In an introductory course, the basic phenomena of electrostatics are often demonstrated with “pith balls” and with a “gold-leaf electroscope”. A pith ball used to be a small, light wad of pith extracted from the twig of an elder bush, suspended by a silk thread. Today, it is more likely to be either a ping-pong ball, or a ball of styrofoam, suspended by a nylon thread – but, for want of a better word, I’ll still call it a pith ball. I’ll describe the gold-leaf electroscope a little later.
It was long ago noticed that if a sample of amber (fossilized pine sap) is rubbed with cloth, the amber became endowed with certain apparently wonderful properties. For example, the amber would be able to attract small particles of fluff to itself. The effect is called the triboelectric effect. [Greek trίboς (rubbing) + ήlektron(amber)] The amber, after having been rubbed with cloth, is said to bear an electric charge, and space in the vicinity of the charged amber within which the amber can exert its attractive properties is called an electric field.
Amber is by no means the best material to demonstrate triboelectricity. Modern plastics (such as a comb rubbed through the hair) become easily charged with electricity (provided that the plastic, the cloth or the hair, and the atmosphere, are dry). Glass rubbed with silk also carries an electric charge – but, as we shall see in the next section, the charge on glass rubbed with silk seems to be not quite the same as the charge on plastic rubbed with cloth.