314 
MR. WHEATSTONE ON NEW INSTRUMENTS AND PROCESSES FOR 
determine the total resistance, consisting of the partial resistances of the rheo- 
rnotor itself, the galvanometer, the rheostat, &c., I have recourse to the following 
simple process. Increasing the resistance of the first circuit by a known quantity 
E ... 
r, the expression becomes ^ + r ; in order that the effect in the second circuit shall 
be rendered equal to this, it is evident that the added resistance must be multiplied 
by the same factor as that by which the electro-motive forces and original resistances 
E 7t E 
are multiplied, for gyqr?. = n R + n ~- The relations of the lengths of the added 
resistances r and n r, which are known immediately, give therefore those of the 
electro-motive forces. 
Experimentally I proceed thus : — I interpose the rheostat and the galvanometer in 
the circuit, and then add, by means of the former, assisted if necessary by the resist- 
ance coils, a sufficient resistance to bring the needle exactly to 45° ; I then ascertain 
the length of wire uncoiled from the brass cylinder of the regulator necessary to 
reduce the deviation of the needle to 40°. The number of turns is the measure of the 
electro-motive force, the number corresponding to that of a standard element having 
been previously determined. 
§ 11 . 
I subjoin a few measures of electro-motive forces obtained by the preceding 
process. 
1. Three elements of different sizes, consisting of copper, a solution of sulphate of 
copper, and a liquid amalgam of zinc, were successively placed in the circuit. The 
number of turns of the rheostat requisite to reduce the needle from 45° to 40° were, 
Small element described in ^ 5 30 turns. 
Copper cylinder 3| inches high and 2J inches diameter . . 30 turns. 
Copper cylinder 6 inches high and 3^ inches in diameter . 30 turns. 
Hence, conformably to the theory, the magnitude of an element occasions no dif- 
ference in its electro-motive force. 
2. Five small elements of copper and amalgam of zinc were charged respectively 
with the following five solutions of copper, the sulphate, the ammonia sulphate, the 
acetate, the per-tnuriate and the nitrate. Though the force of the current produced 
by each element separately was very different, owing to the different conductibilify 
of the solutions, yet, with the exception of the nitrate, all required the same number 
of turns, indicating equal electro-motive forces ; the latter fluctuated between 23 and 
29, owing to some disturbing action probably of the nitric acid on the mercury of the 
amalgam. 
3. The electro-motive forces of a circuit in which 1, 2, 3, 4, 5 similar elements 
were successively placed, were measured. 
