Electric Light at present signifies the production of illumination by heating certain convenient materials to incandescence, an electric current being employed to effect this heating. When a current passes through a given resistance,.; heat is generated in quantity proportional to the resistance and to the square of the current. This tends to raise the temperature of the substance at a rate dependent on its specific heat, its emissivity or power of radiation from its surface, its general dimensions, and the external temperature. But if it be a thin rod offering considerable resistance, currents may be sent through it that will, in course of time, so raise its temperature that it becomes incandescent. Of such a nature is the incandescent lamp, which consists of a filament of specially prepared carbon or carbonised material coiled in a glass vessel, from which the air has been extracted by a very efficient air-pump. Each end of the carbon is joined to a platinum terminal passing through the glass to the outside. The two terminals are connected up with the leads or wires supplying the current. If the air were allowed to remain in contact with the filament when hot, it would supply oxygen that would immediately combine with the carbon, and the whole filament would be quickly burnt. Platinum terminals are used because that metal has almost exactly the same coefficient of expansion as glass when heated, and will, therefore, expand with the glass without cracking it or becoming loose. In course of about 1,500 hours even a carefully made and steadily worked lamp will burst its filament, and a fresh lamp must then replace it. The life of a lamp is prolonged if a low E.M.F. be used with it; but then the light is less brilliant and more yellow, and its efficiency reckoned in the number of candles per watt or horse-power is not so high. On the other hand, a whiter, more brilliant, and more efficient light can be obtained by using a higher E.M.F., but then its life is shorter. The other type of electric light is that known as the arc-lamp. In this case two carbon rods are first brought together, and a current started through them. The resistance at their point of contact is considerable, and they are rapidly brought in that region to a state of incandescence. Moreover, a small quantity of carbon is volatilised, and remains as hot carbon vapour. The rods are then drawn a short distance apart. The current is enabled to flow across because of the carbon-vapour, which, though of high resistance, conducts sufficiently well to prevent a complete falling-off of the current. But its great resistance renders the temperature exceedingly high, and by this means the solid carbon is kept at white heat. The vapour is at a much higher temperature than the solid, but exact estimation of either temperature has not yet been made. The arc light is not kept out of contact with air, and the carbons slowly burn -away; various devices have been introduced for automatic regulation of their distance apart.
Both arc and incandescent lamps, since they depend on the production of heat by current flowing against resistance, may be worked by direct or alternating currents. The current may be supplied direct from a dynamo (q.v.), indirectly with a transformer (q.v.), or from accumulators. The object of the transformer is to bring the current to a suitable condition of strength and potential for the lamps used. Accumulators may be charged by dynamos and then taken off to be worked till they run down again.
There are two general methods of arranging lamps, in series and in parallel. Lamps in series form a continuous chain, the negative pole of one joining on to the positive pole of the next, and so on. Thus the same current passes through the set; and if by chance one lamp happened to fail, the current would be stopped for the set.
Lamps in parallel are arranged so that all the positive poles are connected up with one bar, and all the negative poles with another bar. These two bars are kept at a constant D.P., which therefore exists for each lamp, and so keeps an equal current going through each. If one lamp fails the others still go on. The greater the number in parallel the less the resistance of the set, so that the total current is actually increased by increasing the number of lamps. But then more power is required for the circuit, and a greater proportion of the whole E.M.F. available goes to overcome the resistance of the generator.
Arc lamps in one system are usually put in series, any danger of the whole set going out when one fails being removed by an automatic device for short-circuiting each lamp if its current fails. This means that when the self-regulating mechanism does not act, the current is allowed to flow from one terminal of the lamp to the other by a different path.
Incandescent lamps are generally in parallel sets, each set being worked from the generator by independent lines, or lines in parallel or in series.
But the different methods of arrangement are too numerous to describe in detail. Various installations of different types are continually being laid, the arrangements depending on the amount of lighting to be effected, on the general position of each centre of lighting, and also on the method employed to generate the currents. The largest installation in the world is that of the Electric Supply Corporation, which has its central station at Deptford. The proposed plan is to supply London with alternating-current electricity at a potential of 10,000 volts, to be reduced by transformers to 100 volts. Signor Ferranti, the engineer, has worked wonders in carrying out his designs without any helping precedent. Nevertheless, an unfortunate series of accidents has brought the scheme to the verge of failure, and it is yet uncertain how matters will terminate.
The theoretical defect in these modern systems of electric lighting, or indeed of any artificial lighting, is that so much power is wasted in the production of a large proportion of dark radiation. An arc lamp certainly gives out waves of such frequencies as will produce the sensation of light; but it also gives out an immense amount of dark heat waves, which are practically useless from the light-giving point of view. The light from a glowworm is far more efficient than that from an arc lamp, measuring efficiency as the ratio of the energy of light-giving waves to the total energy supplied. What is required, then, is to give to a material just those vibrations that will cause it to incandesce, and no more; the possibility of so doing has been recently pointed out by J. J. Thomson, Crookes, and Hertz, who have separately worked at electromagnetic radiation. [Light.]