Constant Current Type of Hot- Wire Anemometer. 725 



horizontal, and if, moreover, Y a = velocity of the impressed 

 stream corresponding to the rotation a, we evidently have, 

 since the total cooling effect experienced by the wire is the 

 same in all cases, 



v a =^/v 2 -V a 2 ) 



assuming that the cooling effect due to the walls o£ the tube 

 etc. is the same in all cases. This assumption would be 

 most strictly justified in the case of experiments carried out 

 in a channel of large dimensions compared with the length 

 of the heated wire. Values of v appropriate to various 

 temperatures of the heated wires in the present case were 

 determined in the manner described in a previous paper *. 

 The present results are not strictly comparable with the 

 results obtained in the previous paper owing to the differ- 

 ences in the mounting of the wire, the diameter and lao-aing 

 of the tube, also the difference in the diameter and temper- 

 ature coefficient of the wires in the two cases. In each case, 

 the velocity of the free convection stream, assuming the 

 stream to be at the temperature of the wire, was found to be 

 linearly related to the temperature of the wire, the relation 

 being of the form V = /c(#— 20) where V is the velocity of 

 the free convection current, and 6° C. the temperature of the 

 wire. While the determinations were being made, the mean 

 atmospheric temperature was 20° C. Evidence has pre- 

 viously been given f that the free convection stream is not 

 raised to the temperature of the wire owing to the existence 

 of a stagnant gas film surrounding the wire. The effective 

 velocity of the free convection stream, assuming its temper- 

 ature to be T, the wire being horizontal and at temperature 

 #, is, in the present case, given by the relation 



T-r-273^ 



V=O-026(*- 2 0)(^§). 



For purposes of comparison with results previously given 

 in various calibration curves, in which velocities have been 

 recorded, assuming the impressed air stream to be at 0° C, 

 the values of the effective velocity of the free convection cur- 

 rent for various inclinations of the wire have been calculated 

 as already explained, taking the temperature of the stream as 



* Phil. Mag. vol. xxxix. pp. 518-523 (1920). 



t Ibid. pp. 531-534. See also Langmuir, Proc. Amer. Inst. Elec, Eng, 

 xxxi. pp. 1011-1022. Trans. Amer. Electrochem, Soc. xxiii. p. 293 

 (1913). 



