.Directional Hot-Wire Anemometer. 653 



to 6 '4 cm. per sec, the resistance and consequently 

 the temperature of the second wire is increased by more 

 than that of the first wire is diminished by the same 

 impressed velocity of the air-stream. It is clear, therefore, 

 that over this region of impressed velocities, the whole 

 heating effect experienced by the second wire cannot 

 originate entirely in heat convected directly from the 

 first wire by the stream, but is partly due to heat derived 

 from the free convection current arising from the first 

 wire, and partly to the altered thermal conditions in the 

 neighbourhood of the second wire arising from the dis- 

 turbance of the free convection currents from the two 

 wires by the stream of air. 



The deflexion corresponding to a velocity of about 4 cm. 

 per sec, indicated by P, fig. 1, in the case of a pair of wires 

 of diameter 0101 mm., is seen to be the same whether the 

 wires are mounted vertically or horizontally. The corre- 

 sponding values of the respective resistances are indicated 

 by the points P l5 P l5 P 2 , P 2 , in fig. 2, the points P l5 P 2 

 referring to the vertical arrangement, and P 2 , P 2 to the 

 horizontal arrangement. The equality of deflexion in this 

 case arises owing to the very approximate equality of the 

 difference between the resistances of the first and second 

 wires in the two arrangements (P 1? P 2 ^ P 2 , P 2 ). Equality 

 of deflexion corresponding to a velocity of about 8 cms. 

 per sec, indicated by Q in fig. 1, is seen from fig. 2 to be 

 due to actual equality of the resistances of the respective 

 wires in the two arrangements — i. e., the resistance of the 

 first wire mounted either vertically or horizontally is the 

 same when subjected to the cooling effect of a horizontal 

 stream moving with a mean velocity of about 8 cms. 

 per sec, and similarly for the second wire. In fig. 2 

 the resistance of the second wire, mounted horizon tally 

 or vertically, corresponding to an impressed velocity of 

 the stream of from 8 to 1G cms. per sec, is shown as 

 slightly different in the two cases. It is clear, however, 

 that the experimental results would, within the limits of 

 experimental error — estimated at 0'3 per cent. — be equally 

 well represented by equality of the resistance in the two 

 cases, as shown by the dotted portion of the curve in fig. 2. 

 It will be seen from fig. 2, that whereas when the wires 

 are mounted horizontally, the resistance of the second wire 

 increases very much more than is the case when the wires 

 are mounted vertically, the maximum resistance attained 



