Page:Proceedings of the Royal Society of London Vol 60.djvu/393

366 The Table II below gives the observational results in the case of the liquid air—really, however, of liquid oxygen.

Table II.—Dielectric Constant of Liquid Air (practically Liquid Oxygen).

In Liquid Air. In cold Gaseous Air. Ballistic throw for Ballistic throw for condenser charged to 100 volts. condenser charged to 100 volts.

9-5 — 9-6 — 9-5 — — 6-5 — 6-6 — 6-6 9-4 — 95 — 9-55 — ' — 6-51 — 6-51 9-7 — 9-55 — Mean = 9-54 Mean = 6‘54 Dielectric constant = P495.

With regard to the above-determined dielectric constants for liquid oxygen and liquid air, it may be remarked that these numbers are smaller than those which have been obtained for almost any other solid or liquid substance of which we have been able to find the measured results. It has been already pointed out that large dielectric constant generally accompanies small specific resistance in a dielectric, and vice versa. Hence, as the specific resistance of the liquid oxygen is very large—it being a very fine insulator—it is not surprising to find the dielectric constant very small. As above mentioned, at a very low temperature, the dielectric constant of some other solid dielectrics has been found by us to be very much reduced, and hence an interesting field of research is opened out for the examination of the change produced by low temperatures on the dielectric constants of other wellknown solid insulators, such as paraffin, ebonite, gutta-percha, mica, sulphur, spermaceti, and various frozen liquid insulators, such as the numerous hydrocarbon oils, carbon disulphide, ice, &c.* We hope to

measurements on the changes produced in the dielectric constants of various insulators by heating them. As far as we can see, our initial results at low temperatures for glass and paraffin are consistent w ith his. I t will be interesting to see how this relatively small dielectric constant of liquid oxygen compares w ith that of other dielectrics when these last are cooled to the same tem perature. Profs. J. A. Fleming and J. Dewar. the
 * M r. W . Cassie, M.A., ‘ Phil. Trans.,’ vol. 46, 1889, has given the results of