Discoveries and Inventions of the Nineteenth Century

lengths of wire. The appearance of the spark, with this, as with other large induction coils, may be described as a thick line of light, surrounded by a reddish halo of less brilliancy, and this halo, unlike the line of the spark, had a sensible duration. The reddish glow might be blown aside by a current of air when a series of discharges was taking place, and partly separated from the denser looking line of light. The latter is no doubt formed by intensely heated particles of the metals between which the discharge takes place, while the former is probably due to the incandescence of the oxygen and nitrogen gases in the air. The disc shown in our illustration behind the coil was for carrying six Geissler tubes, to display the pretty experiment of the various colours of the luminous discharge in different attenuated gases. When the coil was first mounted it was provided with an ordinary contact-breaker, but as the strong sparks were found to very soon destroy the contact points, a contact-breaker was substituted on Foucault’s plan. In this, the contacts are made by a platinum tipped wire dipping into mercury, that occupies the bottom of a strong glass vessel and forms part of the circuit. The vessel is filled with alcohol, which is a non-conductor, and it is therefore in the midst of this liquid 507that the contacts are made and broken. This apparatus is shown in the illustration, on the table at the left. A favourite experiment at the Polytechnic was to connect one of the discharging wires of the coil with the back of a large looking-glass, and bring the other wire to the front. In this case the sparks assumed a peculiar appearance, for they became thin and wiry-looking, and divided into many branches. They were very bright, and the noise of the discharges, was crackling and quite different from that produced by the blow of the flaming sparks taken through the air. Their appearance is represented in Fig. 269. The effects in this experiment were probably due to the spark taking a path on the surface of the glass determined by points of moisture or other inequalities. Fig. 269.—Spark on the Looking-glass. Ruhmkorff’s coil has been of great advantage to the electrician, for it supplies a stream of high tension electricity like that of the common machine, but more readily and conveniently. M. Ruhmkorff was the first person to obtain the great prize of £2,000, which the late Emperor of the French (Napoleon III.) directed, in 1852, should be awarded every five years for the most useful application of the voltaic battery. But no award had been made until 1864, when the inventor of the induction coil was properly considered worthy of it. This invention was the means of bringing into notice a new range of interesting phenomena, especially those attending the discharge passed through highly exhausted vessels. Investigations into the circumstances which modify the appearances, and especially into the nature of the stratified discharge in which the vessels are filled with bands or flakes of light separated by dark intervals, have long engaged the attention of some of our ablest physicists. Remarkable results were obtained by Mr. Crookes with very highly exhausted vessels. These showed not only beautiful fluorescent luminous effects, but in them the discharge could produce mechanical actions, and Mr. Crookes was led to regard it as a stream of radiant matter. MAGNETO-ELECTRICITY. When it had been shown that an electric current was capable of evoking magnetism, it seemed reasonable to expect that the reverse operation of obtaining electric currents by means of magnets should be possible. Faraday succeeded in solving this interesting problem in November, 1831, and one of his earliest, simplest, and most convincing experiments 508for the demonstration of the production of electricity by a magnet is represented in Fig. 270. A B is a strong horse-shoe magnet, C is a cylinder of soft iron, round which a few feet of silk-covered copper wire are wound; one end of the wire terminates in a little copper disc, and the other end is bent, as shown at D, so that it is in contact with the disc, but pressing so lightly against it that any abrupt movement of the bar causes the point of the wire and the disc to separate. When the bar is allowed to fall upon the poles of the magnet, the separation occurs, and again when it is suddenly pulled off; and on each occasion a very small but brilliant spark is observed where the contact of the wire and disc is broken. It was in allusion to this experiment that a contributor to “Blackwood’s Magazine” wrote: Around the magnet, Faraday Is sure that Volta’s lightnings play; But how to draw them from the wire? He took a lesson from the heart; ‘Tis when we meet, ‘tis when we part, Breaks forth the electric fire. Fig. 270.—Magneto-electric Spark. Fig. 271.—A Magnet producing a Current. Fig. 272.—Clarke’s Magneto-electric Machine. If a coil of fine insulated wire be passed many times round a hollow cylinder, open at the ends, and the extremities of the wire connected with a galvanometer at some distance, then if into the axis of the coil, A B, Fig. 271, a steel magnet be suddenly introduced, an immediate deflection of the needle takes place; but after a few oscillations it returns to its former position. When the magnet is quickly withdrawn, the needle receives a momentary impulse in the opposite direction. The magnetization and demagnetization of the iron core in the induction coil would, therefore, of itself cause the induced currents already described, for these actions are equivalent to sudden insertion and withdrawal of a magnet. If we suppose 509C, in Fig. 271, to represent, not a magnet, but a piece of soft iron—the reader will remember that this soft iron can be, as often as required, magnetized and demagnetized by simply bringing near one end of it the pole of a permanent magnet (see page 484). Upon this principle many ingenious machines have been constructed for producing electric currents by the relative motions of magnets and of soft iron cores surrounded by wires. 510Clarke’s machine is shown in Fig. 272. A is a powerful steel magnet fixed to the upright. A brass spindle passing between the poles can be made to rotate very rapidly by the multiplying-wheel, E, on which a handle is fixed. There are two short cylinders of soft iron parallel to the spindle, united together by the transverse piece of iron, D, which turns with the spindle. Each bar is surrounded by a great length of insulated copper wire, and the ends of the wires are so connected with springs which press against a portion of the spindle, which is here partly formed of a non-conducting material, that the currents generated in the coils, although in different directions as they approach a pole and recede from it, are nevertheless made to flow in one direction in the external circuit. R R in the figure represent two brass handles, which are grasped by a person wishing to experience the shocks the machine can give when the wheel is turned. When the terminals of the coil are provided with insulating handles and connected with pointed pencils of charcoal, the electric light can readily be produced by expenditure of mechanical effort in turning the handle. The arrangement of the points for this purpose is shown in Fig. 273, and we shall presently see what advantage has been drawn from this experiment on a great scale as a source of light. Fig. 273.—Magneto-electric Light. It will be observed that during the revolution of the armatures, as the wire-covered iron cores are termed, there are two maximum and two minimum points at which the currents are strongest and weakest. These variations may be lessened by increasing the number of armatures and of magnets, and Mr. Holmes arranged a machine with eighty-eight coils and sixty-six magnets, and the connections were so contrived that the currents always flowed in the same direction in the external circuit. This machine required 1¼ horse-power to drive it when the currents were flowing, but much less when the circuit was interrupted, and it was designed for, and successfully applied to, the production of the electric light for lighthouse illumination. Instead of steel magnets which gradually lose their strength, it is obvious that electro-magnets might be employed, but this source of electricity is costly, troublesome, and inconstant. Mr. Wilde hit upon the idea of using a small magneto-electric machine with permanent steel magnets, to generate the current for exciting a larger electro-magnet, and the current from this produced a still more powerful electro-magnet, 511from which a magneto-electric current could be collected and applied. The same idea was subsequently applied in other forms, as by shunting off a portion of the current produced from the mere residual magnetism of an electro-magnet, to pass through its own coils and evoke a stronger magnetism, which again reacts by producing a more powerful current, and so on continually; the limit being dependent only on the mechanical force employed, and on the power of the wires to convey the electricity, for they become very hot, and, unless artificially cooled, the insulating material would be destroyed. The armatures used in Wilde’s, Ladd’s, and other machines of this kind, are quite different in arrangement from those of Clarke’s machine, and are far superior. They are formed of a long bar of soft iron, of a section like this, Ꮋ, and the wire is wound longitudinally between the flanges from end to end of the bar, up one side and down the other. This armature rotates about its longitudinal axis between the pairs of the poles of a file of horse-shoe magnets, either permanent, or electro-magnets excited by the magneto-electric currents. In this case opposite poles are induced along the edges of the bar, and these poles are reversed at each half-turn. The intensity of the induced currents increases with the velocity with which the armature is made to revolve up to a certain point; but because the magnetization of the soft iron requires a sensible time to be effected, and the poles are reversed at every half-turn, it is found that a speed increasing beyond the limit is attended by decrease of the intensity of the current. The intensity in such machines has, therefore, a definite limit. But in a modification of the magneto-electric machine, which has quite recently been invented by M. Gramme, the limit is vastly extended by the ingenious disposition of the iron core and armatures, and his machines appear to solve the problem of the cheap production of steady and powerful electric currents, so that electricity will soon be applied in processes of manufacture where the cost of electrical power has hitherto placed it out of the question. We shall now endeavour to explain the principle on which the Gramme machine depends, and describe some forms in which it is constructed. THE GRAMME MAGNETO-ELECTRIC MACHINE. Fig. 274. Fig. 275.—Gramme Machine for the Laboratory or Lecture Table. Let X, Fig. 274, be a coil of covered wire; then while a bar magnet, B A, is advancing towards it and passing through it, as at M, a current will flow through the coil and along a wire connecting its ends, s s. The current will change its direction as the centre of the magnet is leaving the coil 512to advance in the direction, B A. If A A´ be a bar of soft iron, with the coil fixed upon it, we can still excite currents in the coil by magnetizing the bar inductively. If the pole of a permanent magnet be carried along from A´ to M in a direction parallel to the bar, but not touching it, the part of the bar immediately opposite will be a pole of opposite name, and the advance of this induced pole towards M will be attended with a current in the coil, and its recession by an opposite current. It need hardly be mentioned that the