262 Profs. J. Dewar and J. A. Fleming. Dielectric Constants 



not make a greater difference in dielectric constant than can be 

 properly ascribed to the accompanying change in temperature. 



[Again the following measurements were made at about the same 

 voltage : — 



Dielectric Charging 

 Substance. Temperature. constant. voltage. 



Potassic Bicarbonate .. . — 166*5 2*80 19*81 



Sodic Bicarbonate -166*7 4870 19-8/ 



Perric Chloride -133-8 4*23 19*8 



Sodic Chloride -129-2 14-55 20 2 



Cupric Carbonate -1327 3*42 18*21 



■7/ 



} 



Cupric Sulphate -133*2 16*40 19- 



Lithic Hydrate -198*0 3-23 19* 



Baric Hydrate -196*8 2010 19- 



It is unlikely that polarisation accounts for the differences between 

 the dielectric constants of the above substances, taken pair and pair, 

 when measured at nearly identical temperatures, and very nearly 

 the same voltages. — November 8, 1897.] 



In order to settle the matter finally we propose, however, to re- 

 measure a number of th ose substances which have shown high dielectric 

 values at the low temperature when measured by the galvanometric 

 method at a frequency of 120 but using in all cases an electromotive 

 force of 100 volts. 



If under the larger electromotive force the dielectric values of 

 some electrolytes still remain large, it will be difficult to ascribe this 

 large value to polarization. 



The facts, however, admit of another interpretation. It is clear 

 that the dielectric constants of some substances at low temperatures 

 are vastly more susceptible to change of electromotive force fre- 

 quency than is the case with others, and that the electric strain pro- 

 duced by a given electric stress varies in some cases enormously with 

 the time of imposition of the stress but very little in others, 



Another argument against the view that these high dielectric 

 values are due to polarisation, as ordinarily understood, is as follows : 

 The results of most numerous experiments on water show that the 

 dielectric constant at or near 0° 0., is a number not far from 80. 

 This value is obtained whether the electromotive force reversals are 

 infinitely slow or whether they are very large. The results of the 

 measurement of the electrical refractive index of water even with 

 ether waves only 4 mm. in length, and, therefore, having a fre- 

 quency of about 7*5 X 10 10 , as given recently by Lampa,* indicate 

 a number not far from 9*5 as the refractive index and hence give 

 a dielectric value of 90. There can be no question of polarisa- 

 tion of electrodes in this last case. On the other hand an increase 



* < Wien. Ber./ Tart 2a, p. 587, 1896 ; also p. 1C49, 1897. 



