On the Dielectric Constants of certain Frozen Electrolytes. 299 



" On the Dielectric Constants ot certain Frozen Electrolytes at 

 and above the Temperature of Liquid Air." By J. A. 

 Fleming, M.A., D.Sc, F.R.S., Professor of Electrical 

 Engineering in University College, London, and James 

 Dewar, M.A., LL.D., F.R.S., Fullerian Professor of Che- 

 mistry in the Royal Institution, London. Received May 6, 

 —Read June 3, 1897. 



In a preliminary note on the " Dielectric Constant of Ice and 

 Alcohol at very Low Temperatures "* we have described the method 

 employed by us in determining the dielectric constants of ice and 

 frozen ethylic alcohol at the temperature of liquid air. One diffi- 

 culty which presented itself in our initial experiments was that of 

 the construction of a suitable electrical condenser for use with frozen 

 liquids as dielectrics. The concentric-tube pattern of condenser used 

 by us in the first experiments on ice gave trouble by becoming 

 deformed under the expansive force of the water on freezing, and 

 the subsequent great contraction of the ice on cooling to the tempera- 

 of liquid air tended to break the contact between the metal plates 

 and the dielectric. It was, in addition, difficult to clean out. 



In undertaking, therefore, a more careful study of the dielectric 

 constants of various frozen electrolytes at and above the tempera- 

 ture of liquid air, the first experimental problem which arose was 

 that of the construction of a form of condenser which could be em- 

 ployed with the selected frozen electrolyte as dielectric, and which 

 should comply with the following conditions : — 



1. To allow any required and actual expansion of the dielectric on 

 freezing without deforming the condenser. 



2. To permit the subsequent great contraction of the dielectric 

 substance on cooling to the temperature of liquid air, without break- 

 ing contact between the dielectric and the metal plates. 



3. To be sufficiently compact to be placed in a large vacuum tube 

 of liquid air. 



-t. To be rigid and have high insulation and be able to be taken 

 apart and put together again quickly and accurately. 



After trying several arrangements, we were at last able to design 

 a condenser fulfilling the above conditions in a very perfect way, 

 and which has given in its latest form little or no trouble in work. 



This condenser, which we call a cone-condenser, is constructed as 

 follows : — 



It consists of two hollow cones turned out of hard brass (see fig. 

 1), the outer cone being 14*9 cm. in length inside, 5'1 cm. in diameter 



* See ' Roy. Soc. Proc.,' vol. 61, p. "2, February, 1897. J. Dewar and J. A. 

 Fleming. 



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