Imagini ale paginilor
PDF
ePub
[graphic]

I

FIG. 56.

and are intended to feed a number of arc lamps. The currents, therefore, like those for the Jablochkoff candle, are somewhat dangerous to life, if not properly insulated on the wires, and one or two fatal

[blocks in formation]

accidents have happened with them out of carelessness. The currents from ordinary dynamos constructed to feed one or two arclamps or a number of incandescent lamps are not at all dangerous to life and limb, and hence no word of warning need be given with respect to them. The Edison machine for instance, which we illustrate in Fig. 57, does not generate currents which are injurious to life. Here the

[merged small][ocr errors][merged small][merged small][merged small][merged small]

stout pole-pieces n s, which are bored out to allow the bobbin or armature E to revolve between them. The bobbin wires run lengthwise, like those in the

Siemens machine, and the usual copper brushes collect the currents from the commutator slips. The magnets are excited by this current on its way to the lamps.

Edison has constructed a large dynamo of this kind capable of supplying over 1000 lamps with current; and the Holborn Viaduct with its shops and warehouses is now lighted by it every night.

The latest novelty in medium-sized machines capable of supplying several hundred lamps, is the Ferranti dynamo, in which the bobbins are flat coils or rolls of copper ribbon set round a disc which rotates at a high velocity between opposing magnetic poles. The magnets are excited by a separate machine, for the current obtained is alternating in character, and cannot therefore be used to excite the electromagnets, though it renders the filaments incandescent. Big dynamos are more economical than small ones for lighting large areas; hence the tendency is to construct them in order to compete with gas-lighting. Perhaps the largest yet made is that of Mr. J. E. Gordon. In design it is an inversion of the ordinary type, which we have been describing. The electro-magnets are mounted on an axle and rotated between the coils or bobbins in which the current is excited, a plan introduced by Lontin and others. When the magnets revolve past the coils they induce pulses of current in the latter, which are led away to the lamps. Gordon's dynamo consists of a strong iron disc set round its rim on both sides with electro-magnets. This disc or wheel is mounted on an axle which is coupled direct to a powerful steam-engine, and is revolved at a speed of some two hundred turns per minute between the two sides of a circular iron frame set round with

bobbins of wire. The magnet poles are "north” and "south" alternately, and alternate bobbins on each. side are connected together to form two separate circuits feeding two sets of lamps. The currents in the excited bobbins are collected by brushes, and led by wires to the lamps. These currents are alternating, but the alternations are so rapid, owing to the large number of bobbins, that they have the same effect as a continuous current in the lamps. Such a machine built eight feet in diameter, and weighing about eighteen tons, will feed from 5000 to 7000 incandescent lamps; and the entire works of the Telegraph Construction and Maintenance Company at East Greenwich have been lighted nightly by one for some time past.

The magnets are excited by independent generators, and the strength of current given by the dynamo is regulated by varying the current in the magnets, while keeping the speed of the revolving wheel about the same. The dynamo can therefore be kept going though only a few lights are required of it. The alternating currents which it gives will not charge an accumulator; but accumulators are not so necessary to this generator as to others, because it can be kept going during the day with little waste of power. It is only when the magnets are excited, and strong lighting currents are required, that the power of the driving engine is taxed.

[graphic][merged small][merged small][merged small]

ELECTRIC lamps are of two well-marked kinds; the "are" lamp and the "incandescent" lamp. The arc light was first observed by Davy when he took two pieces of carbon connected to the opposite poles of a voltaic battery and held them so close that they nearly

« ÎnapoiContinuă »