r 240 j 



XX. The Thermal Effect produced by a slow Current of Air 

 flowing past a Series of fine heated, Platinum Wires, and its 

 Application to the Construction of Hot- Wire Anemometers 

 of great Sensitivity, especially applicable to the Investigation 

 of slow Rates of Flow of Gases. By J. S. G. Thomas, 

 DSc. (Lond.), B.Sc. (Wales), A.R.C.S., A.I.C., Senior 

 Physicist, South Metropolitan Gas Company, London*. 



IN a recent communication f a type of directional hot-wire 

 anemometer was described, consisting of two line 

 platinum wires inserted in a pipe or channel parallel to one 

 another, and one behind the other in close juxtaposition, 

 transversely to the direction of flow of gas in the pipe or 

 channel, It was shown that the sensitivity of such a hot- 

 wire anemometer for the measurement of the velocity of a 

 slow-moving stream of gas was considerably greater than 

 that of the Morris type of hot-wire anemometer J, It may 

 be remarked that the Pitot tube is most satisfactorily em- 

 ployed for purposes of anemometry, for the measurement of 

 large velocities, as the measured effect is proportional to the 

 square of the velocity of the stream in which the tube is 

 inserted. The hot-wire anemometer, on the other hand, in 

 which the observed effect is proportional to the square root 

 of the velocity to be measured, appears especially suitable for 

 use in the region of low velocities. Nevertheless, it has 

 b^en shown by King § and others that the hot-wire anemo- 

 meter is an instrument of very high precision, even in the 

 region of high velocities. The difficulties encountered in 

 the measurement of low velocities arise principally owing to 

 the existence of the free convection current of gas in the 

 neighbourhood of the wire due to the heated condition of 

 the wire. In the directional type of hot-wire anemometer 

 the effect of the free convection current is largely eliminated. 

 The directional type of instrument possessing the desirable 

 characteristics referred to, experiments were undertaken to 

 determine whether the sensitivity of this type of anemometer 

 could be still further increased in the region of low velocities, 

 and the present communication details some results obtained 

 in the course of such an investigation. 



* Communicated by the Author. 



t Phil. Mag. vol. xxxix. pp. 525-527 (1920); see also Proc. Phys. 

 Soc. vol. xxxii. pp. 190-207 (1920). 



t Eng. Pat. 25^923/1913. See also B.A. Reports, 1912 and 1920; 

 < Electrician/ Oct. 4, 1912, p. 1056, and Aug. 27, 1920, p. 227. 



§ King, Phil. Mag. vol. xxix. p. 564 (1915). 



