Figure 63 represents an ordinary glow lamp of the Edison-Swan type, where E is the filament, moulded into a loop, and cemented to two platinum wires or electrodes P penetrating the glass bulb L, which is exhausted of air.

Platinum is chosen because it expands and contracts with temperature about the same as glass, and hence there is little chance of the glass cracking through unequal stress.  The vacuum in the bulb is made by a mercurial air pump of the Sprengel sort, and the pressure of air in it is only about one-millionth of an atmosphere.  The bulb is fastened with a holder like that shown in figure 64, where two little hooks H connected to screw terminals T T are provided to make contact with the platinum terminals of the lamp (P, figure 63), and the spiral spring, by pressing on the bulb, ensures a good contact.

Fig. 65 is a cut of the ordinary Edison lamp and socket.  One end of the filament is connected to the metal screw ferule at the base.  The other end is attached to the metal button in the centre of the extreme bottom of the base.  Screwing the lamp into the socket automatically connects the filament on one end to the screw, on the other to an insulated plate at the bottom of the socket.

The resistance of such a filament hot is about 200 ohms, and to produce a good light from it the battery or dynamo ought to give an electromotive force of at least 100 volts.  Few voltaic cells or accumulators have an electromotive force of more than 2 volts, therefore we require a battery of 50 cells joined in series, each cell giving 2 volts, and the whole set 100 volts.  The strength of current in the circuit must also be taken into account.  To yield a good light such a lamp requires or “takes” about 1/2 an ampere.  Hence the cells must be chosen with regard to their size and internal resistance as well as to their kind, so that when the battery, in series, is connected to the lamp, the resistance of the whole circuit, including the filament or lamp, the battery itself, and the connecting wires shall give by Ohm’s law a current of 1\2 an ampere.  It will be understood that the current has the same strength in every part of the circuit, no matter how it is made up.  Thus, if 1/2 of an ampere is flowing in the lamp, it is also flowing in the battery and wires.  An Edison-Swan lamp of this model gives a light of about 15 candles, and is well adapted for illuminating the interior of houses.  The temperature of the carbon filament is about 3450 degrees Fahr—­that is to say, the temperature at which platinum melts.  Similar lamps of various sizes and shapes are also made, some equivalent to as many as 100 candles, and fitted for large halls or streets, others emitting a tiny beam like the spark of a glow-worm, and designed for medical examinations, or lighting flowers, jewels, and dresses in theatres or ball-rooms.

The electric incandescent lamp is pure and healthy, since it neither burns nor pollutes the air.  It is also cool and safe, for it produces little heat, and cannot ignite any inflammable stuffs near it.  Hence its peculiar merit as a light for colliers working in fiery mines.  Independent of air, it acts equally well under water, and is therefore used by divers.  Moreover, it can be fixed wherever a wire can be run, does not tarnish gilding, and lends itself to the most artistic decoration.