| Name of lamp | Watts per candle power | Name of lamp | Watts per candle power |
| Carbon filament | 3 to 4 | Arc lamp | 0.5 to 0.8 |
| Metallized carbon | 2.5 | Mercury arc | 0.6 |
| Tantalum | 2.0 | Flaming arc | 0.4 |
| Tungsten | 1.0 to 1.5 | Nitrogen-filled tungsten | 0.6 to 0.7 |
Incandescent lamps are connected in parallel (see Fig. 254) to wires that are kept at a constant difference of potential of 110 or 115 volts. It is customary to place not more than twelve lamps upon one circuit, each circuit being protected by a fuse and controlled by one or more switches.
295. The Arc Light.—The electric arc light (see Fig. 275) is extensively used for lighting large rooms, also in stereopticons and motion picture machines. The light is intense, varying from 500 to 1700 candle power. The so-called mean spherical candle power of the arc light is about 510. The candle power in the direction of greatest intensity is about 1200. It is produced at an expenditure of about 500 watts. It is therefore more efficient than the incandescent lamp, often taking less than 0.5 watt per candle power produced. The arc light was first devised by Sir Humphrey Davy in 1809, who used two pieces of charcoal connected to 2000 voltaic cells. The arc light requires so much power that its production by voltaic cells is very expensive. Consequently it did not come into common use until the dynamo had been perfected. Fig. 276 shows the appearance of the two carbons in an arc light. If a direct current is used the positive carbon is heated more intensely, and gives out the greater part of the light. The positive carbon is consumed about twice as fast as the negative and its end is concave, the negative remaining pointed.
Fig. 275.—An electric arc light.
Fig. 276.—The appearance of a pair of used carbons.
With alternating currents, the rods are equally consumed and produce equal amounts of light. In the stereopticon, the carbons are usually placed at right angles as in Fig. 277. In the stereopticon as well as in outdoor lighting the direct current is more effective, although the alternating current is often used, since the latter can be produced and distributed more cheaply than can direct currents. In arc lamps, placing an inner glass globe (Fig. 278) about the carbons, decreases the consumption of the carbons materially. The carbon rods of enclosed arc lamps often last 60 to 100 hours.
Fig. 277.—A right-angle electric arc lamp for a stereopticon.
Fig. 278.—An enclosed arc lamp.
The reason why an open arc lamp needs to be "retrimmed" oftener than the enclosed lamp, that is, have new carbons placed in it, is because the carbons "burn" freely, that is unite with the oxygen of the air. In the enclosed arc lamp, the supply of oxygen in the inner globe is limited and is soon consumed, therefore the carbons last many times longer in such lamps.
Some carbon rods have soft cores containing calcium salts. These vaporize in the arc producing the flaming arc light of a bright yellow color, and give more light than the ordinary lamp.