Page:The American Cyclopædia (1879) Volume XI.djvu/23

 MAGNETO-ELECTRICITY 15 peculiar core replaces the double iron armature of Saxton and Clarke ; this is a long cylinder around which a wire is wound lengthwise. The cylinder is made to revolve rapidly be- tween the opposite poles of a series of horse- shoe magnets ; the perpetually reversing mag- netism induced in the core by the magnets is carried in successive currents by the insulated wire coil to the commutator, and thence through the external circuit. In Wilde's machine, fig. 3, the external current from a small Siemens machine, M, is made to pass through a large coil, A B, enclosing a soft iron horse-shoe bar, which is thereby magnetized and acts as a per- manent magnet on a second revolving core, F, larger than but similar to that of the smaller apparatus. The latter core collects a much more powerful current than that first pro- duced, and this can be used to generate a third or higher order of current; but with each such increase of current we increase the power required to turn the cores ; and 'though the heat and light are magnificent, yet in no case can we convert into electrical energy more than a certain per cent, of the mechanical energy consumed. In the machine devised by Ladd in 1867, as shown in fig. 4, a principle has been introduced suggested a short time previously by both Siemens and Wheatstone. Two plates of soft iron, B B', are so placed that if they possess the least initial magnetism, as is ordi- narily the case, then the rotation of the Sie- mens armature, a', collects the currents, which are at once led into the coils about B and B', and thus elevate the original magnetism of the plates to a high degree of intensity. Between the opposite poles of the magnets rotates a second Siemens armature,, which collects the FIG. 4. Ladd's Machine. current for the external circuit. Gramme's ma- chine, invented in 1871, two views of which are given in figs. 5 and 6, differs materially from its predecessors in that it offers a really con- tinuous current instead of rapid alternations. This is effected by using a circular-ring of soft iron, A A, for the core in which the magnetism is to be induced. The coil of wire around the core offers a continuous metallic circuit, di- vided into numerous sections, the ends of the wires in each so connected with radial metallic 523 VOL. XL 2 arms, R R, that as the ring rotates the induced current flows continuously from these arms to certain fixed metallic pieces in frictional con- tact with them, and thence to the external cir- FIG. 5. Gramme's Machine. cuit. By dividing the current, one half may be led back to the exciting magnets, SON, and be used to increase the power of the ma- chine. The effect produced by these machines increases proportion- ately to the velocity of rotation up to an unknown limit ; it also increases with the number of coils encircling the ring core. The machines of the Alliance com- pany have been em- ployed for illumina- ting purposes at some French lighthouses, and those of Wilde F IG. 6. Gramme's Machine, have been similarly employed in Great Britain. The Gramme ma- chine has been used for the illumination of the Victoria tower in London, and in the galvano- plastic works of M. Christofle in Paris. Cur- rents of different Orders. An induced cur- rent, by its action on a third conductor, may produce another current, and this another, and so on. If we call the current of the battery a current of the first order, the first induced current is named that of the second order, and so on. The discovery and investigation of the principle and properties of currents of the dif- ferent orders is mainly due to Prof. Henry. On reflecting a little, it will be evident that these currents cannot be produced immediately by placing several straight wires parallel to each other and passing a current of electricity through one of them ; in this case the battery current would act on the surrounding wires, and simply produce in each of them an induced current of the second order. To obtain, therefore, cur- rents of the different higher orders, we employ a number of flat spirals, through one of which placed horizontally on a table is transmitted the current from the battery. Immediately above this, and separated from it by a stratum of air or a plate of glass, is a second flat spiral, the ends of which are connected with a third