Page:Popular Science Monthly Volume 3.djvu/606

590 the resistance of the coils thus embraced by the friction-disk has to be subtracted from the resistance of the circuit.

The possibility of augmenting the strength of the current by increasing the dimensions of the machine is too obvious to need more than a passing allusion. The effects may also be increased by connecting together several such machines, as galvanic piles are connected, either for intensity or quantity. The quality of the current likewise differs according to the kind of wire surrounding the armature, a short thick wire producing effects of quantity, and a long thin wire, of intensity. It is also easy to see that two horseshoe magnets, instead of one, may be made to act on one ring armature; that is to say, it may be actuated by four poles instead of two, or even by a greater number; always having a friction-disk between each pair of poles. Moreover, the permanent horseshoe magnet may be replaced by electro-magnets, which can be excited by a portion of the current derived from the machine itself, according to the now well-known method. At the beginning of rotation the residual magnetism of these electro-magnets will induce a feeble current in the ring; one-half of this passes round the electro-magnets, the four poles of which react on the armature. Of the four friction-pieces, two carry half the current to excite the electro-magnets, and the machine rapidly attains the maximum effect. From conducting wires attached to the other two friction-pieces a powerful current is available.

A machine of this kind, containing two horseshoe electro-magnets, one for exciting and the other for the exterior current, and having round each pole 7 kilos, of copper wire 3 m.m. diameter, when worked by hand, decomposes water, and fuses 26 centims. of iron wire 9-10ths m.m. in diameter. However slowly the armature is rotated, the needle of a large galvanometer having the wire only once round is deflected, and the effects increase in intensity as the velocity of rotation increases, up to a maximum of 700 or 800 turns a minute, a velocity which is easily obtained when steam is employed.

Such a machine, giving an absolute continuous current of electric force by the mere turning of a wheel, is of value outside the physical laboratory. It is available—(1) for medical purposes; (2) for telegraphy; (3) for electro-plating, gilding, etc.; (4) for military purposes, signalling, explosions, etc.; (5) for chemical decompositions; and (6) for electric illumination.

A large machine, which has lately been exhibited in London, driven by a 2½-horse-power engine, produced a light equal to 8,000 candles; a copper wire about 1¼ m.m. in thickness, suspended between the poles, became instantly red-hot with a revolution of little over 300 in a minute. Larger machines are being made that will probably give a light equal to 25,000 candles.

This machine has lately been examined by the French Société ď Encouragement, and, in accordance with therecommendation of the