OF ARTS AND SCIENCES. 155 



through the galvanometer. It is hardly necessary to say, that the 

 width between the pieces m and n is made slightly greater than the 

 width of j, to prevent a direct current from the battery through 

 the galvanometer. 



The method above described has been found very satisfactory for 

 the purpose of obtaining a single swing of a not too heavy pendulum; 

 it is essential that the stop e should be somewhat elastic ; a short 

 piece of soft rubber tubing slipped over the pin answers well. 



The upper pole of the magnet is brought quite near the upper part 

 of the pendulum bob to strengthen the induction. 



The motion of the magnet is so slight, that its direct effect upon 

 the galvanometer is negligible. No great care is necessary in the 

 release, as, even if the bob is brought up against the stop f, so as to 

 rebound, the time of describing the second and third quarters of the 

 complete vibration is altered by an inappreciable amount. 



With the apparatus described a great many measurements were 

 made, and these all gave concordant results. The following tables 

 give, in microcoulombs, the charges which variously arranged batteries 

 of water cells gave, in the times named, to condensers of different 

 capacities. Each number in the body of a table is the mean of a set 

 of closely agreeing observations. In the cases of many of the com- 

 binations of cells we have used a much greater number of different 

 capacities than is indicated in the tables, but the results given below 

 are representative. 



Whereas in studying certain other cells we have found it desirable 

 to use times shorter than the ten-thousandth of a second, it is evident, 

 from the results in Table L, that in the present instance it was not 

 worth while to use charging times smaller than the hundredth of 

 a second. 



