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



The Table II below gives the observational results in the case of 

 the liquid air really, however, of liquid oxygen. 



Table If. 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 = ]'495. 



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 interest- 

 ing field of research is opened out for the examination of the change 

 produced by low temperatures on the dielectric constants of other well- 

 known 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 



* Mr. W. Cassie, M.A., ' Phil. Trans.,' vol. 46, 1889, has given the results of 

 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 tempera- 

 tures for glass and paraffin are consistent with his. It will he interesting to see 

 how this relatively small dielectric constant of liquid oxygen compares with that of 

 other dielectrics when these last are cooled to the same temperature. 



