824 



ELECTRICITY. 



will not answer well where large supplies are re 

 quired. The essential parts of an instrument for 

 procuring large supplies of electricity for the purposes 

 of experiment, are the electric, the rubber, the prime 

 conductor, the insulator, and the machinery for setting 

 the electric in motion. The electric, by the excita 

 tion of which the electricity is to be developed, may 

 be made of various substances. Polished glass has, 

 however, received the preference. Its form is thai 

 of a hollow cylinder, or of a flat circular plate, re- 

 volving upon a horizontal axis. The cushion is 

 usually made of soft leather, generally basil skin, 

 stuti'ed with hair or wool, so as to be as hard as the 

 bottom of a chair, but yet sufficiently yielding to ac- 

 commodate itself, without much pressure, to the sur- 

 face of the glass to which it is applied. The prime 

 conductor is a cylindrical tube, each end terminating 

 in a hemisphere. There is no advantage in its being 

 made solid, for the electricity is only contained at the 

 surfaces. It may be made of thin sheet brass or 

 copper, or tin, or of pasteboard covered with gold 

 leaf or tinfoil. Care must be taken that its surface 

 be free from all points and asperities ; and the per- 

 forations which are made in it, and which should be 

 about the size of a quill, for the purpose of attaching 

 wires and other kinds of fixtures, should have their 

 edges well rounded and smoothed oft". In order to 

 render the arrangement of these parts more intelligi- 

 ble, we will describe two of the simplest and best of the 

 machines. The glass cylinder, E F, Fig. 2 is from 8 to 

 16 inches in diameter, and from 1 to 2 feet long, sup- 

 ported, for the purpose of insulation, on two upright 

 pillars of glass, D C, which are fixed firmly in a wooden 

 stand, A B. Two hollow metallic conductors G, I, 

 equal in length to the cylinder, and about one-fourth 

 of its diameter, are placed parallel to it, one on each 

 side, upon two insulating pillars of glass, J, K, which 

 are cemented into two separate pieces of wood capa- 

 ble of sliding out and in, and furnished with screws, 

 M and N, so as to place the conductors farther from 

 or nearer to, the cylinder. To one of these conductors 

 the cushion, H, is attached, which is nearly of the 

 same length with the conductor. Its pressure against 

 the cylinder is regulated by an adjusting screw, M, 

 adapted to the wooden base, on which the glass pil- 

 lar tliat supports the conductor is fixed. From the 

 upper edge of the cushion there proceeds a flap of 

 thin oiled silk, which is sewed on the cushion about 

 a quarter of an inch from its upper edge. It extends 

 over the upper surface of the glass cylinder to within 

 an inch of a row of metallic points, proceeding, like 

 the teeth of a rake, from a horizontal rod, which is 

 fixed to the adjacent side of the opposite conductor. 



This figure will make the constniction of that part of 

 the machine more intelligible. The line D represents 

 the side of the conductor next to the cylinder. E is 

 a wire inserted into the conductor, and screwed into 

 the rod A B. Into this rod pointed wires are fixed 

 whose ends nearly touch the surface of the cylinder 

 C C. The motion of the cylinder, which is given by 

 a simple handle, must always be in the direction of 

 the silk flap. That part of the cushion which comes 

 in contact with the glass cylinder, should be coated 

 with an amalgam of tin, zinc, and mercury, mixed 

 with hog's lard. The amalgam should be placed uni- 

 formly over the cushion, until level with the line 

 l( Tmed by the seam which joins the silk flap to the 



face of the cushion. No amalgam should be placed 

 over this line, nor on the silk flap ; and it is even 

 requisite to wipe the silk flap clean whenever the 

 continued motion of the machine should have soiled 

 it by depositing dust or amalgam on its surface. 

 The l>est amalgam is formed by melting together one 

 ounce of tin and two ounces of zinc, which are to be 

 mixed, in a melted state, with six ounces of mercury, 

 and agitated in an iron or thick w<x)den box until 

 cold. It is then to be reduced to very fine powder 

 in a mortar, and mixed with a sufficient quantity of 

 hog's lard to form it into a paste. The mode in 

 which the electrical machine just described acts, 

 will readily be understood. The friction of the 

 cushion against the glass cylinder produces a transfer 

 of electric fluid from the former to the latter ; that 

 is, the cushion becomes negatively and the glass 

 positively electrified. The fluid which thus adheres 

 to the glass, is carried round by the revolution of the 

 cylinder ; its escape is at first prevented by the silk 

 flap which covers the cylinder, until it comes to the 

 immediate vicinity of the metallic points, which, 

 being placed at a small distance from the cylinder, 

 absorb nearly the whole of the electricity as it passes 

 near them, and transfer it to the prime conductor. 

 Positive electricity is thus accumulated in the prime 

 conductor, while the conductor connected with the 

 cushion, being deprived of this electricity, is nega- 

 tively electrified. If both these conductors are in- 

 sulated, this action will soon have reached its limit ; 

 for when the cushion and its conductor have been 

 exhausted of their fluid to a certain degree, they 

 cannot, by the same force of excitation, supply any 

 further quantity to the glass. In order to enable it 

 to do so, we must replenish it, or restore to it a 

 quantity equal to what it has lost. This is done by 

 destroying the insulation of the cushion through the 

 means of a metallic chain or wire, extending from 

 the ball on the conductor,G, to the earth, which is the 

 *reat reservoir of the electric fluid. The prime con- 

 ductor will now be supplied with a constant stream of 

 3ositive electricity. If it be our object, on the other 

 land, to accumulate negative electricity by the same 

 nstrument, we have only to insulate the conductor to 

 which the cushion is attached, and to connect the 

 Drime conductor, I, with the ground, in order to allow 

 .he fluid to escape from it as soon as it is collected 

 rom the cylinder. The fluid will thus continue to 

 )e drawn, without interruption, from the negative 

 conductor, as it now meets with no impediment to its 

 discharge on the opposite side of the machine. That 

 the quantity of positive electricity produced in one 

 conductor is exactly equal to that of the negative 

 electricity in the other, is proved by the fact, tliat, if 

 the two conductors are connected by a wire, no signs 

 if electricity are obtained in any of the conductors 

 on turning the machine. The machine now described 

 s that most commonly used, but when experiments 

 are to be performed which require an expeditious 

 supply of electricity, and that in large quantities, 

 he plate machine, represented in figure 3, is used, 

 t may be observed, however, that its advantage over 

 he cylinder machine is, in some degree, counterba- 

 anced by the great expence of its erection and its 

 lability to be broken. A B is a stand in which the 

 wo uprights, F & E, are firmly fixed, and kept steady 

 >y a cross piece at the top. Through the middle of 

 hese uprights there passes a strong wooden axle car- 

 rying the circular plate of glass, D E F, which ought 

 a be truly ground and polished; in size it may vary 

 rom fifteen inches to two feet in diameter, and in 

 thickness from one to two inches. The plate is made 

 to revolve on its centre by means of the handle at- 

 tached to the end of its axis. A pair of rubl)ers 

 similar to that in the cylinder machine,) art made to 



