264 Profs. J. Dewar and J. A. Fleming. Dielectric Constant* 



For the 500-olim galvanometer. . . . 373 cm. 



100-ohm „ 67 „ 



,, 4-ohm „ .... 0*55 „ 



Hence, the sensibilities are in the ratios of these deflections, and the 

 deflections of the 100-ohm galvanometer must be multiplied by 5 5, 

 and tliose of the 4-ohm galvanometer by 67 8, to reduce their scale 

 readings to equivalent deflections in terms of the 500-ohm galvano- 

 meter. 



The 500-ohm galvanometer was then used with the condenser and 

 vibrator, as described in one of our previous papers.* The con- 

 denser having gaseous air as dielectric, the scale deflection for a 

 frequency of 120 and an electromotive force of 97 volts was 3*2 cm. 

 when corrected for capacity of leads. 



The condenser then had its dielectric space filled with the 5 per 

 cent, solution of rubidic hydrate, and was frozen in liquid air. The 

 dielectric constant was next measured, using an electromotive fore e of 

 17'8 volts and the 500-ohm galvanometer. The value of the dielec- 

 tric constant found, when corrected for the capacity of the leads, was 

 65*6. The mean corrected scale deflection was 38*5 cm. for 1 7'S 

 volts. This, reduced to its equivalent for 97 volts, is 210 cm. and 

 210/3-2 = 65-6. 



In the next place the same experiment was repeated employing 

 the 100-ohm galvanometer and an electromotive force of 79 volts. 

 Applying the necessary corrections to the observed scale deflection 

 of 23'5 cm., and reducing to the equivalent deflection on the 500-ohm 

 galvanometer, gave 160 cm. as the value of the reduced deflection. 

 Hence 160/3 "2 = 50 is the dielectric constant. The rather con- 

 siderable difference between these values (65*6 and 50) is not a matter 

 for surprise, having regard to the extreme steepness of the dielectric 

 curve of the rubidic hydrate solution at about the temperature of 

 liquid air. As we were merely desirous of determining whether a 

 considerable increase of electromotive force would greatly diminish 

 the large dielectric value, we did not trouble to put in operation the 

 rather elaborate platinum thermometer arrangements for determining 

 the exact temperature of the dielectric. A reference to the dielectric- 

 temperature curve of rubidium hydratef will show that even one or two 

 degrees of temperature change in the neighbourhood of —185° C, or 

 — 200° pt. makes a very considerable alteration in the dielectric value. 

 The result, however, ascertained is that changing the electromotive 

 force from T434 volts to 17*8 or 79 volts does not bring down the 

 dielectric constant from a large value to a small one. 



In the same way the 5 per cent, solution of potassic hydrate was 

 tested. 



* ' Boy. Soc. Proc./ vol. Gl, p. 300, 1897. 

 t Ibid., p. 378. 



