﻿538 Royal Society :— 



train or network of conductors, let the galvanometer and battery be 

 interchanged, another arrangement is obtained which will probably 

 be useful for a very different, although reciprocally related object. 

 Hence, as soon as I learned from Mr. Mance his admirable method 

 of measuring the internal resistance of a galvanic element (that de- 

 scribed in the first of his two preceding papers *), it occurred to me 

 that the reciprocal arrangement would afford a means of finding the 

 resistance of a galvanometer-coil, from a single deflection of its own 

 needle, by a galvanic element of unknown resistance. The resulting 

 method proves to be of such extreme simplicity that it would be in- 

 credible that it had not occurred to any one before, were it not that 

 I fail to find any trace of it published in books or papers, and that 

 personal inquiries of the best-informed electricians of this country 

 have shown that, in this country at least, it is a novelty. It consists 

 simply in making the galvanometer-coil one of the four conductors of 

 a Wheatstone's bridge, and adjusting, as usual, to get the zero of 

 current when the bridge-contact is made — with only this difference, 

 that the test of the zero is not by a galvanometer in the bridge 

 showing no deflection, but by the galvanometer itself, the resistance 

 of whose coil is to be measured, showing an unchanged deflection. 

 Neither diagram nor further explanation is necessary to make this 

 understood to any one who knows Wheatstone's bridge. 



" On a Constant Form of Darnell's Battery." By Prof. Sir William 

 Thomson, F.R.S. 



Graham's discovery of the extreme slowness with which one liquid 

 diffuses into another, and Fick's mathematical theory of diffusion, 

 cannot fail to suggest that diffusion alone, without intervention of a 

 porous cell or membrane, might be advantageously used for keeping 

 the two liquids of a Daniell's battery separate. Hitherto, however, 

 no galvanic element without some form of porous cell, membrane, or 

 other porous solid for separator, has been found satisfactory in practice. 



The first idea of dispensing with a porous cell, and keeping the two 

 liquids separate by gravity, is due to Mr. C. F. Varley, who proposed 

 to put the copper plate in the bottom of a jar, resting on it a saturated 

 solution of sulphate of copper, resting on this a less dense solution of 

 sulphate of zinc, and, immersed in the sulphate of zinc, the metal zinc 

 plate fixed near the top of the jar. But he tells me that batteries on 

 this plan, called "gravity-batteries," were carefully tried in the late 

 Electric and International Telegraph Company's establishments, and 

 found wanting in economy. The waste of zinc and of sulphate of cop- 

 per was found to be more in them than in the ordinary porous-cell bat- 

 teries. Daniell's batteries without porous cells have also been tried in 

 France, and found unsatisfactory on account of the too free access of 

 sulphate of copper to the zinc, which they permit. Still, Graham's 

 and Fick's measurements leave no room to doubt that the access of 

 sulphate of copper to the zinc would be much less rapid, if by true dif- 

 fusion alone, than it cannot but be in any form of porous-cell battery 

 with vertical plates of copper and zinc opposed to one another as are 

 the ordinary telegraphic Daniell's batteries which Mr. Varley finds 

 * Phil. Mag. April 1871, pp. 314, 318. 



