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LXXIV. The Directional Hot- Wire Anemometer : Its Sensi- 

 tivity and Range of Application. Bij J. S. Gr. Thomas, 

 M.Sc. (Londj, B.Sc. (Wales), A.R.C.S., A.I.C., Senior 

 Physicist, South Metropolitan Gas Company, London *. 



Introduction. 



IN a recent communication f the author described a type 

 of hot-wire anemometer, which in addition to affording 

 a ready indication of the direction of flow of gas in a pipe, 

 was, on account of the almost complete elimination from 

 the observed effect, of the influence of the free convection 

 currents arising from the heated wires, especially applicable 

 to the determination of the velocity of a very slowly moving- 

 current of gas. 



The directional hot-wire anemometer consists of two fine 

 platinum wires mounted parallel, and one behind the other 

 in close juxtaposition, transversely to the direction of flow 

 of the gas in the pipe or channel. The wires constitute 

 two arms of a Wheatstone bridge (see fig. 7 later), the 

 remaining; arms of which are formed of a resistance of 

 1000 ohms and an arm capable of adjustment. A constant 

 current is maintained in the bridge, and the battery terminals 

 are connected through a rheostat to the appropriate ends of 

 the platinum wires so that the maximum heat is developed 

 in these wires. A galvanometer is inserted in the bridge in 

 -the usual manner. Further details concerning the method 

 of mounting the wires etc. will be found in the papers 

 referred to, wherein it is shown that as the velocity of 

 the gas-stream to which the heated wires are exposed is 

 gradually increased from zero, the galvanometer deflexion 

 increases until a critical value of the velocity of the gas 

 stream is attained. With further increase in the impressed 

 velocity of the gas stream, the galvanometer deflexion 

 very slowly diminishes. The direction of the galvanometer 

 deflexion is reversed on reversing the direction of flow of 

 the gas current, and the instrument affords an indication 

 of the direction of flow even for extremely high values of 

 the velocity of the gas stream. In the papers referred to, 

 the maximum value of the galvanometer deflexion corre- 

 sponded with a mean velocity of the gas stream equivalent 

 to about 5 cm. per sec. The directional type of hot-wire 

 anemometer possessing certain advantages in the region 

 of low velocities — greater sensitivity, more complete elimi- 

 nation of the effect of the free convection current, greater 



* Communicated by the Author. 



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

 Soc. vol. xxxii. Part 3, pp. 196-207. 



