﻿The Clark Cell when Producing a Current. 271 



unshielded value. The other terms are reduced to 1/b'lst, 

 l/101st, l/141st, &c, parts of their unshielded values. 



In this case the potential inside is very little less than 

 when there was no iron, A x being — A/a x 2n for all values of t 

 until t is small ; of course A x is when / is 0. 



It is easy to extend the reasoning to several enveloping 

 cylinders of iron, or to the ease of any distribution of inducing 

 potential which does not vary in a direction parallel to the 

 axis. 



XXXI. The Clark Cell when Producing a Current. By 

 S. Skinner, 31. A., Lecturer at Clare College, and Demon- 

 strator at the Cavendish Laboratory , Cambridge*. 



§ 1. Introduction. 



rj^HE electromotive force of the Clark cell when not pro- 

 -»- ducing any current has been frequently determined, and 

 has been found to be of so constant a value that it is now used 

 as a standard. These values of E.M.F. are for the cell when 

 its poles are not united by a conductor. In the following an 

 account is given of a series of experiments made on cell 

 when producing currents, with the view of ascertaining, 

 (1) how far the total electromotive force round the circuit 

 differs from that of the open cell, and (2) how this new value 

 for the E.M.F. changes when the current is maintained. If 

 these quantities can be accurately measured, it follows that a 

 cell might be used for producing currents of known value. 

 Some experiments have been made on subject (2) by Threlfall 

 and Pollock (Phil. Mag. November 1889), and their results 

 are compared with mine in Sect. 8. 



§ 2. The Cells. 



The experiments were made on three cells, all much larger 

 than the ordinary Board-of-Trade pattern of Clark cell. 



Cell B, made July 1891, is the cell no. 90 described on 

 p. 558 of the paper by Mr. Glazebrook and the author, Phil. 

 Trans. 1892, A. The area of its exposed zinc surface is 

 approximately 14*4 square centimetres. 



Cell L, made November 1892, is a much larger cell fitted 

 up in a cylindrical jar 22 centim. high and lo centim. in 

 diameter. The zinc plate exposes an area of about 95 square 

 centimetres. 



Cell N, made July 1893, is intermediate in size and 

 exposes about 29 square centimetres of zinc surface. 



* Communicated by the Physical Society: read June 22, 1894. 



