[ 505 ] 



XLIX. The Hot-wire Anemometer : its Application to the In- 

 vestigation of the Velocity of Gases in Pipes. By J. S. G. 

 Thomas, M.Sc.{Lond.), B.Sc.(Wales), A.B.C.S^ A.I.C.* 



[Plates X.-XIIL] 



THE possibility of utilizing the cooling effect experienced 

 by a fine heated platinum wire, when immersed in a 

 stream of fluid, as a method of practical anemometry has been 

 placed on a sound theoretical basis by King f. Morris £ has 

 examined the characteristics of wires of various kinds for 

 use in this connexion, and has described a number of 

 methods of employing the hot wire for the same purpose. 

 Both these investigators § and others || are responsible 

 for types of so-called hot-wire anemometers to be employed 

 for the measurement of the velocity of air-currents. The 

 author has recently had occasion to examine the possibility of 

 the use of such instruments in an investigation connected with 

 the flow of gases through pipes and orifices, upon which he is 

 at present engaged, and the present paper contains an account 

 of certain interesting results obtained as the result of such 

 examination. Of the various types of hot-wire anemometers 

 available, the type due to Morris and described by him 

 in Eng. Pat. 25,923/1913, was found on examination to 

 be the most suitable for the purpose of the investigation. 

 This type of anemometer is constituted of four equal wires 

 of the same material — platinum by choice — composing the 

 four separate arms of a "Wheat-stone bridge. One pair of 

 alternate arms of the bridge is shielded by means of sur- 

 rounding tubes ; the resistances being all adjusted to equality 

 initially at any temperature, the bridge remains balanced at 

 any other temperature. The bridge-wires being inserted 

 into a stream of fluid, the balance of the bridge is upset, the 

 unshielded arms alone being subjected to the cooling effect of 

 the fluid current, and the galvanometer deflexion serves, after 

 calibration of the instrument, to measure the velocity of the 

 stream. Morris, in the calibration of his instruments, 

 employed air-currents produced in a vertical wind channel, 

 the stream of air passing vertically downwards therein, at 



* Communicated by the Author. 



t Phil. Trans. Roy. Soc, A. 520, 214, pp. 373-432 (1914). Phil. 

 Mag., 1915, p. 570. 



I British Association, Dundee, 1912; Electrician, Oct. 4, 1912, p. 1056; 

 Engineering, Dec. 27, 1912. 



§ King, Eng. Pat. 18,563/1914; Morris, Eng. Pat. 25,923/1913. 



|| See e. <j. King, Phil. Trans., he. cit. p. 404. 



Phil. Mao. S. 6. Vol. 39. No. 233. May 1920. 2 L 



