426 M. W. Holtz on a new Electrical Machine. 



about half a turn distant, are two metal rods, e and/, which I 

 will call conductors, held by insulating supports. At their free 

 ends they are provided with binding- screws for fastening wires ; 

 at the other, with which they approach the glass disk, with radial 

 cross bars : these latter in turn are provided with a great number 

 of fine points \ an inch long, which stand quite near the disk, 

 but without touching it. These series of points are opposite the 

 external parts of the coatings. 



For the sake of greater insulation, it is best to coat the disks 

 with a solution of shellac. By the working of the apparatus, 

 the rotating disk especially becomes a conductor ; for it gradu- 

 ally gets covered with an exceedingly fine dust, which appears to 

 consist of carbon, and to be formed from the decomposition of 

 the resin. After three or four hours' continuous activity the 

 action perceptibly decreases ; and then, if a further decrease is 

 injurious, the disk is taken out and the dust removed by a 

 moistened pad. From time to time, a new coating of shellac 

 is desirable, to keep the action at the same level. 



The principle on which the apparatus works presupposes a cer- 

 tain electrical excitation ; and as this is not effected by the rota- 

 tion of the disk alone, it must be given by a previously electrified 

 body. For this a thin plate of vulcanite, about 4 inches broad 

 and twice as long, is well adapted, especially when the surface, 

 which is sometimes a good conductor, has been removed by scra- 

 ping with a knife. This becomes electrical when it is laid on a 

 table and rubbed with fur, and more easily if this is done alter- 

 nately on both sides. The electrified surface is brought near to 

 one of the coatings, while the disk rotates like the hand of a 

 watch, and the conductors are connected with each other or with 

 the earth. The two coatings then immediately take opposite 

 electrical charges, whose intensity increases, accompanied by a 

 crackling noise, till after a few minutes a definite and provi- 

 sionally constant maximum value is obtained. At the same time 

 a continuous electrical current is established, with which, as 

 long as it is not completely broken, experiments can be made. 



For this purpose a discharger is very convenient, which con- 

 sists of three insulated brass supports. If the two outside ones, 

 a and c (fig. 2, Plate V.), are connected with the conductors, or 

 one of them with the earth, the striking-distance between a and 

 b can be varied by means of the moveable wires m and n ; while 

 between b and c, instead of the wire /, the body which is to be 

 exposed to the action of the current can be inserted. 



The current loses its continuity if the wires m and n are not 

 in perfect contact ; for the tone which accompanies the lumi- 

 nous phenomenon shows that it consists of a great number of 

 small discharges. If these discharges are to be intensified at 



