Page 66 - Teach Yourself Electricity and Electronics

46 Measuring devices

3-2 Circuit for demonstrating
how a galvanometer
indicates relative current.

Electrostatic deflection

Electric fields produce forces, just as do magnetic fields. You have probably noticed this
when your hair feels like it’s standing on end in very dry or cold weather. You’ve proba-
bly heard that people’s hair really does stand straight out just before a lightning bolt hits
nearby; this is no myth. Maybe you performed experiments in science classes to ob-
serve this effect.
The most common device for demonstrating electrostatic forces is the electro-
scope. It consists of two foil leaves, attached to a conducting rod, and placed in a sealed
container so that air currents will not move the foil leaves (Fig. 3-3). When a charged
object is brought near, or touched to, the contact at the top of the rod, the leaves stand
apart from each other. This is because the two leaves become charged with like electric
poles—either an excess or a deficiency of electrons—and like poles always repel.

3-3 A simple electroscope.

The extent to which the leaves stand apart depends on the amount of electric charge.
It is somewhat difficult to actually measure this deflection and correlate it with charge
quantity; electroscopes do not make very good meters. But variations on this theme can