1 2 Profs. J. Dewar and J. A. Fleming. On the Dielectric 



The difference between the mean galvanometer readings, viz., 8*31 

 and 8*39, in the two last cases, is due to the fact that the condenser 

 capacity is changed by the cooling, owing to the fact that the plates 

 contract. The actual difference due to the temperature of the air 

 changing from 20° C. to —185° C. is very much smaller, and does not 

 sensibly affect the capacity as far as the accuracy of the present 

 measurements is concerned. 



There is, however, another correction which needs to be applied, 

 and that is for the capacity of the leads and of the contact maker. 

 Experiment showed that this was equivalent to 4' 15 mm. deflection 

 of the galvanometer, and hence the amount has to be deducted from 

 all mean galvanometer readings. 



The final result then is as follows : — 



The specific inductive capacity, K, of the ice at —185° C. is very 

 nearly equal to the ratio of (22'917— 0*07— 0'415) to (8*39-0-07- 

 0-415), or K = 2-83. 



The correction O07 is the correction for the capacity of the glass 

 separators, which is equal to 5x0*17 = 0'85 per cent, of the total 

 capacity, and hence the remanent capacity of the condenser is 8'39 — 

 8-39x0-0085 = 8-39— 0*07. The correction 0-415 is the correction 

 for the capacity of the contact maker and leads. 



A second determination was made at a lower voltage, the details of 

 which are as follows in Table VII. 



Table VII. — Brass Condenser filled with Distilled Water and frozen 

 into Ice. Ice reduced to —185° C. by use of Liquid Air. 



Deflection of 

 galvanometer 

 when in charge 

 circuit. 



Deflection of 

 galvanometer 

 when in discharge 

 circuit. 



Volts on 

 terminals of 

 condenser. 



Mean 

 galvanometer 

 deflection. 



8 -35 cm. 

 8-35 „ 



8-60 cm. 

 8-60 „ 



36-1 volts 

 36 -1 „ 



8 -48 cm. 

 8-48 „ 



Mean galvanometer deflection = 8'48 cm. 

 Mean voltage == 36' 1 volts. 



Mean deflection for voltage of 101*1 = 23*75 cm. 

 Corrected value of the deflection = 23*75-0-07 = 23-68. 



Specific inductive capacity of ice at —185° C- = ^o^f_ (wvf = ^ *^* 



Hence the mean value of the dielectric constant of ice at —185° C. 

 for slow reversals of electromotive force is a number not far from 

 2-9. 



We have in the same manner examined ethylic alcohol frozen 

 and reduced to —185° C. 



