive and we use it to show small amounts of charge, while the Braun type is used for larger amounts of charge—such as the net charge on a glass rod when it is rubbed with silk (see Triboelectric Effects).

The Braun electroscope has a light needle pivoted on a horizontal axis. The needle is balanced in a vertical position. The action is similar to that of the "gold-leaf" type in that, when support rod and needle are charged similarly, electrostatic forces cause the needle to rotate to a new equilibrium position proportional to the amount of charge.

Setting it up:
The movement of the Braun electroscope needle may be made visible to the class by video projection. Use back lighting by placing a photographic light box (slide-viewer) behind the electroscope—this will give a high-contrast silhouette image.

Comments:
The electroscope may be an old instrument, but its elegant simplicity makes it a must for electrostatic experiments.

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