Potential-difference to employ with Incandescent Lamps. 307 
More recently one of our students, Mr. Robertson, has 
been making efficiency-experiments on some Edison lamps 
obtained from France ; and by trial he has found some that 
give 16 candles at about 100 volts, and are therefore pre- 
sumably of the same type as those employed in the life- 
experiments given by M. Foussat in the ‘ Hlectrician’ for 
January 31st. 
Mr. Robertson’s results from one such lamp are as 
follow :-— 
TABLE I. 
Candles, or 6(v). ee | Volisiorn |. . Watt. 4) or v. Watts. 
3 84 56°62 
4 87 63°51 
5 87°83 66°03 
6 90:5 69:05 
7 93°d 73°74 
10 96°75 79°34 
11 97°61 81-01 
12 98:04 82°35 
14 98-9 85°55 
16 100-6 90°54 
18 | 101-5 92°37 
20 103-6 96°66 
22 105-4 100:10 
24 107-5 104-28 
26 109:2 108-11 
28 110 111:10 
30 111-4 11363 
35 114 118-56 
40 115-2 122-11 
We are therefore now in a position to take up the problem 
in a more direct manner, without making any assumption 
beyond this—that Mr. Robertson’s 16-candle 100-volt French 
Edison lamp is of the same kind as M. Foussat’s 16-candle 
100-volt French Edison lamp. 
Solving the problem graphically, in the way previously 
described, and using the same values of p, n, and H—viz. 
cost of a lamp 5s., number of hours of burning per year 560, 
and annual cost of an electric horse-power for those 560 hours 
£5,—we obtain the curve AAA to represent the cost per 
candle per year as regards renewal of lamps, B B B the cost 
per candle per year as regards power, and the resultant curve 
CCC as the total cost per candle per year. 
The minimum value of this cost appears from the curve 
CCC to be at about 101°4 volts, and to equal about 11d. per 
candle per year. If the potential-difference be maintained 
constantly down at 98:7 volts, or up at 104 volts, then the 
cost becomes ls. per candle per year. It. will also be seen 
from the curves that the yearly cost for renewals of lamps is 
