1909-10.] The Efficiency of Metallic Filament Lamps. 
555 
XLI. — The Efficiency of Metallic Filament Lamps. By R. A. 
Houstoun, M.A., D.Sc., Ph.D., Lecturer in Physical Optics in the 
University of Glasgow. Communicated by Professor A. Gray, F.R.S. 
(MS. received April 25, 1910. Read November 7, 1910.) 
Practical men are accustomed to measure the efficiency of a glow-lamp in 
watts per candle or, in other words, by the quantity of energy which must 
be supplied to it for every mean spherical candle-power of light it gives. 
The energy supplied is measured by an ammeter and voltmeter, and the 
mean spherical candle-power is obtained on the photometric bench by com- 
paring the light given in different directions with that of some standard 
source. The watts per candle together with the life and initial cost of the 
lamp determine its commercial value. 
On the other hand, we may regard the lamp as an energy-transforming 
device, and measure its efficiency by the percentage of electrical energy it 
transforms into light. Let Q = total electrical energy consumed per second, 
R the total energy radiated per second, and L the total luminous energy 
radiated per second. Then we make the following definitions : — 
L 
Q 
= luminous efficiency, 
h 
R 
= radiant efficiency of the lamp. 
R is somewhat less than Q, as all the energy is not lost by radiation, but 
some by conduction and convection. This proportion is, however, so small 
that it is usually neglected. Dr C. V. Drysdale * states that he found the 
convection loss to be not more than 2 or 3 per cent, of the total 
energy consumed. W. Wedding j- finds it to he very much larger; but, 
on account of the wide difference between his results and those of other 
investigators, it is impossible to avoid the suspicion that he makes an error 
somewhere. It should be stated, though, that there is nothing in his book 
itself to justify this suspicion. 
For the purpose of defining L, it is usual to take 760 /u/ul as the upper 
* Charles Y. Drysdale, “ On Luminous Efficiency and the Mechanical Equivalent of 
Light,” Proc. Roy. Soc ., A, 80 (1907-08), p. 19. 
t W. Wedding, fiber den W irkungsgrad und die praktische Bedeutung der gebrauch- 
lichsten Lichtquellen ; published by R. Oldenbourg, Munich and Berlin, 1905. 
