ELECTRICITY. 



513 



Pracrfca! 

 Elect ridty. 



Vn Ma- 

 rum's elec- 

 trifying 

 machine. 



PLATE 



3,+, *. 



4. Description of I'an Marum's Electrifying Machine. 



The most perfect machine, perhaps, that has ever 

 been constructed, is that which was made under the di- 

 rection of Van Marum, and of which we have given a 

 perspective view and a section, in Figures 1, 2, 3, 4, 

 and 5. of Plate CCXLVIII Great ingenuity is dis- 

 played in the construction of this machine, not only 



CCXLVIII ! , L. /* i* A.* / i * 



Figs 1 2 ln preserving all its parts from a dissipation of electri- 

 city, but also in the method of obtaining positive and 

 negative electricity at pleasure. The plate of glass 

 AB, 31 inches in diameter, with all its apparatus, is sup- 

 ported upon a single column E, at the upper end of 

 which are two brass collars I, I, each of which is shewn 

 separately in Fig. 4. On these collars rests the axis 

 MX, which carries a counterpoise of lead L, to prevent 

 too much friction on the collar I nearest the handle or 

 winch W. The rubbers are shewn at TO, n, Fig. 1 . being 

 invisible in the section in Fig. 2. Each pair is sup- 

 ported in a horizontal position upon the glass pillars ef, 

 and is attached to balls O, P. The whole of this appa- 

 ratus is minutely and correctly shewn in Fig. 3. where 



0, P are the balls, and m, n the rubbers, with the glass 

 plate moving between them. Receiving conductors 

 C, D, each ot which is six inches long, and two and a 

 half in diameter, are fixed at the ends of the semicircle 

 CGI), which is attached to an axis g, that turns on the 

 ball G, so that the conductor CGD can be turned round 

 into any position. A copper tube h H, terminating in 

 a ball H, moves like a radius upon the stem of a ball 

 h Z inches in diameter, which being screwed into the 

 conductor G, serves to confine the tube A H in any re- 

 quired position. Attached to the extremity of the 

 bearing piece K, is an arc c I d of brass wire, ha]f an 

 inch in diameter, which has a motion of rotation round 



1, so that it can be turned like the conductor CD into 

 any position. Hence by this apparatus, the receiving 

 conductor CGD can be brought either directly opposite 

 to the rubbers m, n, or placed at right angles to them ; 

 and the same can be done with the small conductors c, 

 d, which are shewn in Fig. 1. as nearly in contact with 

 the rubbers m, n, while the conductors C, D have a ver- 

 tical position. By this ingenious contrivance, positive 

 and negative electricity can be produced at pleasure, 

 without any trouble. In the position represented in 

 Fig. 1. the conductor G will give positive electricity, 

 because the rubbers m, n communicate with the ground 

 by means of the semicircular wire eld, and the vertical 

 rod KK (Fig. 2.). But when negative electricity is 

 wanted, the arc eld is moved into a vertical position 

 away from the cushions, while the conductor CGD is 

 moved into a horizontal position, so as to leave the re- 

 ceiving conductors C, 1) opposite to the rubbers m, n. 

 In order to prevent all dissipation or loss of electricity 

 along the surface of t)i glass supports, a mahogany cap 

 T covers the metallic caps into wl.ich the supports are 

 cemented. The lower extremity of the supports is like- 

 wise guarded by a hollow ring of mahogany, covering 

 the metallic socket into which the support is iiiM-rted. 



Fig. 5. is a section of the moving part of the branch 

 clil. 



Fig. 3. shews the method of constructing the rubbers, 

 which have at their extremities four pieces of gum lac 

 a, l>, a, b. 



From this brief description, our readers will l>e ena- 

 bled to form a correct idea of the excellence of this in- 

 genious machine ; but for a minute account ot all its 

 p.irt~, we imi>t refer to the Journal de P/iytitjue for 



TOL.VUI. FART II. 



April 1789, and Feb. 1791 ; or to Nicholson's Journal, Fraction! 

 vol. i. p. 8i, 4to. ^"^ 



5. Description of Wolff's Electrifying Machine. 



The machine used by M. Wolff, of Hanover, is near- Wolff's 

 ly the same as that of Van Marum, which we have just electrifying 

 described, but it differs from it essentially in the con- macuine ' 

 struction of the rubbers. The rubbers, which are 5-J 

 inches long, 1^ broad, and \ of an inch thick, are 

 made of dry walnut wood, socked in amber varnish. 

 They are covered with a piece of thick woollen, on 

 which is a piece of fine neat's leather. When the leather 

 is fastened to the wood, it is wetted, and pressed be- 

 tween two boards till it is dry. It is then covered with 

 another piece of leather a little broader, having its rough 

 surface towards the glass. A piece of silk is next ap- 

 plied accurately to this leather, after the silk has been 

 heated and besmeared with butter of cacao, and a great 

 quantity of Kienmayer's amalgam. The leather is next 

 covered with amber varnish. Amalgam is spread over 

 this, and when the varnish is dry it is smoothed with a 

 burnisher. This operation is repeated several times. 

 When the rubber is very dry, and has been brought by 

 pressure to touch the glass in all points, the leather, 

 after being coated with amalgam, is covered with a 

 piece of white paper of the same length as the leather, 

 and about half an inch broad, so as to cover the seam 

 by which the silk is joined to the leather. The paper 

 is then fastened to the wood, above and below, accord- 

 ing as it is on the ascending or descending side of the 

 plate. A more minute account of this construction, and 

 of the advantages which its author supposes it to pos- 

 sess, will be found in Gilbert's Annalen der Physick, 

 180'2, and Nicholson's Journal, vol. vii. p. 124. 



6. Descrij>lion of Musnier's Electrifying Machine. 



This electrifying machine consists of a plate .of glass Musnier's 

 moving vertically, and differs only from those of the electrifying 

 common form in having the four cushions insulated up- mac ' line * 

 on pillars of glass, and connected with the upper ends 

 of these pillars by semicircular springs. A drawing and 

 description of the machine will be found in the Mem. 

 Acad. Par. 1 772, p. 502 ; and in Bertholon, DC I' Elcc- 

 Iricite du corps Jiumaiii, vol. ii. p. 220. 



7. Description of Si Julien's Machine. 



This machine consists of three plates of glass put in St Julien's 

 motion by a winch driving three-toothed wheels. The Machine, 

 winch is fixed on one end of the axis of the largest 

 wheel, and at the other end of the axis is fixed the 

 middle plate of glass. Above and below this wheel 

 is another wheel, upon the axis of which is a plate of 

 glass, so that by turning the winch all the three plates 

 are put in motion. Each plate is rubbed by four rub- 

 bers fixed to the upright supports, between which the 

 plates move. The conductor is supported upon two 

 glass rods, and the extremity of it nearest the machine 

 has three anus like a fork, the plane of which is verti- 

 cal. From the upper prong proceeds two long pointed 

 wires, which receive the electricity from the upper plate 

 of glass. From the middle prong proceed other two 

 similar wires, which draw the fluid from the middle 

 plate, and from the lower prong proceeds other two 

 wires, to take the electricity from the lower plate. Each 

 of the plates of glass is 1 5 inches in diameter; and the 

 machine- is, said to have been much more powerful than 

 a bingle plate machine thirty indies in diameter. A 

 3 T 



