368 TENTH ANNUAL REPORT OF 



more tlian three times as strong as the other. When the current is 

 well closed, a zinc and carbon element will effect as much as a Dan- 

 iells element of three times as great a mean surface. 



When the resistance is very great, the ratio is different ; then the 

 strength of the current is proportional to the electro-motive force, or 

 as 470 to 824. In this case, by increasing the surface of the zinc and 

 copper element, but little would be gained. Two Daniell's elements 

 wouhl have to be united to obtain the same effect as with one zinc 

 and carbon element. 



The effect of a zinc and carbon battery can be attained in all cases 

 with a Daniells battery by giving to single elements of the latter a 

 three-fold surface, and using twice as many of them as would be re- 

 quired of zinc and carbon elements. 



What has been said of the zinc and carbon battery holds good for 

 Grove's battery, since the constants are nearly the same in both. 



As a conclusion of this section we present the description of a few 

 instruments which have been used for measuring, in the course of the 

 previous experiments. 



§ 26. Bheostats. — To accomplish a gradual change of the resist- 

 ance in the closing circuit of an electfo-motor within the desired 

 limit, without being obliged to open the circuit, several instruments 

 have been proposed, chiefly by Jacobi and Wheatstone. Jacobi called 

 his instrument agometer. The descriptions are to be found in Pog- 

 gendorff 's Annalen, LIV 340, and LIX 145. An instrument of this 

 kind is very costly, and therefore will not be generally employed, es- 

 pecially since Wheatstone' s instruments, constructed for the same 

 object, besides answering the purpose equally well, are far simpler 

 and more convenient in manipulation. In my treatise on physics 

 (Lehrbuch der Physik 3 te., aufl. 2 ter. Bd., S. 193) I have described 

 Wheatstone' s rheostat with thick wire, which is to be used when the 

 resistance of the closing conductor is not very great. But when the 

 entire resistance in the battery is very considerable, a great length of 

 this thick wire would have to be wound or unwound to produce a 

 sensible change in the strength of the current ; consequently, in such 

 ca«es a rheostat with a thin wire must be used, and which, of course, 

 must have a different construction. 



Wheatstone's rheostat with thin wire is represented in Fig. 18. g 



is a cylinder of dry wood about 6 inches long and 1| in diameter ; h 



Fig. 18. jg a cylinder of brass having the same 



y-^^^y^X ■• dimensions. The axes of the two cylin- 



r*^ .'~ ' ; (lers are parallel. A screw-thread is cut 



^ -'^^ in the wooden cylinder, and at its end 



(the one seen in the figure) there is a brass 



ring to which the end of a long and very 



fine wire is fastened. This is so wound. 



upon the wooden cylinder as to fill all the 



screw-threads, and its other extremity is 



then fastened to the opposite end of tlie 



brass cylinder. The omall brass columns 



J and k, designed for clamping the wires, 



