344 TENTH ANNUAL REPORT OF 



added. Now insert a Grove's element into the same circuit, and so 

 regulate the entire resistance that the needle stands again at 45°. 

 To hring it down to 40° the resistance must he increased hy (say) 

 fifty turns of the rheostat ; then the electromotive force of Daniell's 

 hattery is to that of Grove's as 30 to 50. This is evidently the sim- 

 plest process for determining the ratio of the electro-motive forces of 

 difierent hatterios. 



VVheatstore used a multiplier as a rheometer, and on that account 

 had to insert a considerahle resistance to make the current of the 

 hydro-electric elements weak enough. Under these circumstances, of 

 course, only a rheostat with a thin wire can he used. 



Although this method was originally designed for a multiplier, it 

 may he also used with any other rheometer, as the torsion galvanom- 

 eter, tangent compass, &c. But with these instruments, which ad- 

 mit of stronger currents, the current used need, of course, not he very 

 weak, and therefore a rheostat with a thicker wire can be used. 



This method of Wheatstone gives us the values of electro-motive 

 force measured by the length of wires required to effect the retrogres- 

 sion of the needle ; hence these numbers are dependent on the in- 

 dividuality of the galvanometer and the rheostat. 



As examples of his method, Wheatstone adduces the following 

 measurements. Three small Daniell's batteries* of unequal size were 

 in succession brought into the circuit. To revert the needle from 45° 

 to 40°, the following number of turns of the rheostat Avere necessary : 



Copper cylinder 1^ inch high, 2 inches diameter, 30 turns. 



" u 3| u u 21 " " 30 " 



a a 6 u i( 31 u cc 39 a 



Thus the electro-motive force, according to the theory, is independ- 

 ent of the size of the pair of plates. 



When batteries o.' 1, 2, 3, 4, 5 equal elements were used as eleo- 

 tro-motors in succession, the following results were obtained : 



1 element required 30 turns. 



2 " '' Gl " 



o 



li 



91 



4 " " 120 " 



5 '' " 150 " 



Thus the electro-motive force of the battery is, as theory indicates, 

 proportional to the number of pairs of plates. 



1 have determined by this method the electro- motive force of a 

 Daniell's, a Grove's, a Stohrer's, and a Deleuil's element, using for 

 this j)urpose the tangent compass, and a rheostat with thick wire. 



For bringing the needle back from 15° to 10°, I found as follows: 



With Daniell's element, 9 turns. 

 " Grove's " 13 " 



" St6hrer's " 13.6 " 

 " Deleuil's " 15.1 " 



~' The elements were somewhat differently constructed from those of the ordinary Dan- 

 iell's battery. The porous clay cell contained only liquid zinc amalgam, and it, as well as 

 the cylinder of copper surrounding it, stood in a solution of sulphate of copper. 



