Comparative tests of carbon, metallized carbon and tantalum filament lamps . gy to maintain the same tem-perature than the carbon; that is, the lower emissivity of the tan-talum filament gives it a better efficiency than the carbon at thesame temperature. This is in addition to the fact thattantalum has a greater atomic weight and a higher vaportension point than carbon, thus allowing it to be operated at ahigher temperature with the consequent better efficiency. Since the metallized filament has a higher emissivity thanthe carbon filament it must require a greater input of energy persquare in
Comparative tests of carbon, metallized carbon and tantalum filament lamps . gy to maintain the same tem-perature than the carbon; that is, the lower emissivity of the tan-talum filament gives it a better efficiency than the carbon at thesame temperature. This is in addition to the fact thattantalum has a greater atomic weight and a higher vaportension point than carbon, thus allowing it to be operated at ahigher temperature with the consequent better efficiency. Since the metallized filament has a higher emissivity thanthe carbon filament it must require a greater input of energy persquare inch of surface to maintain a given temperature. Experi-ment shows that the input of the metallized lamp is about 460watts and the carbon about 410 watts per square inch of filamentarea at a temperature 1720° C. To give, as it does even at equaltemperatures, a better watt per candle efficiency it must then giveoff a greater proportion of light energy to heat energy than thecarbon lamp. Since the carbon filament is approximately though AMRINE—COMPARATIVE TESTS OF LAMPS 31. only approximately, equivalent to the theoretical solid black bodythis fact seems to show that the greater efficiency of the metallizedfilament must be due, at least in part, to a sort of selective ra-diation. That is, it radiates either a greater proportion of itsenergy within the range of the visible spectrum than a blackbody or a smaller proportion in the invisible range. In Fig. 25and 26 the dotted curves show the radiation from a solid blackbody. Fig. 25 shows the curve for a body having at the sametemperature almost the same radiation outside the visible spectrumbut a much greater radiation within, while in Fig. 26 is shown thecurve for a body having practically the same radiation within the 32 ILLINOIS ENGINEERING EXPERIMENT STATION
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