PHOTOMETRY AND ELECTRIC LIGHTING. 337 



The development of the enclosed arc lamp was due, however, 

 chiefly to the attempt to devise an arc lamp which would operate 

 efficiently across standard 1 10- volt mains. Efficient operation in 

 this case depends upon the use of a long arc with a small current 

 so that the greater part of the supply voltage may be expended 

 in the arc, the ballast being barely large enough to make the arc 

 stable. It is impossible however to operate a long low-current 

 arc in the open air, because such an arc is blown out by the 

 least draft of cold air ; but when the arc is enclosed in a 

 nearly air-tight bulb this difficulty is avoided. Thus an So-volt 

 enclosed arc may be satisfactorily operated across I lo-volt mains 

 in series with a ballast which absorbs about 30 volts, and a 1 50- 

 volt enclosed arc may be operated satisfactorily across 2 20- volt 

 mains in series with a ballast which absorbs about 70 volts. These 

 figures are exemplified in commercial enclosed arc lamps for 1 10- 

 volt circuits and for 22O-volt circuits respectively, I lO-volt lamps 

 usually take from 5*^ to 6j^ amperes and 2 20- volt lamps 

 usually take from 2^ to 3 % amperes, according to the adjustment 

 of the spring which opposes the pull of the electromagnet that 

 operates the lamp mechanism. 



Another advantage of enclosing the arc of a lamp is that the 

 noise of the arc is thereby muffled. This is especially important 

 when alternating current is used. 



The enclosed-arc lamp is now used in all new installations, 

 except those in which the luminous-arc lamp is employed. 



Arc lamp rating. An arc lamp should be rated on the basis 

 of voltage, current, and watts. In fact this mode of rating is 

 generally used with arc lamps of recent design, such as enclosed- 

 arc lamps and luminous-arc lamps, and the manufacturer gener- 

 ally specifies a certain consumption of watts per spherical-candle 

 of light emitted. Old style open carbon-arc lamps have however 

 long been rated in candle power. Thus the open carbon-arc lamp 

 which has 47 volts between its terminals with 9.6 amperes flowing 

 through it has an accepted nominal rating of 2,000 candle power, 

 although, as a matter of fact, its spherical-candle power is about 700. 



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