a rapid succession of Electrical Discharges. 335 



with the inner and outer coatings of the jar, which latter is also 

 in connexion with the coil. The apparatus being thus arranged, 

 if the pendulum be made to swing, the two arms will describe an 

 arc over the periphery of the paper; and if the coil be set in 

 action, discharges will pass between the points, marking the arc 

 with perforations. If, now, the disc be turned slowly on its axis, 

 a series of curves will be described by the perforations from the 

 oscillating points. The number of perforations in each curve 

 may be counted ; and if reckoned from the top of one curve to 

 the top of another, we get the number per second ; or if from 

 the top of one curve to the bottom of the same one, the number 

 in a half-second. The rapidity of the discharges will depend 

 upon the nature of the inteiTupting spring employed in the coil, 

 and as many as 100 to 200 per second may be obtained. It is 

 necessary that the paper should be rather slight for this purpose, 

 and not turned too slowly, otherwise each discharge will not 

 make a new perforation for itself, but some will occasionally pass 

 back over the paper through the perforation made by the last 

 discharge. A little observation will, however, soon obviate this. 



Having now the means of counting the number of discharges 

 of the coil per second, their equivalent in glass surface was easily 

 calculated, and the following is the result. 



I took one of my cylinder machines of 11 inches diameter, 

 having a 10-inch rubber, and which in ordinary excitation will 

 give six straight sparks between 3 and 4 inches long, from a con- 

 ductor 5 inches in diameter and 18 inches in length, for every 

 revolution of the cylinder, and a strong current of zigzag sparks 

 between 9 and 10 inches in length, from a 2-inch brass ball. 

 The quantity of surface rubbed by this machine to produce a 

 discharge equivalent to a single discharge of the coil, was about 

 16 square feet; therefore when the coil gave 150 discharges per 

 second, it was equal to 2400 square feet of glass surface per second; 

 and as the machine could not be comfortably worked at the rate 

 of more than 3 square feet per second, the coil was equivalent to 

 the action of 800 such machines, and when giving discharges at 

 a more rapid rate, could be made equivalent to 1000 of them. 

 A 24-inch ))latc machine, having four rubbers of 5 inches in 

 length, is just equivalent in actual power to one of my 11 -inch 

 cylinders ; therefore the same number of such machines would 

 be required to do the work of the coil. During my late visit to - 

 London I was desirous of obtaining the equivalent of the coil in 

 Hteain electricity, and through the kindness of my friend Mr. 

 Pepper, who generously placed the powerful hydro-electric machine 

 of the Royal Polytechnic Institution at my disposal, I was able 

 to accomplish this. The machine having been powerfully ex- 

 cited, the steam being at 90 lbs., and the full number of jets iu 



