Dynamo-machine and Lamp. 479 
work ; and finally, the almost entire absence of sparks at the 
commutator brushes, even when the machine is doing its maxi- 
mum of duty. 
Test of the Lamp. 
The photometric tests were made by the well known method . 
of Bunsen. The lamp, which was furnished us, was No. 858, 
one of the old pattern with carbon of charred paper, which had 
been used by Professors Rowland and Barker. 
An arrangement was adopted by which the current employed, 
the difference of potential of the lamp wires and the resistance 
of the lamp could be measured, continuously, during the pho- 
tometric observations, The current was measured by the de- 
position of copper in an electrolytic cell. The resistance was 
ound by making the lamp one arm of a Wheatstone’s bridge, 
suitably proportioned. The difference of potential was meas- 
ured by a Thomson’s high resistance galvanometer connected 
with the Jamp terminals. 
During the first test, which lasted twenty minutes, the mean 
photometric intensity “ broad-side on” was 13°8 candles. Cal- 
culation shows the mean illumination to be about 73 per cent 
of the maximum, hence, the mean illumination was about 10° 
candles. 
The resistance of the lamp, while shining, was found to be 
99°6 ohms; and the difference of potentials at its terminals was 
74°33 volts. (producing a current of 0-75 weber); hence the 
lamp was consuming 0°075 of a horse power. Or, adopting our 
previous result for the available efficiency of the dynamo- 
machine, we find that one horse-power measured at the Edison 
dynamometer would maintain a number of lamps represented by 
78° . . 
the formula 0-075x<100 10°5, nearly. 
During the second test, which lasted thirty minutes, the mean 
candle power of the lamp was 10-7 candles, calculated as be- 
fore. 
The resistance was- eee enn eeene cee 99°7 ohms. 
The difference of potential - ..-------- 76°5 volts. 
Tne Carmen 6 ee 0°76 weber. 
Accordingly the lamp was consuming 0-077 of a horse-power 
—or one horse-power applied as before, would maintain 10-2 
lamps at this candle power. 
Taking the mean of these results we find that one horse- 
power applied at the dynamometer would produce in a lamp of 
this pattern and dimensions a light of 107 candles; or about 
187 candles if we estimate the energy actually developed in 
the lamp in terms of horse-power. 
Mr. Edison kindly put everything we required at our dis- 
posal, and himself, as well as Mr. Upton and his other assist- 
ants, aided us in every possible way. 
