ELECTROMOTIVE FORCE.] ELECT 11 I C I T Y 101 charge on the prime conductor can be raised. We can never get a longer spark from the machine than the length of the interval between the collector and the cushion or the axis, as the case may be. The limiting potential can, however, be increased by insulat ing the axis of the machine, or making the axis itself wholly, or partially of insulating material, and by using only one rubber and one collector, and placing them at the extremities of a diameter. The machine of Lc Roy, often called Winter s machine (fig. 57), is V rue s j;li inc. l- tion of V Jers, FIG. 5". Le Roy s machine. constructed on this pattern. We get, of course, ctctcris paribus, only half as much electricity per revolution with a machine of this kind as with Ramsden s ; but the spark is longer, in consequence of the greater insulation between the cushion (A) and the collector (B). The cylinder machine, also called Nairne s machine, was one of the first machines in which all the essential parts of the modern frictional machine appeared. It consists of a glass cylinder, which can be turned about a horizontal axis by a multiplying gear, or (as is now more usual) by means of a winch handle simply. The cushion is affixed to one horizontal metal cylinder, and the collector to another. It is necessary to insulate the axis in this machine, owing to its proximity to the ends of the conductors. Positive or negative electricity can be obtained with equal readiness by insulat ing either of the conductors, and connecting the other with the earth. Those who desire inore minute information concerning the functions of the different organs of the frictional machine, are referred to Mascart, torn. ii. 834, &c. In the same place will be found a description of the famous machine with double plates con structed by Cuthbertson for Van Marum, and still to be seen in Teyler s Museum at Haarlem. A description of another of Van Marum s machines will be found in the article "Electricity" in the Encyclopaedia Metropolitans. We take this opportunity of calling the scientific reader s attention to that article, which con tains a great quantity of very valuable matter. Much of the work of the earlier electricians that we have been obliged to pass over in silence is fully described there. Electric machines have also been constructed of less costly mate rials than glass or even vulcanite of cloth and paper, for instance for an account of these, see Riess, Bd. ii. 936, 937. Many experiments have been made on the electrification of sifted powders. We have already, in describing Lichtenberg s figures, alluded to some cases of this kind. As a rule, either the results are very uncertain, or the conditions of the experiment very com plicated, so that the experiments are, in most eases, more curious than valuable, from a scientific point of view. Such as desire it will find abundant indications of the sources of information in Riess, Bd. i. 938 sqq., and Encij. ^[ctrop., art. " Electricity," 193 sqq. One ease of this kind, however, was so famous in its day, that we ought to mention it. In the year 1840 a workman at Newcastle, having accidentally put one hand in the steam which was blowing off at the safety valve of a high-pressure engine boiler, while his other hand was on the lever of the valve, expe rienced a powerful electric shock in his arms. Armstrong investi gated the matter, and was led to construct his famous hydroelectric Machine. This apparatus consists simply of an insulated boiler for generating high-pressure steam, fitted with a series of nozzles, kept cool by a stream of water. The steam issues from these nozxles and impinges on a conductor armed with points for collect ing the electricity. The boiler gets electrified to a high potential, and a torrent of dense sparks may be drawn from it. The machine far surpassed any ordinary electrical machine in the quantity of electricity furnished in a given time. By means of it water was decomposed, and the gases collected separately. It was reserved for Faraday to trace the exact source of the electromotive force. He demonstrated, by a series of ingenious experiments, that the electrical action arose from the friction of the particles of water in the condensed steam against the wood of the nozzles. 1 Jtcmaining Ca;s. Of these the most important are atmospheric Miscel- electricity, 2 which belongs properly to meteorology, animal elec- laneous tricity, comprehending the study of the properties of the electrical results, fishes, and the electric phenomena of nerve and muscle. "We have already indicated the literature of the former subject, and the latter belongs, for the present at least, to physiology. Evaporation, com bustion, and hi fact chemical action generally, have been brought forward by some experimenters as sources of electromotive force. About the last of all there is, of course, in one well-known case no doubt. As to the experiments generally alluded to under the other two heads in particular, those of Laplace and Lavoisier, Volta, Pouillet, and others there has been considerable difference of opinion, and we need not occupy space here with fruitless dis cussion of the matter. 3 Similar remarks apply to the electrification caused by pressure, cleavage, and rupture. Machines founded on Induction and Convection.* The oldest Electro- electric machine on this principle is the electrophoras of Yolta. phorus. 1775. This consists of a plate of resinous matter (now usually vulcanite) backed by a plate of metal, and a loose metal plate, which we may call the collector, fitted with an insulating handle. The vulcanite is electrified by flapping it with a cat-skin, the collector is placed upon it, uninsulated for a moment by touching it with the finger, 5 and then lifted by the insulating handle. The collector plate is then found to be charged (positively) to a high potential, and sparks of some length may be drawn from it. The explanation of the action of the electrophorus is simple enough, if we keep clearly in view the experimental fact that the surface elec trification of a non-conductor, like vulcanite, will not pass to a metal plate in contact with it under ordinary circumstances. If the surface density of the electrification be very great, discharge to the metal may no doubt take place ; and if the collector be kept for a very long time in contact with the vulcanite, it is s.iid that it may become negatively electrified. In the normal state, however, the negative electricity of the vulcanite remains upon it, and the thin layer of air intervening between it and the collector forms the dielectric in a condenser of very great capacity, so that a quantity of electricity collects on the lower surface of the condenser very nearly equal to that on the vulcanite. The difference of potential between the plates is very small (just as in Volta s condensing elec troscope, see above, p. 34). When the collector is raised it carries away the positive charge the potential of which, owing to the decrease in the capacity of the collector, rises enormously. It is to be noticed that the potential of the charge on the vulcanite rises to a corresponding extent. This remark partly explains the re markable fact that, when the collector is kept on the excited vulcanite, its electrification may be kept for a long time (for weeks under favourable circumstances), whereas it speedily dissipates if the vulcanite be left uncovered. According to Riess, the fact that a plate of metal laid on an excited piece of glass tends to preserve its electrification was discovered by Wilcke in 1762. ; If each time we charged the collector it were discharged by con tact with the interior surface of a hollow conductor A, it is obvious that we could raise A by a sufficient number of such contacts to as high a potential as we please, provided it were sufficiently well insulated. This remark brings Volta s electrophorus into the pre sent category of electrical machines. In the rest of the induction machines to be described the excited dielectric is dispensed with, and an electrified conductor substituted in its place. The earliest apparatus that involved the principle of such machines appears to have been Beunet s doubler." The principle of this Beimet n apparatus may be explained thus. Let A and C be two fixed discs, doubler. and B a disc which can be brought at will within a very short dis tance of either A or C. Let us suppose all the plates to be equal, and 1 Exp. Res., ser. xviii. 2075. 2 See Riess, 1028 5177., and Thomson s papers in Kejwint already alluded to ; also Ency. Metrop., art. "Electricity," 219, for biblio graphy of older investigators. 3 See Riess, 943 sqq. 4 This highly-descriptive title is Sir William Thomson s. 5 In most modern specimens this is rendered unnecessary by a brass pin, which is in metallic connection with the metal backing of the vulcanite, and comes up flush with the surface of the vulcanite, so as to touch the collector when it is I M situ.
6 J hil. Trans., 1787.