When a heavy current is sent through a small copper wire, the copper melts. If the same experiment is made with a platinum wire, the wire will not melt, but will become intensely hot, and glow with a very bright light. Similar incandescence can be produced also in some other substances. The incandescent lamp (Fig. 1) consists of a glass bulb, into the base of which there are sealed two platinum wires which carry a loop of carbon filament. The air is exhausted from the bulb before it is sealed. After the lamp is screwed into the base there is but one gap, in the circuit
FIG. 2- Turbo-Generator at the Schuylkill Station of the Philadelphia Electric Company The capacity of this machine is 35,000 K.W., which means that it is capable of supplying energy for the total electrical demands of a city of about 125,000 population.
at G. This is closed when the lamp is turned on. When the proper current is sent through the carbon filament, it becomes incandescent, but does not burn, as there is no air in the bulb. After a lamp has been used for some time, part of the carbon becomes deposited on the inside of the bulb, and absorbs a great deal of the light sent out by the filament. When this has happened, the best economy is to replace the lamp with a new one.
