358 Profs. J. A. Fleming and J. Dewar. On the 



** On the Dielectric Constant of Liquid Oxygen and Liquid 

 Air." By J. A. FLEMING, M.A., D.Sc., F.R.S., Professor of 

 Electrical Engineering in University College, London, and 

 JAMES DEWAR, M.A., LL.D., F.R.S., Fullerian Professor 

 of Chemistry in the Royal Institution, &c. Received 

 December 8, Read December 17, 1896. 



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 pres- 

 sure, 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 0*001 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, 



