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.

ELECTRIC lamps are of two well-marked kinds; the "arc" 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

touched each other. To his surprise a brilliant arch or "are" of white light sprang across the air gap between the two carbons, as illustrated in Fig. 58, and so intense

was the temperature of the light that gold, platinum, and other refractory metals, fused and ran like wax when they were dipped into it. The explanation of the arc light is that the electric energy streaming across the open space from one carbon to another tears away the particles from the carbon connected to the

positive pole of the generator and heats them up to a very high temperature, the highest known on earth, thereby rendering them intensely luminous.

An arch of light is also produced by the electric discharge in a vacuum, or speaking more strictly in a rarefied gas. When the sparking current from an induction coil is employed, a rich luminous haze, often of great beauty, is observed between the discharging points. The colour of the haze varies with the kind of gas in the bulb and the degree of rarefaction. The effect, which is probably a mimic aurora borealis, just as the electric spark is a mimic lightning flash, is much feebler than the arc in open air between two carbon points.

It has not hitherto been practically employed for lighting, but the intrinsic loveliness of the light has called forth numberless bulbs of fantastic design and variegated hues, for display at lectures. These are generally known as Geissler tubes.

The incandescent light, on the other hand, is produced by heating up a continuous solid conductor to a very high temperature by passing the current through it; and it was observed soon after the invention of the voltaic battery, and generally shown by connecting the poles of a battery through a piece of fine platinum wire. As distinguished from the "arc" light, it is absolutely steady, thanks to the solid nature of the heated body, and moreover the light is richer and mellower for the same reason. Every solid body becomes luminous at a temperature of about 1000° Fahr., and can be made to yield all the rays of the solar spectrum by increasing its temperature still higher; so that we have only to send sufficient current