Directional ffot- Wire Anemometer. f>49 



greater sensitivity of the vertical arrangement might con- 

 ceivably be due to the higher initial temperature to which 



the wires are raised by the bridge current when installed 

 vertically. It will, however, be clear from the sequel, 

 and may be seen from fig. 1, that very little variation of 

 sensitivity in the region of low velocities accompanies a 

 change of the initial temperature to which the wires are 

 raised. Thus in fig. 1, in the case of wires of diameter 

 0*202 cm., it will be seen that when the wires are vertical, 

 the calibration curves in the region of impressed velocities 

 between and 3 cm. per sec. are practically identical 

 when heating currents equal to 3*3 amps, and 3*15 amps, 

 are employed, the initial temperatures of the wires in the 

 two cases being 540° 0. and 480° C. approximately. The 

 initial greater sensitivity of the vertical arrangement, com- 

 pared with the horizontal, is to be attributed to the relative 

 disposition of the free convection currents to the wires 

 in the two cases. With the wires vertical, the hot free 

 convection current from each wire completely surrounds 

 the same, whereas, with the wires horizontal, the heated 

 free convection current rises directly away from each wire. 

 In the case of the vertical arrangement, an impressed 

 stream of low velocity would disturb the free convection 

 current so that the central portion of the wire would 

 no longer be completely protected by the hot ascending 

 current of air. With increase of impressed velocity, the 

 shielding effect of the free convection current would be 

 reduced still further. The transfer of heat from the wire to 

 the stream being in this case initially effected through the 

 surrounding ascending free convection current, in which, 

 moreover, the radial temperature gradient for some distance 

 — up to something of the order of three times the radius 

 of the wire* — is small, it is to be anticipated that any 

 disturbance of the protecting free convection current by 

 means of an impressed air-flow would produce a com- 

 paratively considerable reduction in the temperature of 

 the first wire of the vertical pair. In the case of the 

 horizontally disposed pair of wires, the heated free con- 

 vection currents pass transversely over the wires, and it 

 is evident that the first wire of the pair would experience 

 a smaller cooling effect due to a slow air-stream than 

 would be the case with the wires mounted vertically. The 

 various effects can be readily discussed by the aid of fig. 2 



* See Thomas, ' An Electrical Hot-Wire Inclinometer/ Proc. Phvs. 

 Soc. 1920. 



Phil. Mag. S. 6. Vol. 40. No. 239. Nov. 1920. 2 U 



