1871.] On a Constant Form of Daniel I' s Battery, 



253 



IV. " Modification of Wheatstone's Bridge to find the Resistance of 

 a Galvanometer- Coil from a single deflection of its own needle." 

 By Prof. Sir William Thomson, F.R.S. Received January 

 19, 1871. 



In any useful arrangement in which a galvanometer or electrometer and 

 a galvanic element or battery are connected, through whatever trains 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 re- 

 sistance of a galvanic element (that described in the first of his two pre- 

 ceding 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 re- 

 sistance. The resulting method proves to be of such extreme simplicity 

 that it would be incredible 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 explana- 

 tion is necessary to make this understood to any one who knows Wheat- 

 stone's bridge. 



V. "On a Constant Form of DanielFs Battery." By Prof. Sir 



William Thomson, F.R.S. Received January 19, 1871. 



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 with- 

 out some form of porous cell, membrane, or other porous solid for sepa- 

 rator, 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 solu- 

 tion 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 



