Two centuries and a half ago, Gilbert recognised that the property of
attracting light substances when rubbed, be their nature what it may,
is not peculiar to amber, which is a condensed earthy juice cast up by
the waves of the sea, and in which flying insects, ants, and worms lie
entombed as in eternal sepulchres. The force of attraction (Gilbert
continues) belongs to a whole class of very different substances, as
glass, sulphur, sealing-wax, and all resinous substances--rock crystal
and all precious stones, alum and rock-salt. Gilbert measured the
strength of the excited electricity by means of a small needle--not
made of iron--which moved freely on a pivot, and perfectly similar to
the apparatus used by Haüy and Brewster in testing the electricity
excited in minerals by heat and friction. “Friction,” says Gilbert
further, “is productive of a stronger effect in dry than in humid air;
and rubbing with silk cloths is most advantageous.”
Otto von Guerike, the inventor of the air-pump, was the first who
observed any thing more than mere phenomena of attraction. In his
experiments with a rubbed piece of sulphur he recognised the
phenomena of repulsion, which subsequently led to the establishment
of the laws of the sphere of action and of the distribution of
electricity. _He heard the first sound, and saw the first light, in
artificially-produced electricity._ In an experiment instituted by
Newton in 1675, the first traces of an electric charge in a rubbed
plate of glass were seen.