The latest improvement has been that from the magneto-electric to the dynamo-electric machine. It is due to both Dr. Siemens and Sir C. Wheatstone. Induced currents are directed through the coils of the electro-magnets which produce them, increasing their magnetic intensity, which in its turn strengthens the induced currents, and so on, accumulating by mutual action until a limit is reached. _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 _NN__{1}, _SS__{1}. [Illustration: Fig. 164.]