4:66 Peirce and Willson — Measurement of the 



ance for a definite short time, less than one ten-thousandth of a 

 second. 



We found that if we assumed the electromotive force of the 

 battery to be the same during the short interval when the poles 

 were closed as it was just before this interval when the poles 

 were open, the value of the internal resistance given by the 

 method of alternating currents would not account for our results 

 and was therefore useless as a basis for computing the quantity 

 of electricity which the battery could furnish under slightly 

 different conditions. We thought it worth while, therefore, 

 to make some direct measurements of the difference of poten- 

 tial between the poles of a battery while they were connected 

 for short times by shunts of various resistances. By means of 

 the apparatus described in a paper on the charging of con- 

 densers, which we published* a short time ago, we were able 

 to keep the battery shunted for almost any interval at pleasure 

 from 0*3 sec, down to 0*0001 sec, and during this interval to 

 charge a condenser of suitable capacity by connecting its poles 

 to the poles of the battery and then disconnecting them.' The 

 charge which the condenser received could then be measured 

 at leisure by the help of a ballistic galvanometer. 



Our method of procedure was generally this. We first 

 charged the condenser at the open poles of the cell and measured 

 the charge, we then charged it with the poles connected 

 by a shunt of known resistance and measured the charge, and 

 finally we again took observations with open battery poles to 

 see whether the cell had become "fatigued" by the treatment 

 to which it had been subjected. 



Even though no sign of " fatigue " was shown by the process, 

 we did not feel sure that the electromotive force of the cell 

 had been the same in all the observations. Some previous 

 experiments with water cells seemed to show that when the 

 poles of such cells are connected, the electromotive force falls 

 in a very small fraction of a second to a value which depends 

 upon the current which the cell is delivering, and this value 

 then very slowly diminishes as the time goes on. If the poles 

 are closed for a short time only, say for half a second, the bat- 

 tery almost instantly acquires its old electromotive force when 

 the circuit is broken. If, however, the circuit is kept closed 

 for a number of minutes the battery becomes " fatigued " and a 

 comparatively long time must elapse before it acquires again 

 its old strength. For purposes of computation, however, we 

 made the usual assumption that the electromotive force of the 

 cell when its poles are closed for a very short interval only is 

 the same as the electromotive force with open poles, and we 

 computed B from the formula 



* Proceedings of the Am. Academy of Arts and Sciences, for 1889, pp. 146-163. 



