The following is another method of ascertaining the internal resistance of a battery cell.
A circuit is formed, consisting of the battery cell, a rheostat, and a galvanometer, and the strength _C_ is noted on the galvanometer. A second cell is then joined with the first, so as to form one of double the size, and therefore half the resistance, and then by adding a length _l_ of the rheostat, the strength is brought to what it originally was, _C_.
Then if _E_ is the electromotive force, and R the resistance of cell, _r_ the resistance of the galvanometer, and other parts of the circuit, the strength _C_ in the one case is C = E / (R + _r_), and in the other = E / ((1/2)R + _r_ + _l_), and since the strength in both cases is the same, R = 2_l_, i.e., the internal resistance of the cell is equal to twice the resistance corresponding to the length _l_ of the rheostat wire.
The comparative electromotive force of a battery may be determined by means of a double shunt differential galvanometer in the following method, as recommended by Mr. Latimer Clark.
"This can only be done relatively in terms of some other standard battery. First determine the resistance of the standard and of the other cells to be measured; then insert the shunt plugs at _A_ and _D_, Fig. 97, and also at _C_ and _B_, and join up the standard cell in circuit with a resistance coil to the terminals _A_ and _D_, and unplug the resistance coils until a convenient deflection is obtained, say 15°; note the sum of the resistances in circuit, including that of the battery galvanometer, resistance coil and connecting wires; now change the battery for another, and by unplugging the resistance coils bring the needle again to the same deflection, 15°; having again found the total resistance in the circuit, the relative electromotive force will be directly proportional to these resistances."
The electromotive force of a battery may also be measured statically by means of Thomson's quadrant electrometer, the poles of the battery being connected with the two chief electrodes of the instrument, in which arrangement no current will pass, and the electromotive force will be directly indicated by the difference of potential observed.