the Hot-wire Anemometer, 521 



In the case of the smaller impressed velocity v 1? the effective 

 cooling velocity is v c — i?i, and with the larger impressed 

 velocity r 2 , the effective cooling velocity is v 2 — v c . As the wire 

 is brought to the same temperature by these two effective 



velocities, w r e have, numerically,^ — 1\ = v 2 — v c ,i.e. v c = -^— — -. 



The free convection current corresponding to any deflexion to 

 the left of the axis of volumes is thus seen to be represented 

 by the poinL midway between the two points at which the 

 curve is cut by the ordinate through that deflexion. This 

 affords an accurate means of determining the exact positions 

 of the points P, Q, R, &c, and also enables the variation 

 with temperature of the free convection current to be very 

 accurately determined. The temperature of the wire corre- 

 sponding to any deflexion to the left of the axis of volumes 

 increases as the deflexion increases. The free convection 

 current from the wire therefore increases in the same 

 direction. Bearing in mind how the free convection current 

 is determined from the curve, this fact explains the want of 

 symmetry of the respective curves about the horizontal lines 

 through P, Q, R . . . respectively. The upper half is blunt 

 compared with the lower half, as the lines joining the mid- 

 points referred to above slope upwards to the left. In 

 particular, reference may be made to the fact that the length 

 of ordinate intercepted at the origin represents twice the free 

 convection current from the wire in the absence of any 

 impressed velocity. In Plate XII. fig. 12 are given the form 

 of the air-flow deflexion-curves for values of the electric 

 current equal to O'G and l'l amp. respectively, in the case both 

 of an upward and downward air-current. The galvanometer 

 shunt was different in the case of the two currents employed. 

 The curves for other values of the electric current ranging- 

 from 0'6 to 1*2 amp. are omitted for the sake of clearness. 

 They all show the same characteristics as those illustrated. 

 The characteristics of the portions of the curves to the 

 left of the axis of volume have been already discussed. 

 To the right of this axis, it is seen that, for the same 

 value of the electric current, commencing with a maximum 

 difference between the respective ordinates for upward 

 and downward flow corresponding to zero deflexion, the 

 difference thereafter continuously diminishes, the curves 

 approaching more nearly with increasing flow in the tube. 

 This approach is attributable to the fact that corresponding 

 to an impressed velocity V a of the air-stream, the actual 

 velocity in the neighbourhood of the wire is in the case of 

 the upwardly directed stream Vi + v e , while in the case 

 Phil. Mag. S. 6. Vol. 39. No. 233. May 1920. 2 M 



