292 Mr. J. N. Hearder on the Amount of Electricity developed 



Great care was necessary to avoid discrepancies which would 

 otherwise arise from variations in the quaUty of the amalgam; 

 and when different amalgams were used^ their relative value was 

 always previously ascertained by having recourse to a given 

 standard of measm-ement, and all subsequent experiments were 

 reduced to this standard. The discharging interval was always 

 kept as small as was consistent with the character of the experi- 

 ment, since, with high charges, the conductor, partaking of the in- 

 tensity of the jar, takes up less and less of the electricity from 

 the cylinder as the charge increases, and consequently much of the 

 electricitythus excited passes roundupon the surface of the cylinder 

 to the rubber again, and the results are therefore inaccurate. 



Although my experiments will serve to show the superiority 

 of cylinders over plates, with regard to the quantity of electricity 

 excited by equal rubbed surfaces, I do not pretend to offer a 

 correct solution of the problem, as I am not at all sure of the 

 real cause of the extraordinary difference. The experiments 

 were undertaken more with a view of analysing the actions of 

 each class of machines under different modifications, than of 

 comparing them with each other. 



The first experiments were made with a view to ascertain the 

 connexion between the length of the rubber and the quantity of 

 electricity developed. A cylinder machine was therefore fitted 

 with four separate rubbers, whose lengths were respectively 

 1 inch, 2 inches, 4 inches, and 8 inches. Each rubber was fur- 

 nished with a silk flap, which reached half round the cylinder to 

 the conductor on the other side, and of a width corresponding to 

 the rubber to which it belonged, the smallest being 1 inch wide, 

 and the largest 8 inches. The collecting points of the conductor 

 were also made moveable, so that just so many might be used as 

 would be suited to the width of the end of the silk flap near to 

 which they were applied. The discharging interval of the coated 

 glass was varied in different sets of experiments, but in all cases 

 it was observed that the quantity of electricity developed was in, 

 as near as possible, an exact ratio with the length of rubber. 

 The length of rubber being multiplied by the circumference of 

 cylinder, and this again by the number of turns, gave the actual 

 amount of surface submitted to friction ; and it thus appeared 

 that the length of the silk flap being the same, the quantity of 

 electricity developed was just in proportion to the length of the 

 rubber or the surface rubbed. 



The next point which it appeared interesting to examine was 

 the part which the silk flap plays in the excitation of the machine. 

 Much more depends upon this than is usually supposed, and 

 accordingly the same cylinder exhibits very different results 

 with silk flaps of different kinds. Without going into detail of 



