DETERMINING THE CONSTANTS OF A VOLTAIC CIRCUIT. 
313 
§ 9 . 
When a perfectly constant element, a galvanometer and a rheostat are placed in a 
circuit as in fig. 1, the resistance of any interposed body may be ascertained in the 
following way. Observe the point at which the needle stands ; then remove the 
body, the resistance of which is to be measured, from the circuit, and, by means of 
the rheostat, add a sufficient length of wire to bring the needle again to the same 
point. The number of standard units corresponding to this added length will be the 
measure. 
It is a point of importance to determine the resistance of the wire of the galvano- 
meter employed in the experiments ; to ascertain this by the above method an auxi- 
liary galvanometer would be required, but when a second galvanometer is not at 
hand, recourse may be had to the following process. Take two rheomotive elements 
exactly equal both in electro-motive force and resistance; place one of them in the 
circuit fig. 1, and observe accurately the deviation of the needle; then interpose also 
the other element and bring the needle again to the same point by means of the 
rheostat. The equivalent of the wire uncoiled X, will be the measure of the resistance 
of the galvanometer wire g plus that of the connecting wires r. Subtracting r from 
X, the resistance of g will be determined, 
E 2 E , _ 
R + r + ^r 2R + r + ^T A ' ’ $ r ' 
The resistance of a galvanometer wire or any other interposed resistance may be 
still more accurately ascertained by means of the instruments described in § 16. 
§ 10. Process to ascertain the Sum of the Electro-motive Forces in a Voltaic Circuit. 
The rheostat affords a most ready means of ascertaining the sum of the electro- 
motive forces active in a voltaic circuit, without requiring for this purpose the aid of 
a rheometer graduated to indicate proportional forces, or having recourse to the 
tedious process of counting the oscillations of a needle, employed by Fechner in his 
investigations. To save time and trouble in this operation will be of great importance 
to the future progress of electro-chemistry, on account of the great number of expe- 
riments of this kind which yet remain to be made, and also from the fluctuations in 
the electro-motive forces of many circuits from chemical and other actions, which 
render observations requiring considerable time to make completely valueless. 
The principle of my process is as follows : — In two circuits, producing equal rheo- 
metric effects, the sum of the electro-motive forces divided by the sum of the resist- 
E Ft E 
ances is a constant quantity, i. e. -jr = ^ ; if E and R be proportionately increased 
or diminished, F will obviously remain unchanged. Knowing therefore the proportion 
of the resistances in two circuits producing the same effect, we are able immediately 
to infer that of the electro-motive forces. But as it is difficult in many cases to 
