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Gray, Stephen

insulators electrical electricity conductors

( c. 1666–1736) English physicist: distinguished between electrical conductors and insulators.

Gray was a dyer’s son who began to follow the same trade but after meeting (the Astronomer Royal) was attracted to astronomy and obtained a job as an observer at Cambridge for a year. Back in London, he experimented with electrical devices, and in 1729 (when he was over 60) he made a major discovery. He had electrified a glass tube by friction and found by chance that the electricity was conducted along a stick or thread mounted in a cork inserted in one end of the tube. Led by this, he found that a string resting on silk threads would conduct for over 100 m, but if the string was supported by brass wires the transmission failed. Based on this, he distinguished conductors (such as the common metals) from insulators (such as silk and other dry organic materials). In the 18th-c, experiments with electricity were difficult; primitive electrostatic machines, changes in humidity and induction effects easily led to confusion. Gray also observed that two spheres of the same size, one solid and one hollow, had the same capacity for storing electric charge.

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