Rowland and Barker— Efficiency of Edison's Electric Light. 339 
When the lamps were reversed, the result was 38540 foot 
pounds for No. 580, and a power of 13°5 or 1071 candles mean. 
The mean of these two gives, therefore, a power of 3513 foot 
ounds per minute for 11°6 candles, or 109°0 candles to the 
orse power. 
To test the change of efficiency when the temperature varied, 
we tried another experiment with the same pair of lamps, and 
also used some others where the radiating area was smaller, 
and, consequently, the temperature had to be higher to give 
out an equal light. 
We combine the results in the following table, having calcu- 
lated the number of candles per indicated horse power by 
taking 70 per cent of the calculated value, thus allowing about 
30 per cent for the friction of the engine, and the loss of 
energy in the magneto-electric machine, heating of wires, ete. 
As Mr. Edison’s machine is undoubtedly one of the most efficient 
now made, it is believed that this estimate will be found prac- 
tically correct. The experiment on No. 817 was made by 
observing the photometric power before and after the calori- 
meter experiment, as two equal lamps could not be found. As 
the fiber was round, it gave a nearly equal light in all direc- 
tions as was found by experiment. 
Smee bee | MOS he 
Lampsused in| Photometric power | SS | #40: | bed | 3S 232 12°" Qub] Sago 
nae peo | Sot | Boa | Bats |GSSkok! Base 
388/458 | 85 | BEES |[eaeSSe| 2588 
measured z= S25 wes, agoo |a&Boes Amo & 
Calor. Photo- perpen- ‘vied 25 2 of P) sae gs z2 acho8) a8 
sera) aris | Ara | eee | des | ate | Eee EagP*| SOF 
201 | 580 | 175 13:1 | 2-57 | 1°75 | 3486 
s80 |) 201) 135°) 101 a apart OP A . 
01 | 385 28-9 | ana | 2 5181 ; 
aoa | 880) 446 | 335 6 | 2°29 | aggs-| 7043 | 128 a 
09 | 19-0 143 | 281 | 1°14 | 2483 : : 
809 | 850 | 12-2 92 | 2-79 | 1°54] 3330° t eis tua abe 
Bal 17-2 | 9-73 | 1°28] 2708} 209°6 | 18-1 92 
made either cheap enough or durable enough, there is no 
reasonable doubt of the practical success of the light, but this 
point will evidently require much further experiment before 
the light can be pronounced practicable. 
n conclusion, we must thank Mr. Edison for placing his 
entire establishment at our disposal in order that we might 
form a just and unbiased estimate of the economy of his light 
