The latest development in electric incandescent lamps is the “half-watt” lamp. The watt is the standard of electrical energy, and it is the rate of work represented by a current of one ampere at a pressure of 1 volt. With continuous currents the watts are found very simply by multiplying together the volts and the amperes. For instance, a dynamo giving a current of 20 amperes at a pressure of 50 volts would be called a 1000-watt dynamo. With alternating currents the calculation is more complicated, but the final result is the same. The ordinary form of tungsten lamp gives about one candle-power for every watt, and is known as a one-watt lamp. As its name suggests, the half-watt lamp requires only half this amount of energy to give the same candle-power, so that it is very much more economical in current. In this lamp the tungsten filament is wound in a spiral, but instead of being placed in the usual exhausted bulb, it is sealed into a bulb containing nitrogen gas. The increased efficiency is obtained by running the filament at a temperature from 400° to 600° C. higher than that at which the filament in the ordinary lamp is used. In spite of the great advances in artificial lighting made during recent years, no one has yet succeeded in producing light without heat. This heat is not wanted, and it represents so much waste energy. It has often been said that the glow-worm is the most expert of all illuminating engineers, for it has the power of producing at will a light which is absolutely without heat. Perhaps the nearest approach to light without heat is the so-called “cold light” invented by M. Dussaud, a French scientist. His device consists of a revolving ring of exactly similar tungsten lamps. Each of these lamps has current passed through it in turn, and the duration of the current in each is so short, being only a fraction of a second, that the lamp has not sufficient time to develop any appreciable amount of heat. The light from the ring of lamps is brought to a focus, and passed through a lens to wherever it is required. Electric incandescent lamps are made in a variety of sizes, each one being intended for a certain definite voltage. If a lamp designed for, say, 8 volts, is used on a circuit of 32 volts, its candle-power is greatly increased, while the amount of current consumed is not increased in proportion. In this way the lamp becomes a more efficient source of light, but the “over-running,” as it is called, has a destructive effect on the filament, so that the life of the lamp is greatly shortened. In the Dussaud system however the time during which each lamp has current passing through it is so short, followed by a period of rest, that the destructive effect of over-running is reduced to the minimum; so that by using very high voltages an extremely brilliant light is safely obtained with a comparatively small consumption of current. It might be thought that the constant interchange of lamps would result in an unsteady