100 ELECTRICITY [ELECTROMOTIVE FORCE. Fric- tional series. Wilcke. Faraday. Feclet s experi ments. Contact of con ductor withnon conduc tor. Fric- tional ma chines. These hypothetical results tally very well with the electrical phenomena observed when non-conducting bodies are lightly rubbed together ; and the above is nearly the explanation that most physicists of the present day would probably give (if they gave any) of what is called the " Motional generation of electricity." All experimenters are agreed that equal quantities of positive and negative electricity appear in this case as in every other case of electrical separation ; an experiment to prove the contrary would have to be very demonstrative indeed before it would now be accepted as conclusive. A single case of exception would revolu tionize our fundamental ideas completely. The reader should consult on this point Faraday s Experimental Researches, series xi. IT ii. The other consequences of our hypothesis are by no means so finnly established. One of these is that we ought to be able to arrange non-conducting bodies in a series such that any body rubbed with one below it in the series becomes positive, and rubbed by one above it negative. Many electricians have attempted to establish such electro motive series, but the experimental conditions (see the admirable remarks of Riess, Rcibungselcctricifat, 907) are so complicated that nothing absolute has been attained. Yet it would appear that, if we could make sure that we were always dealing with definite materials under definite surface conditions, electromotive series could be constructed in which every different body would have a fixed position. As it is, the body bearing the same name in the lists of different experimenters was in all probability not exactly of the same material in all cases, and (we might say certainly) was not under the same surface conditions. We refer the reader to Riess (I.e. ) for an admirable resume of the work of different elec tricians in this department. Mascart has given a very interesting account of the matter (t. ii. 834, &c.) from a more modern point of view. From these sources, together with indications in Young s Lectures on Natural Philosophy, the reader will be able to follow up the literature of this somewhat uninviting department of elec tricity. We give two instances of frictional electromotive series which may be useful in giving the reader a general idea how different bodies stand. The following is Wilcke s series 1 (1758) : Glass, woollen cloth, feathers, wood, paper, shellac, white wax, ground glass, lead, sul phur, metals. Faraday 2 gives cat and bear skin, flannel, ivory, feathers, rock crystal, flint glass, cotton, linen, white silk, the hand, wood, shellac, metals (iron, copper, brass, tin, silver, and platinum), sulphur. To which Riess adds (in order) the highly negative bodies gutta- percha, electrical paper, 3 collodion, gun cotton. Considered as evidence for the contact hypothesis, the experiments of Peclet seem to be important. He used an apparatus which was virtually a Nairne s machine (see below, p. 101), in which the rubber could be varied at will. His general conclusions are quite in accordance with the contact theory. He found, for instance, that for the great majority of materials the quantity of electricity gener ated was independent of the pressure and of the breadth 4 of the rubber, and varied as the angular velocity of the cylinder, and it even appeared to be the same for rolling friction as for sliding fric tion, so long as the material of the rubber was unchanged. Besides the case of two non-conductors, we might consider the case of a conductor and a non-conductor in contact. Much of what has just been said would apply to this case also, an excellent , example of which is furnished by a frictional electrical machine of the ordinary construction when the cushions are well furnished with amalgam. This is the place to give a short account of these time- honoured pieces of electrical apparatus. For &*history of them we cannot do better than refer to Mascart 5 (I.e.), who has devoted much attention to the theory as well as the history of electrical machines in general. A very common form of machine, called Ramsden s, is pictured in fig. 56. It consists, like all other frictional machines, essen tially of three parts (1) the nibbed or moving body, (2) the rubbers, and (3) the collectors and prime conductors. In the pre sent instance the rubbed body is a disc of glass, which can be turned about a horizontal axis by means of a suitable handle. The efficiency of the machine depends very much on the quality of the glass of which the disc is made. According to Mascart, glass of old manufacture is superior to the more modern specimens, owing to the smaller proportion of alkali in the former ; it appears, however, that the disc improves in most cases with age and use. Many Ramsden s electrical machine. 1 According to Riess, the earliest. 2 Exp. Res., 2141. 3 The parchment-like paper obtained by treating ordinary paper with concentrated sulphuric acid. 4 That is. the dimension of the rubber perpendicular to the axis of rotation. s A few notices of the earlier machines will be found in the Histori cal Sketch. other materials have been proposed to replace glass, which is some what costly when large discs arc required. Ebonite has been tried a good deal of late, and has great advantages so far as its electrical pro perties are concerned ; but it has the disad vantage that it warps very readily if heated incautiously, and its surface will not keep good for any length of time. Owing to decom position under the action of light, a layer of sul phuric acid forms on the surface, after which it is very difficult to restore the electrical virtue so remarkable in the new material, although wash ing with hot water or immersion in a blast of steam are said to be effective in some degree. The rubbers consist of two rectangular pieces of wood, hinged to sup ports attached to the framework of the ma chine, and fitted with springs and screws, so that they can be made to clip the plate with any required pressure. The rubbing surfaces are usually formed of leather, stretched as smooth and flat as possible (oiled silk is sometimes used, but it is not so durable). Before the leather cushions are fit for use, they must be carefully coated with amalgam. The amalgam most commonly used is Kienmayer s, which is a composition of two parts of mercury with one of zinc and one of tin. A great variety of different compounds of this kind have been used by different electricians, bisulphide of tin being a general favourite. The amalgam must be powdered as finely as possible, all grit being carefully removed. The cushions arc then to be lightly smeared with lard, and worked together till the surface is very smooth and the greasiness almost gone ; then the amalgam is to be carefully spread over them, and the surfaces again worked together till a uniform metallic surface is attained ; 6 they are then ready for use. The amalgam aids the action of the machine in two ways, first, by presenting a surface which is highly negative to glass ; secondly, by allowing the negative electricity evolved by friction to flow away without hindrance from the points of contact. In order to secure the second of these advantages still more perfectly, the cushions should be carefully connected by strips of tinfoil, or otherwise, with knobs, which can be put to earth during the action of the machine. The collectors are two stout Tiietal forks bestriding the glass disc at the ends of a horizontal diameter. They are armed, on the sides next the glass, with rows of sharp points, which extend across the rubbed part of the disc. The prime conductor in the specimen we are describing forms a metal arch rising over the framework of the machine, and insulated from the sole by two glass pillars. Various forms are given to this part of the machine, according to the fancy or convenience of the experimenter. One important thing to be seen to is, that there be no salient points on it which might facilitate the dissipation of electricity by brush, convective, or spark discharge. After what has been said, the action of the machine requires little explanation. The disc, electrified positively by contact with the amalgam, carries away a positive charge, whose potential rises rapidly as it leaves the cushion, so high, in fact, that there is a tendency to discharge to the air, which is prevented by covering the excited parts of the disc by pieces of oiled silk. When the highly charged glass comes opposite the points of the collector, owing to the inductive action, negative electricity issues from the points and neutralizes the chargc-d plate, which at this point is vir tually inside a closed conductor. The result of this is that the prime conductor becomes positively charged. The neutralized parts of the disc now pass on to be rubbed by the other cushion, and so on. The electricity goes on accumulating in the prime conductor until the potential is so great that discharge by surface conduction, or by spark, takes place between the collectors and the cushion, or between the collectors and the axis. If it is desired to obtain negative electricity from a machine with a glass disc, we have simply to connect the prime conductor to earth, insulate the cushions, and collect the electricity from them. We have said that there is a limit to the potential to which tho
6 Mascart, J.c.