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



and Mi, M 2 are the coefficients of mutual induction of the circular 

 ends of the sheet and the helix. 



Hence the calculation of this force reduces itself to a double 

 application of the formulae for the coefficient of mutual induction of 

 a circle and coaxial helix. 



It is hoped that this may form a useful method of calculating the 

 constant Of current weighers designed to measure current in absolute 

 units. 



u A Note on some further Determinations of the Dielectric 

 Constants of Organic Bodies and Electrolytes at very Low 

 Temperatures." By James Dewar, M.A., LL.D., F.R.S., 

 Full erian Professor of Chemistry in the Royal Institution, 

 and J. A. Fleming, M.A., D.Sc.,*F.R.S., Professor of Elec- 

 trical Engineering in University College, London. Re- 

 ceived October 28, — Read December 9, 1897. 



In several previous communications* we have described the inves- 

 tigations made by us on the dielectric constants of various frozen 

 organic bodies and electrolytes at very low temperatures. In these 

 researches we employed a method for the measurement of the 

 dielectric constant which consisted in charging and discharging a 

 condenser, having the given body as dielectric, through a galvano- 

 meter 120 times in a second by means of a tuning-fork interrupter. 

 During the past summer we have repeated some of these determina- 

 tions and used a different method of measurement and a rather 

 higher frequency. In the experiments here described we have adopted 

 Nernst's method for the measurement of dielectric constants, using 

 for this purpose the apparatus as arranged by Dr. rTernst which 

 belongs to the Davy-Faraday Laboratory. The frequency of alterna- 

 tion employed was 350 or thereabouts, whereas in all our formerly 

 described experiments with the galvanometer method it was 120. 



The electrical details of the arrangement employed in Nernst's 

 method are as follows : — A Wheatstone's bridge is formed (see 

 diagram), two sides of which consist of variable resistances, R x , R 2 , 

 which are usually liquid resistances contained in \J -tubes. The other 

 two sides of the bridge consist of two sliding condensers of variable 

 capacity, C l5 C 2 , which are shunted by adjustable liquid resistances, 

 E s , R±. The bridge circuit contains a telephone, T, as detector. 

 The alternating currents are furnished by an induction coil, T. 

 An experimental condenser, X, the dielectric of which can be made 



* See Fleming and Dewar, ' Eoy. Soc. Proc.' (1897), vol. 61, pp. 2, 299, 316, 

 358, 368, and 381. 



