Page:Proceedings of the Royal Society of London Vol 60.djvu/385

358 The exceedingly high insulating properties of liquid oxygen and liquid air indicate that these bodies are dielectrics, and possess a dielectric constant or specific inductive capacity which it is necessary to determine. We have, therefore, lately made some measurements which have enabled us to assign a number representing, in all probability, a close approximation to these constants.

The remarkable non-conducting quality of these liquid gases for electricity enabled us to employ a method which, generally speaking, is not applicable to liquids other than those of very high specific resistance, or insulating power.

The method used by us in these experiments consists in the employment of a small condenser composed of metal plates which can be plunged beneath the surface of the liquid gas, and the capacity of this condenser measured when the dielectric between the plates is first gaseous air at ordinary temperature and pressure, and is next replaced by the liquid oxygen or liquid air. In order to determine the capacity of this condenser, which is necessarily small and of the order of O’OOl microfarad, we adopted the well-known device of charging the small condenser with a high potential (100 volts) and then discharging it into a much larger, well insulated mica condenser, having a capacity of about 0*5 microfarad. This process was repeated ten times, and the larger condenser was then discharged through a standardised ballistic galvanometer. A specially constructed and highly insulated key was employed to charge the small condenser by means of a battery of fifty small lithanode secondary cells; and then to discharge it into the larger condenser. The success of this method depends entirely on the absence of sensible leakage in the condensers, and it is essential to show that the small condenser loses no sensible portion of its charge by leakage or conduction during the interval which elapses between disconnecting it from the battery and connecting it to the large condenser, which acts as a reservoir.

In these experiments the small condenser consisted of seventeen plates of carefully flattened aluminium, about 1 mm. in thickness; each plate being 5 cm. wide by 15 cm. long. In order to separate the plates, small distance pieces of crown glass were employed,