[ 304 ] 
XXXIV. The most Economical Potential-difference to employ 
with Incandescent Lamps. By Professors W. H. AyRTon, 
F.R.S., and JOHN Parry, M/.E.* 
f oes subject in connection with which the accompanying 
paper is a small contribution is one of considerable 
commercial importance. It has long been known that the 
luminous power of an electric lamp increased much more 
rapidly than the power electrically expended on it; or, that 
the number of candles per horse-power increased as the lamp 
was made to become brighter and brighter. Perhaps the 
earliest experiments on the subject were those published by 
Sir William Thomson in 1881, and those made by our students 
in 1880. Since that time (that is, during the last five years) 
tests of the efficiency of various types of incandescent lamps 
have been made over and over again by various persons, 
without perhaps its being clearly realized that such efficiency- 
experiments by themselves gave us no idea of the commercial 
value of any particular lamp. 
It is not sufficient to know that when a lamp is giving out 
a certain number of candles it absorbs so much power per 
candle, and when giving out a much larger number of candles 
it absorbs so much less power per candle; but what must be 
known in addition is the life of the lamp at each of these two 
candle-powers, before we can decide whether it is more eco- 
nomical to use the lamp with the filament at not much more 
than a dull red or when brilliantly luminous with a bluish tint. 
For with the filament at a comparatively low temperature, 
although the efficiency of the lamp is low, its life will be 
great ; whereas if the temperature of the filament be high, the 
large efficiency will be to some extent balanced by its short 
life and the consequent large cost due to lamp renewals. 
In the ‘ Electrician’ for January 31st of this year M. Foussat 
gives a table of values of the life ofa 100-volt Edison lamp for 
different potential-differences; and M. Foussat has assured us 
that these numbers were obtained experimentally, and not from 
calculation according to any theory. ‘That the results should 
lie so nearly in a curve is undoubtedly at first sight a little 
striking ; but if it be remembered that they are stated to 
represent the results obtained from the averages of a very 
extended series of tests, their great regularity is not more 
striking than is a curve showing graphically the result of 
mortality tables. We asked M. Foussat whether he had also 
the results of experiments on the efficiency of the same type 
of 100-volt lamps, as our own experiments on the efficiency 
* Communicated by the Physical Society, having been read at the 
Meeting on February 28th, 1886. 
