Conductors and Insulators
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-323
2025-11-27
32
Demonstration. - Wind one end of a copper wire a meter long around the knob of an electroscope, and attach a brass ball to the other end. Rest it on a glass support. Charge the proof plane from a charged body and touch the ball with it. The leaves of the electroscope will instantly diverge. Discharge the electroscope by touching it with the finger. Replace the wire by a silk thread, and no effect will be seen when the ball is charged. Replace the silk thread by a damp cotton thread, and the leaves will diverge gradually when the ball is charged.
Bodies like the wire, which carry electrical charges readily, are called conductors; while those like silk, which carry them with difficulty, are called insulators, nonconductors, or dielectrics. There are no substances that are perfect conductors, neither are there any that are perfect insulators; but the following table gives what are usually classed as conductors and insulators, all arranged in the order of their conductivity.
The conductivity of bodies depends upon their physical condition, temperature and moisture having a decided effect. Glass becomes a conductor at 200° C. Air under normal pressure is a good insulator, rarefied air is a poor one. Pure water is a very poor conductor, but is rendered a good conductor by the addition of a little salt or a few drops of acid. Whether bodies are conductors or not also depends upon the character of the electrical charge. A single layer of cotton is insulation between two wires carrying the current for an electric bell, but is no protection whatever against the sparks from a charged glass rod.
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