Electromotive Force of a Constant Voltaic Cell. 413 



dried at a gentle heat. The solution contained about 5 per 

 cent, of cadmium iodide and '0032 grm. of free iodine in 

 1 cubic centim. A little solid iodine was added to the cell 

 to keep up the strength of the iodine solution. 



It is sufficient to state that the weight of cadmium used 

 and the weight of combined iodine found corresponded to the 

 formation of cadmium iodide, as the result of the reactions in 

 the cell. Probably about one third of the cadmium iodide 

 formed was due to the direct action of the iodine on the cad- 

 mium plate. 



After setting up the cell its E.M.F. was measured on the 

 electrometer against a standard Latimer-Clark. This mea- 

 surement, it was found, was not affected by setting the plates 

 in motion. 



The plate being now started moving, the cell was connected 

 to the galvanometer, and a deflection obtained corresponding 

 to *1054 ampere. The external resistance did not amount to 

 *5 ohm, showing the internal resistance of the cell to be some- 

 where about 10 ohms. I next proceeded to measure this 

 internal resistance by interpolating known resistances in the 

 circuit. 



Now, as is well known, it is impossible to measure the 

 internal resistance of an ordinary voltaic cell in this way, 

 because every variation in external resistance alters the E.M.F. 

 of the cell. It was necessary therefore to determine whether 

 internal resistance, as calculated from different sets of two 

 interpolated resistances, was the same. 



It is to be noted here that the calculation of the resistance 

 in the rest of the circuit by this method is not capable of great 

 accuracy, as is at once obvious from an examination of the 

 formula used. 



= x; 



where x is the unknown resistance, r, r' the interpolated 

 resistances, and i, i' the respective current measurements. In 

 fact, when the resistance of the whole circuit is varied one 

 third of its whole amount, an error of 1 per cent, in the 

 galvanometer-reading becomes about 10 per cent, in the cal- 

 culated resistance of the circuit. Keeping this in mind, the 

 differences in the following numbers are within the errors of 

 experiment ; and it is further to be noticed that they are quite 

 irregular, and that the resistance of the cell shows no tendency 

 to increase with diminishing external resistances. 



