Siemens' Electric Light.—The following is a description of Messrs. Siemens Brothers' dynamo-electric light apparatus, which, for use on board ship against boat torpedo attacks, &c., is equal, if not superior, to any similar apparatus yet produced, and which is extensively used in the German and other European navies. This apparatus was one of many others experimented on by Dr. Tyndal and Mr. Douglas, M.I.C.E., for the Trinity House.
Dr. Tyndal says: "I entirely concur in the recommendation of Mr. Douglas, that the Siemens machine recently tried at the South Foreland be adopted for the Lizard. From the first I regarded the performance of this handy little instrument as wonderful. It is simple in principle, and so moderate in cost that a reserve of power can always be maintained without much outlay. By coupling two such machines together, a great augmentation of the light is moreover obtainable."
Principle.—When a closed electrical circuit is moved in the neighbourhood of a magnetic pole, so as to cut the lines of magnetic force, a current is generated in the circuit, the direction of which depends upon whether the magnetic pole is N or S; it also depends on the direction of motion of the circuit, and according to the law of Lenz, the current generated is always such as to oppose the motion of the closed circuit.
All magneto-electric and dynamo-electric machines are based on the principle stated above, and are subject to many modifications.
The name dynamo-electric machine is given to it, because the electric current is not induced by a permanent magnet, but is accumulated by the mutual action of electro-magnets and a revolving wire cylinder or armature. It is found that, as the dynamic force required to drive the machine increases, so also does the electric current; it is therefore called a dynamo-electric machine.
Description.—In the machine here described, of which [Fig. 164] is an elevation, [Fig. 173] a part elevation, and [Fig. 165] a longitudinal section, the electric current is produced by the rotation of an insulated conductor of copper wire or armature coiled in several lengths, 8, 12, 16, &c., up to 28, and in several layers, longitudinally, upon a cylinder with a stationary iron core nn' ss', so that the whole surface of the armature is covered with longitudinal wires and closed at both ends, as in [Fig. 165]. This revolving armature is enclosed to the extent of two-thirds of its cylindrical surface by curved soft iron bars NN1, SS1.
The curved bars are the prolongations of the cores of the electro-magnets E E E E. They are held firmly together by screws to the sides or bottom of the cast iron frame of the machine, making it compact and strong.
The coils of the electro-magnet form with the wires of the revolving armature one continuous electric circuit, and, when the armature is caused to rotate, an electric current (which at first is very feeble) is induced by the remanent magnetism in the soft iron bars and directed through the collecting brushes into the electro-magnet coils, thus strengthening the magnetism of the iron bars,[V] which again induce a still more powerful current in the revolving armature.