100 TKANSACTIOXS OF THE [DEC. 21, 



lamps have been their short life and lack of economy; this is not 

 true of them in all cases. 



The first public test of the life of incandescent lamps was 

 made by the Franklin Institute in the early months of 1885. 

 (Jou)'. of the Franklin Inst., Sept., 1885.) The record of 

 these tests is given in a i)ami)hlet of some 130 pages, and with a 

 detail which renders it impossible, in our limited time, to do more 

 than gather from its averages such general lessons as we may 

 learn. 



From the efficiency test, which was preliminary to the pro- 

 longed duration test, we find that 194-.1 spherical candles were 

 realized per electrical horse power. 



Spher. candles. 



Edison's 97 volt lamps— per El. II. P. . . . 1G9.2 



Stanley's 96 " " " " " . . . 189.1 



44 '' " u a u ... 216.1 



Woodhouse & Eawson's 55 volt lamps — per El. II. P. 209.0 



55 " " " *' " 210,8 



White's 50 volt lamps— per El. II. P. . . 182.6 



Weston's 110| " " '■ '■ '• . . . 209.8 



70 " " <i i( u ^ _ ^ lg(j_3 



Average " " " . . .194.1 



The committee was forced by the different forms of carbon 

 filament used to take the illuminating power of the lamps from 

 all points, and to call the mean the spherical intensity of illumi- 

 nation. This procedure perhaps gives a better idea of the prac- 

 tical value of the incandescent lamp because it is customary to 

 place these lamps in any position that convenience may dictate. 



I have already, I trust, convinced you that the incandescent 

 lamp, by reason of its smaller quantity of light and better distri- 

 bution, is worth at least twice as much as the arc light. 1 have 

 also told you that one electrical horse-povver costs with Weston's 

 incandescent dyiuimo electric machine about 4.9 lbs. of ordinary 

 anthracite. Therefore 1 lb. of coal will give about 40 candles 

 by the incandescent lamp, and this is equivalent to 80, and 

 probably many more, candles by the arc light, whenever we have 

 to use our eyes for any purpose save guarding our footsteps. 



You will recall tliat under assumjitions most favorable to the 

 arc light, I showed you that we probably do not get more than 

 75 candles per lb. of anthracite from the shaded arc light. Had 

 the committee on arc lights obtained the spherical intensity of 

 illumination of these lamps, their showing could have been made 

 much less favorable than the one given. The present method 



