414 Messrs. J. J. Thomson and H. F. Newail. [June 16, 



either A or C, and the condenser and electrometer were then charged ; 

 the cells were then disconnected, and the fall of potential as noted by 

 the electrometer readings was observed at equal intervals. A metro- 

 nome was used to beat half-seconds ; the position of the spot of light 

 on the scale was noted every 5 seconds generally, and a curve was then 

 plotted with the time for abscissae and the potentials as denoted by 

 the electrometer deflection for ordinates. 



In preliminary experiments, in which a sample of benzene (later 

 experiments showed it to be very impure) was used in the condenser, 

 it was found desirable to introduce an air condenser, so as to diminish 

 the rate of fall of potential, and to get more reliable readings. The 

 air condenser consisted of two worked brass plates, held very slightly 

 apart by three dots of shellac. One plate was attached to the inner 

 cylinder, the other connected to earth with the outer cylinder of the 

 experimental condenser. 



From time to time throughout the course of the experiments, which 

 were continued through about three months, the apparatus was tested 

 for leaking (i) through the electrometer, (ii) through the key, 

 (iii) through the air condenser, (iv) through the attachments of the 

 experimental condenser. 



To get rid of dirt and dust from the liquids to be placed in the 

 condenser, very careful filtering was necessary ; bat generally after 

 ten or twelve times, first through Swedish filter-paper, and finally 

 through a tight plug of " glass-wool," as much as could be done by 

 filtering had been done. 



Difficulties were at first met with on account of what were probably 

 chemical impurities in the samples used. We finally decided to use 

 much smaller quantities of the liquids in the condenser. There was 

 scarcely ever more liquid than would half immerse the inner cylinder, 

 that is, more than would extend 4 inches up the straight part of it ; 

 but there was never less than would extend three- eighths of an inch 

 up the straight. 



Mode of Entering Results of Experiments. 



The potential of the inner cylinder was noted at equal intervals of 

 time (generally five seconds) from the deflection of the electrometer 

 needle. These deflections were recorded in tables, and the ratios of 

 succeeding deflections were deduced. Two curves are drawn, one 

 showing the fall of potential with the lapse of time, the other showing 

 the value of the ratio of successive values of the potential. The mean 

 value of the ratios is found, and a line drawn with this value. This 

 cuts the curve of ratios, and shows at a glance the departures from the 

 constant ratios, which would obtain if the leak through the dielectrics 

 took place according to Ohm's law. 



