Section III., 1913. [69] Trans. R.S.C. 



An Application of the Photo- Electric Effect to the Measurement of the 

 Thermal Conductivities of Different Gases. 



By F. C. AsBURY, B.A., 

 Assistant Demonstrator in Physics, University of Toronto 



Presented by Prof. J. C. McLennan 



(Read May 28, 1913) 



A well-known experiment to illustrate the difference in the con- 

 ductivity for heat of various gases, consists in filling two incandescent 

 lamp bulbs with hydrogen and carbon dioxide respectively and heating 

 the filaments by means of an electric current. When maintained at 

 the same potential difference, the filaments, if possessing the same 

 resistance when cold, differ greatly in appearance. That surrounded 

 by carbon dioxide at atmospheric pressure glows fairly l^rightly, 

 whilst that enclosed in hydrogen at the same pressure is just suffici- 

 ently luminous to be visible. In this manner the comparatively 

 great conductivity for heat of hydrogen is strikingly exhibited. 



As far as the author has been able to find out, no quantitative 

 results have been obtained for this experiment. It was, accordingly, 

 thought worth while to investigate the conditions a little more care- 

 fully, and especially to observe the variation, under change of pres- 

 sure, in the intensity of the light emitted from the glowing filament 

 when the latter was surrounded by different gases. A photo-electric 

 method was used to determine the intensity of the light from the 

 filament. 



I. Experimental Method. 



Although at first an ordinary incandescent lamp filament was 

 employed, it was soon found preferable to use a platinum wire. This 

 was placed axially in a glass bulb, containing, in succession, various 

 gases, and heated by an electric current. The difference in potential 

 between the ends of the jjlatinum wire was kept constant and the 

 variation in the intensity of the visible radiation was observed as the 

 pressure of the surrounding gas was reduced. 



Now we see that, at ordinary pressures, if the thermal conductivity 

 of the gas surrounding the filament be comparatively large, heat will 

 be conducted away freely and the wire will glow only dimly; if, how- 



