﻿when Producing a Current. 273 



§ 4. Measurement of the Internal Resistance of the Cells. 



The important determination of the resistance of the cells 

 was made by the method of Opposition, and by the use of an 

 Alternating Current. By opposing any two of the cells, the 

 resulting electromotive force was so small that, making them 

 an arm in a Wheatstone's arrangement, there was no deflexion 

 in the bridge-galvanometer. The bridge was fed with an 

 alternating intermittent current by means of the commutator 

 used by Mr. T. C. Fitzpatrick in his electrolytic measurements, 

 B.A. Report, 1886, p. 328. 



In this method the commutator is arranged to supply the 

 currents in the galvanometer-circuit always in the same 

 direction, so that an ordinary sensitive mirror-galvanometer 

 can be used. The drum of the commutator has on each 

 circular face eight insulated sectors of brass, those on one 

 side being larger than those on the other. The larger sectors 

 are connected by brushes to the battery-circuit, and the smaller 

 to the galvanometer-circuit. They are arranged so that the 

 battery connexion is always closed before that of the galvano- 

 meter, and is always broken after that of the galvanometer. 

 A more complete description of the commutator, which works 

 excellently, will be found in the paper already mentioned. 

 Two or three Leclanche cells were used as the battery. 



The method of connecting two cells together so as to have 

 small resulting electromotive force is particularly applicable 

 in the case of Clark cells, for they may be obtained of nearly 

 equal electromotive force. In the case of the cells we are 

 discussing, as mentioned in § 2, the greatest difference was 

 never more than 2 in 5000. 



The errors to which this method of measuring battery- 

 resistance is liable' are two : first, from self-induction, and 

 second, from polarization at the electrodes. The only con- 

 ductor containing self-induction of any magnitude is the 

 galvanometer- coil, and as the method is a zero method this 

 does not matter. The second error is eliminated by using an 

 alternating current. However, following Fitzpatrick's method, 

 tests were specially made for polarization by varying the ratio 

 of the arms and by varying the speed of the commutator. 

 For instance, on one occasion the resistance was between 11*0 

 and 11*1 with the usual speed for working; when the speed 

 was more than doubled, the resistance retained the same value, 

 The three cells were measured in pairs, and the results in 

 legal ohms are given in the following table, where C is the 

 resistance of the connecting wires. 



The words " Before," " After," refer to the relation of these 

 measurements to those recorded in § 6. 



