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Published online by Cambridge University Press: 15 September 2014
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 comparing 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.
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page 556 note * It has been shown recently by R. A. Houstoun and Logie, J., Phys. Zs., xi. p. 672 Google Scholar, that an aqueous solution of ferrous ammonium sulphate absorbs the dark heat much more effectively than water alone while allowing the light to pass. Nevertheless, it is not satisfactory enough as a means of separating the dark heat from the visible radiation.
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page 557 note † “Energy in the Visible Spectrum of the Hefner Standard,” Physical Review, xvii., 1903, p. 302.
page 557 note ‡ “On the Luminous Efficiency of the Carbon Filament,” Physical Review, xx., 1905, p. 160.
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