the heat increases also, but at a faster rate, so that if the conductor is a wire a point is reached when it becomes red-, or white-hot, and gives out light. This is, indeed, the principle on which incandescent lamps become sources of light. The wire may be a carbon filament, or a filament of tantalum, tungsten, or other highly refractory metal. Whatever the substance used for the filament of a lamp, it is subject to the same influence as any other conductor—it gets hot when traversed by a current. There is, however, this difference in degree. In the lamp we desire to produce heat at a high temperature, for only then do we get light as well as heat; in a conductor used for the purpose of transferring electricity from the source where it is generated to the apparatus where it is utilised, we desire to generate as little heat as possible. There is an advantage in producing a high temperature in the filament of the incandescent lamp, but there is no advantage whatever in producing heat in the wires that carry the current to the lamp. On the contrary, there is an objection to it; not only does the generation of heat mean a dissipation of energy, that is, of something which costs money, but it may be a positive danger, since
