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432 RICHARDSON'S BLUEBOOK OF PROJECTION
perfect insulators. There are no perfect insulators. Dry sulphur at normal temperatures is very nearly perfect. A block of sulphur an inch square, under an electro-motive force of one volt, will allow only a few thousand negatrons to pass through it in one second. It therefore constitutes an admirable insulator. A block of copper an inch square, with the same electro-motive force applied, will pass many millions times millions times millions of negatrons in one second's time.
The amount of hindrance any conductor offers to the passage of negatrons is called the (17) resistance of the conductor. It is measured, in ohms, by comparison with a standard resistor made of mercury.
Nature of Conduction
Why some substances hinder the flow of negatrons more than others is still very much a mystery. But it is noteworthy that all the good conductors have only one negatron in the outermost layer of their atoms. It may be that those substances that have many negatrons in the outside layer of their atoms repel the free negatrons that constitute an electric current by reason of the mutual repulsion of like charges.
(18) Conductors are heated by the passage of current. The effect of a negative charge moving through the balanced positives and negatives of a system of atoms apparently sets the atoms .into vibration at a frequency which constitutes heat. When the resistance is great the substance of a conductor undergoes internal vibration at a still higher frequency and in consequence it may emit visible light.