THE SMIIHSONIAN INSTITUTION. 



345 



The electro-motive force of Deleuil's battery was determined by the 

 ciicniical method at the same time. The results of these determina- 

 tions have been given in a previous table. Of the six measurements 

 of Deleuil's battery, the last three belong to this series. 



After these determinations, it is easy to reduce the number of turns 

 necessary to revert the needle from 15° to 10°, to the unit of electro- 

 motive force described above. We have — 



hence 1 



15.1 turns, eq[uivalent to 823 of electro-motive force; 



54,51 



Thus the values determined by revolution of the rheostat expressed 

 in our unit, are as follows : 



For Daniell's battery 490. 

 '' Grove's '' 709. 

 '' Sl6hrer's " "741. 



The zinc of Daniell's battery during the last measurement was in 

 stronger sulphuric acid, for which case the direct measurement had 

 given the value 486. The electro-motive force of Stohrer's battery was 

 previously found somewhat greater. The numbers for Grove's bat- 

 tery differed considerably, on which account no dependence can be 

 jilaced upon them. 



In the same manner as here, values of electro-motive force, con- 

 nected with the individuality of the instrument, may be reduced to 

 our unit, provided the corresponding factor has been determined. 



To determine the resistance of the element, Wheatstone has given 

 several methods, the first of which only we will present here. 



Place the galvanometer and rheostat in the circuit, and so adjust 

 the latter that the needle of the former stands at a given point. The 

 force of the current S is — 



Sizr 



E 



Fig. 12. 



denoting by E the electro-motive force, by g 

 the resistance of the multiplier, by E the 

 whole of the remaining resistance in the cir- 

 cuit. This arrangement is rendered clear by 

 Fig. 12, g representing the galvanic element, 

 Jc the rheostat, n the multiplier. 



Making a branch to the current passing 

 through the galvanometer, by a wire whose 

 resistance is exactly equal to the resistance of 

 the multiplier, one-half of the current will 

 reach h from a through v, the other half will 

 pass through the galvanometer to h. The 

 resistance between a and h is now just half as great as before, when 

 only the multiplier was present; hence the power of the undivided 



current is nov,- — 



E^_ 



B + igr 



