Electrical Resistance of Insulators

Electrical Resistance of Insulators.

In a recent issue of the Comptes Rendus, M. G. Foussereau described some experiments made to determine the resistance of certain insulators. We take the following abstract of the article from the Journal of the Chemical Society for March: ''The resistances were determined in the manner previously described by measuring the time required to charge a condenser at a given difference of potential across the particular substance. The resistance of porcelain varies with the temperature, in the same manner as that of the more highly insulating varieties of glass; it is 751 at 60°, and 0.052 at 180°. (All the resistances except those of phosphorus are given in millions of megohms per cubic centimeter). The resistance of sulphur, previously fused and then allowed to cool slowly in the prismatic condition, is 7.39 at 112.1°, and 3,930 at 69°; below the latter temperature the conductivity is inappreciable. When the same sulphur is allowed to remain at the ordinary temperature for some time, so that it devitrifies, its conductivity gradually increases; in one case the resistance after one day was 1,170 at 17°; after two days, 705 at 17°. Octahedral sulphur shows no trace of conductivity at the ordinary temperature, and the conductivity only becomes appreciable at about 80°. It would appear from these facts that sulphur has a higher resistance when crystalline. When sulphur melts, its resistance diminishes to one-fortieth its original amount. If the temperature is gradually raised from 114° to 150°, the resistance gradually diminishes as the temperature rises, and it varies between these limits in the ratio of 9 to 1. When the temperature rises to 160°, the sulphur becomes darker colored and pasty, and its resistance increases.

If the liquid is again cooled to the melting point, it retains a resistance higher than the original resistance, and the difference is greater the higher the temperature to which the sulphur has been heated, the longer it has been kept at this temperature and the more quickly it has passed 155° in the process of cooling, this being about the temperature at which the inverse change takes place. By repeatedly heating the sulphur in this way to a high temperature, its original resistance at the melting point can be increased twelvefold. When sulphur is allowed to cool in the prismatic form and is then re-melted, it shows a diminished resistance at the same temperatures. Similarly, when octahedral sulphur is repeatedly crystallized in the prismatic form, its resistance at 115° is gradually reduced to one-half. These results agree with Gernez' supposition that one variety of sulphur fully acquires the properties of another variety, only after repeated crystallizations. The resistance of carefully dried ordinary phosphorus is 84,000 megohms at 15°, and 15,600 megohms at 42°; it is of the same order of magnitude as that of' liquid sulphur. The resistance of liquid phosphorus is 3.32 megohms at 25° (this is the temperature given in the original paper — apparently it should be 45°, and 0.84 megohm at 100°.