﻿752 Prof. J. P. Knenen on the 



the scale of the electrometer used, and sometimes less than 

 this. A farther separation of the plates by about the four- 

 hundredth part of a millimetre will generally be sufficient to 

 make the effect completely disappear; and if A be now 

 gradually screwed down, the charging up of A will re- 

 appear, and will be again entirely removed by bringing the 

 plates into contact. The range within which this current is 

 produced corresponds to about two divisions of the head of 

 the micrometer-screw, that is to about "001 cm. Why the 

 deflexion shows sometimes the whole of the contact difference 

 and sometimes only part of it may be explained by assuming 

 that when part only is shown there is metallic contact 

 between the plates, whose resistance is comparable with the 

 resistance of the air between the plates. Let the former 

 resistance be R x and the latter R 2 . R 2 is the ratio of the 

 current through the air to the potential difference between 

 the plates, which for the sake of simplicity we will assume 

 to be constant. If C denote the capacity of the pair of plates 

 and the electrometer and V the contact difference, it is easy 

 to show that the potential v indicated by the electrometer at 

 a time t after the pair of quadrants in connexion with A is 

 insulated is given by 



r , _ Ri + R 3 \ 



If R x is very small in comparison with R 2 , v will never 



acquire an appreciable value : but if K 1 be very great in 



comparison with R 2 , the final value of v will be V the contact 



R V 

 difference. In general, the final value of v is -^ — ^5 . 



Experiments on the various points here referred to are in 

 progress with a view of obtaining quantitative results. 



LXXIII. On the Deduction of Thermodynamical Belati 

 By Professor J. P. Kuenen, University of Leiden 



ons. 



1 



SI 



N a recent number of this journal t Dr. Houstoun deduces 

 a relation between linear extension and torsion. At first 

 „.ght this relation might seem to be incorrect, as in the proof 

 no account appears to have been taken of the heat-effects 

 which accompany the elastic changes. The author starts 



* Communicated bv the Author. 



t R. A. Houstoun, Phil. Mag. [6] xxii. p. 740 (1911). 



