Resistance of Insulators

The Resistance of Insulators.

M. Foussereau, who recently devised the method of measuring high resistances of insulators by the time necessary to charge a condenser through them to a given difference of potential, has applied the method to determine the resistance of different insulators, and his results were communicated to the last meeting of the French Academy of Science by M. Jamin. The resistance of porcelain is thus found to be 751,000,000 megohms at 60° C., and 0.052,000 megohm at 150° C. per c. cm. Were a curve of resistance at different temperatures thus prepared, a useful pyrometer might be the result. Sulphur gave a resistance of 7,390,000 megohms at 112.1° C., and 3,930,000,000 megohms at 69° C. per c. cm. of substance. Below the last temperature its conductivity ceases to be measurable. Left to itself, the sulphur takes a vitreous texture on cooling, and on cooling to an average temperature of, say, 17° C, the resistance becomes 1,170,000,000 megohms after one day, and 705,000,000 megohms after two days. An octahedral crystal of sulphur at ordinary temperature gave no sign of condiictivity, but at 80° C. conductivity begins to appear. Crystallization, therefore, seems to render sulphur more insulating, a fact which is worth bearing in mind. In passing to the liquid state, sulphur becomes at once four times more conducting. Ordinary phosphorous has a resistance much lower than the above substances, Solid phosphorous has a resistance of 84,000 megohms at 15° C. per c. cm., and 15,600 megohms at 42° C. This great fall assimilates it to sulphur. The specific resistance of liquid phosphorous is 2.30 megohms at 25° C. and 0.34 megohm at 100° C.