Open arcs have been classified as follows:

Full Area, 2,000 candle-power taking 9.5 to 10 amps or 450-480 watts.

Half Area, 1,200candle-power taking 6.5 to 7 amps, or 325-350 watts.

These candle-power ratings are much too high, and run more nearly 1,200 and 700, respectively, for the point of maximum intensity and less than this if the mean spherical candle-power be taken. For this reason, the ampere or watt rating is not used to indicate the power of the lamp. It is now recommended that specifications for street lighting should be based upon the illumination produced. This point is considered later under the topic of street lighting. Enclosed arcs use from 3 to 6.5 amperes, but the voltage at the arc is higher than for the open lamp. Table X gives some data on enclosed arcs on constant-potential circuits.

Efficiency

The efficiency of arc lamps is given as follows:

Direct-Current Arc (enclosed) 2.9 watts per candle-power.

Alternating-Current Arc (enclosed) 2.95 watts per candle-power.

Direct-Current Arc (open) .6-1.25 watts per candle-power.