Constant Current Type of Hot- Wire Anemometer. Ill 



respective free convection currents arising from the two 

 exposed wires owing to their small difference of temperature 

 when exposed to the convective effect of an impressed 

 stream of gas. 



In the type of hot-wire anemometer investigated in detail 

 by King *, the heat-loss from the wire due to forced con- 

 vection was ascertained by adjusting a measured current 

 through the wire so as to bring its resistance to a value 

 corresponding to a predetermined temperature. A Kelvin 

 double bridge was employed, and the arm opposite the 

 sensitive exposed platinum arm was of manganin of negli- 

 gible temperature coefficient. In the Morris type of instru- 

 ment, the arm of the bridge opposite to the exposed arm is 

 constituted of a wire similar in all respects to the latter, but 

 shielded by means of a surrounding tube from the cooling 

 effect of any impressed gas stream. It is clear that, with 

 this latter arrangement, the balanced condition of the bridge 

 is, except in so far as a difference exists in the respective 

 free convection currents from the two wires owing to their 

 different respective dispositions to their immediate sur- 

 roundings, independent of the actual temperature of the 

 fluid medium in their neighbourhood, so long as this temper- 

 ature is the same in each case. It has, however, been shown 

 by the author f , that the balanced condition of the bridge is, 

 owing to the difference in the cooling effects experienced by 

 the exposed and shielded wires, due to their respective free 

 convection currents, dependent to some extent upon the 

 heating current employed in the bridge. It appeared, there- 

 fore, desirable to investigate the possibility of constructing 

 a null-deflexion type of hot-wire anemometer in which the 

 bridge current was maintained constant, the deflexion 

 produced by an impressed gas stream being annulled other- 

 wise than by increasing the heating current through the 

 arm of the bridge exposed to the stream. The present paper 

 details some of the results obtained in the course of such an 

 investigation. 



In a previous paper J, attention has been directed to the 

 fact that the resistance of a fine heated wire varies con- 

 siderably when the wire, through which a constant current 

 passes, is inclined at various inclinations to the horizontal. 



* Phil. Trans. A. 520, vol. ccxiv. p. 385 (1914). 

 f Phil. Mag. vol. xxxix. pp. 511, 528, fig. 16 (1920). 

 % Pro<\ Phys. Soo. vol. xxxii. part 5, pp. 291-314 (1920). Also Phil. 

 Mag. vol. xl. loc. cit. 



Phil. Mag. S. 6. Vol. 41. No. 245. May 1921 . ? B 



