PHOTOMETRY AND ELECTRIC 'LIGHTING. 325 



distributed over-head so as to be as much as possible out of the 

 field of vision, and walls and ceiling are usually yellowish-white. 

 Under these conditions about 0.2 of a spherical-candle is required 

 for each square foot of floor area to give a degree of illumination 

 that would be considered satisfactory in a reception room or in a 

 lecture hall. If the ceiling is very high a greater candle-power 

 is required inasmuch as the area of the walls is increased. If the 

 ceiling and walls are very dark, if they are made of stained oak 

 " or cherry paneling, for example, effective illumination may re- 

 quire 0.4 or o. 5 of a spherical-candle per square foot of floor area. 



Street lighting by arc lamps* Arc lamps consuming about 

 500 watts each are usually placed 200 or 300 feet apart in Amer- 

 ican cities, and the height of the lamps above the street is usually 

 20 or 30 feet. In many cases, especially in streets having shade 

 trees, smaller units placed nearer together give much better 

 illumination. 



Old style open-arc street lamps for all night burning have two 

 sets of carbons, and when one set burns out the other set is 

 thrown into circuit automatically. Such lamps are trimmed 

 daily, one man being able to trim about 80 lamps per day. 



The number of hours of operation per year of street lamps 

 varies greatly in different cities. Thus the all-night schedule, 

 lamps operated every night from 30 minutes after sunset to 30 

 minutes before sunrise, aggregates 3,950 hours per year in New 

 York City ; the aggregate time varies slightly with latitude. In 

 the so-called moonlight schedule the lamps are operated about 

 2,100 or 2,200 hours per year. 



134. The electric lamp. -- The production of light by means of 

 the electric current depends upon the heating by the current of a 

 portion of an electric circuit to incandescence, and the electric 

 lamp consists essentially of an element which is so heated, and 

 in some cases, of special regulating devices. Electric lamps may 



*See Electrical Engineer' 's Pocket Book, H. A. Foster, D. Van Nostrand, 1903 

 (third edition), for information on arc lamps and on arc-lighting schedules, pages 

 393-402 and 414-422. 



