296 Ml". J. N. Hearder on the Amount of Electricity developed 



the useless length of silk flap increases the adhesion and friction 

 to such a degree, as not only to render it exceedingly laborious 

 to turn the machine, but also to hazard the fracture of the glass 

 necks of the cylinder, all of which is avoided by the short silk 

 flaps. 



The experiments detailed in the present paper have been con- 

 fined to machines with only one rubber. 



Having determined the conditions necessary to produce the 

 strongest electrical excitation in the cylinder machine before 

 mentioned, I was desirous of comparing cylinders of different 

 sizes with each other, and I accordingly adopted the same stand- 

 ard of comparison, viz. a uniform length of discharge from the 

 same Leyden jar. In this way I have compared the action of 

 machines varying from 6 inches to 12 inches diameter, and 

 have found that when they arc all constructed with the same 

 care, and the same conditions are as nearly as practicable main- 

 tained in all, they afford, within moderate limits, the same quan- 

 tity of electricity for equal surfaces rubbed; but any considerable 

 inequality in the surface of the glass which shall prevent the 

 rubber from fitting nicely, or the silk flap from lying smoothly, 

 will produce a considerable deterioration in power, amounting to 

 10 or even 20 per cent., as compared with true and smooth 

 cylinders; thus I have a 12-ineh cylinder which has an irregular 

 surface, which excites very little more electricity than a 9^ inch 

 cylinder the surface of which is remarkably true and smooth, 

 although either cylinder is fitted to work in the same frame, and 

 with the same rubber and flap, and both are mounted with equal 

 care. 



In experiments of this kind, it is absolutely necessary that 

 the same amalgam should be used, and that those experiments 

 which are to be compared with each other should be performed 

 within moderately short intervals, as the quality of amalgam un- 

 dergoes remarkable changes in the course of a few months. I 

 have frequently set by a sample as weak and useless, which in 

 the course of a few months has become remarkably energetic, 

 and vice versa, a choice sample has often during the same 

 period become almost useless ; indeed it often happens that in 

 making amalgam, two samples will vary as much as 50 per 

 cent, in their qualities, although made at the same time and with 

 every possible provision to make them exactly alike; hence ex- 

 periments made at different times, and with different samples of 

 amalgam, cannot fairly be compared with each other. 



The amalgam which I usually employ, consists of 9 parts of 

 mercury, 5 of zinc, and 3 of tin, the two latter being fused in an 

 iron ladle, and the mercury then added and well stirred. This 

 mixture is then put into a wooden box and shaken for half an 



