Dielectric Constant of Liquid Oxygen and Liquid Air. 363 



It will be seen that the mean galvanometer throw, when the con- 

 denser was immersed in liquid oxygen, was 10'903 centims., and the 

 mean throw, when raised into the gaseous oxygen, was 7'146 centims. 



One matter which we felt it important to examine, was whether 

 there was any correction needed for the change in the dielectric con- 

 stant of the glass separators with temperature. 



Since these glass separators had a total surface of nearly 1 per 

 cent, of the area of the metal plates, the condenser may be regarded 

 as consisting of two condensers joined in parallel, one consisting of a 

 glass dielectric condenser having an effective surface of 1, and the 

 other a condenser having a liquid or gaseous oxygen dielectric 

 having an effective area of 99. In the course of these experiments 

 we have therefore examined the effect of low temperature upon the 

 dielectric constants of glass, paraffined paper, and mica. We find 

 that on cooling these bodies to 182 C. they experience a marked 

 reduction in dielectric constant. The dielectric constant of a certain 

 specimen of crown glass was reduced by 2T4 per cent, by cooling to 

 the temperature of liquid air or to 185 C. The dielectric con- 

 stant of paraffined paper was reduced by 28'4 per cent, under the 

 same circumstances.* We are engaged in a systematic examination 

 of the influence of very low temperatures on the dielectric constants 

 and specific resistances of the principal dielectric bodies. The crown 

 glass used as separators in the construction of our small condenser 

 had a specific inductive capacity of about' 6'0 at ordinary tempera- 

 ture, and this at the low temperature would be reduced to nearly 5'0. 

 Hence in estimating the capacity of the condenser, as constructed, 

 there comes in as we have seen a correction from the presence of the 

 glass. We selected glass in the first instance rather than ebonite or 

 sulphur, as we thought it probable we should use the same con- 

 denser in determining other dielectric constants, and we wished to 

 construct the separators of a material which was very rigid and not 

 easily acted upon by oils or other liquids. 



Taking the formula above given, we can deduce from the observed 

 results the required constant, for, we have 



0___ K + Q-05 1030 



V' ~ 1'05 1019 ' 



* 



and hence substituting for t the observed ratio , we find 



i'd\j' 



K = 1-491 



* By another method we have found that for the glass of a glass te&t-tube the 

 dielectric constant was decreased 22*2 per cent, by cooling to the temperature of 

 liquid air. Under the same circumstances a certain specimen of mica decreased 

 only 3'01 per cent, in dielectric constant. 



