In the newest Siemens’ machine, represented in fig. 280c, the Gramme principle is made use of, as the revolving coil is of large diameter, and it consists of a copper cylinder, on which are wound a number of juxtaposed coils like those of a galvanometer. The revolving cylinder is surrounded by the poles of a system of electro-magnets excited by the whole of the induced current being passed through their coils. In a paper describing this machine, Siemens first made use of the term “dynamo-electric machine,” and this expression, contracted to the single word DYNAMO, has since been universally employed to designate machines of this kind. The modifications in the forms and arrangements of the different dynamos that have been invented in late years are endless, and every week patents are granted for further improvements and fresh combinations of the parts. It would be quite beyond the scope of this work to enumerate all the forms of the dynamo that have been favourably spoken of; but we shall content ourselves by adding a drawing of the Brush dynamo (Fig. [280d]), which has been so largely used for electric lighting in the United States. In this dynamo we have a Gramme ring, but the number of coils on it is reduced to eight, the intervals being filled up with pieces of iron, and the ring revolves in a vertical plane between the poles of two double oblong electro-magnets, which are arranged with poles of the same name opposite to each other. The commutators shown in the nearer part convert the alternately reversed currents generated in the coils into a direct continuous one. They are formed with bundles of wires, as in the Gramme machine.
Fig. 280d.—The Brush Dynamo.
Fig. 280e.—Siemens’ Regulator.
But the providing of a cheap and efficient source of current electricity, although an absolutely necessary step, would not have been capable of bringing about the present development of electric lighting, unless the appliances by which the current is made to manifest itself as light had not also been brought nearly to perfection. The conditions required to maintain a steady light from a current of electricity passing between carbon points have been already explained on page [497], and a representation of Dubosc’s electric lantern and regulator is shown. The regulator systems that have been invented since it became obvious that the light of the electric arc admitted of practical application on the large scale are very numerous. The earlier forms of regulator, which were used only for scientific purposes—such as lantern projections on screens, experiments on light, etc.—were complicated in their arrangements and uncertain in their action, for great variations in the light sometimes took place, and occasionally it would, indeed, be extinguished, and then again shine out as brightly as before. Nearly all the regulators that have come into use depend upon movements controlled by electro-magnetic actions produced automatically as the distance between the carbon changes. It would, however, lead us too far into the technicalities of the subject to explain minutely the mechanism of any particular form of the mechanical regulators, and the results depend so often upon the minute details, that it would be difficult to trace the action without a set of large and complete drawings. Perhaps the regulators that have been most used are those of Serrin, Siemens, Brush, Thomson, Houston and Edison. But nearly every inventor has produced different forms of his apparatus; Siemens, for instance, has patented eight or ten regulators. Fig. [280e] shows the mechanism of one of the last named inventor’s regulators, in which the two actions required for the separation and approach of the carbons are determined respectively by the vibrations of the rocking lever, M Y L, actuated by the electro-magnet, E, and the simple weight of the upper carbon-holder, A A. When the lamp is not in circuit, the lever, L, is thrown back by a spring, the tension of which is regulated by the screw, R, so that the catch, Q, is disengaged from the wheel, I. The train of wheels is then free to revolve by action of the rack, A, supporting the weight of the upper carbon, until the motion stops by the carbons touching each other. Now let the lamp be connected up, and the current will pass from C, through the electro-magnet, the mass of the apparatus, and return by the wire connecting the lower carbon-holder with Z. The carbon points will glow, but the magnet then attracting M moves the lever, L, the piece, Q, engages the wheel I, pushing it one tooth forward. But this movement of the lever establishes a contact at X, so that the current abandons the electro-magnet, to pass the shorter way, and M being no longer attracted, the lever is pushed back by the spring, the contact at X is broken, and the magnet being again excited the lever turns as before, and Q pushes I round the space of another tooth. These alternating actions succeed each other with great rapidity, and effect the separation of the carbons through the train of wheels acting on the racks. These movements continue until, in a second or two, the separation of the carbons has become so great, that the current passing through the electro-magnet is no longer able to operate against the weight of the upper carbon-holder, and this happens when an arc of proper size is produced, this required result being brought about by proper adjustment of the parts of the apparatus, marked by the letters R, K and X. But as the carbons are consumed, the increase of the length of this arc further weakens the current, until the spring attached to the lever, L, prevails over the attractive force of the electro-magnet on M, and thus withdraws the catch, Q, altogether, when the wheels being free to turn, the weight operates to bring the carbons nearer together, until, with the lessened resistance, the energy of the current is restored, and Q again comes into play to arrest the approximating movement. It may be seen, from the above explanation, that this lamp is automatic; in other words, when it has once been properly adjusted, it is lighted by merely completing the circuit. For fixing the carbons properly in their holders there are, of course, other regulating screws. How very nearly perfection the automatic regulation of the arc electric lamp has been brought by such contrivances as these, will be obvious to all who have noticed the steadiness that has been attained in all the modern installations.
Fig. 280f.—Jablochkoff Candle.
An ingenious plan was devised by Jablochkoff for dispensing with all mechanism for regulating the distance of the carbons. This invention is known as the electric candle, and is of great interest from the fact that it was with this arrangement that the electric light was, for the first time, practically employed for street and theatre illumination. This was in 1878, when visitors to Paris, during the Exhibition, were astonished by the splendid displays in the Avenue de l’Opéra, at the shops of the Louvre, and at some of the theatres. Then it was shown, for the first time, that electric lighting was not merely a scientific curiosity, but a new and formidable rival to gas. The Jablochkoff candles were also subsequently used in the electric lamps on the Thames Embankment. The principle of the contrivance will be understood from fig. 280f. Two carbons, C and D, are placed parallel at a little distance apart, and the space between them is filled up with plaster of Paris, kaolin, or some similar material, through which the current will not pass, but which burns, fuses, volatilises, or crumbles away by the heat produced by the passage of the current between the two carbons. These carbons are, of course, fixed in insulated holders, and to start the candle a small tip of carbon paste is made to connect the carbons at the top. The Jablochkoff candles must be used with currents rapidly alternating in direction. The reason for this is, that otherwise one of the carbons (the positive one) would be consumed quicker than the other, and that would cause the distance between them to increase, until it became so great that the current would cease to pass, and the light would go out. In order to obtain such alternating currents with the Gramme machine, a special apparatus had to be devised to change its direct into alternately reversed currents; but, dynamos intended to supply electric lights are now made without commutators, and they supply rapidly succeeding currents in opposite directions. In certain types of dynamos, again, the armature coils are stationary, and it is the field magnets that are made to revolve, and in these cases, not even a sliding contact is required, but the end of the armature coils are directly and permanently connected with the main circuit. But as these dynamos are self-exciting, the electricity induced in a few of the armature coils is collected apart from the main circuit, and passed through the electro-magnets of the machine itself, after the alternate currents have, by means of a commutator, been converted into one direct continuous current.
