282 Dr. J. S, G. Thomas on the Forced Convection of 



section of the tube and originating in the wire in question, 

 but also in part by that similarly accumulated due to the 

 other wire of the pair. This accumulation of a compara- 

 tively stagnant layer of heated air extending over a com- 

 paratively large distance in the upper region of the flow tube, 

 is an additional factor complicating the application of the hot- 

 wire anemometer to the investigation of the velocities (and 

 more especially of low velocities) of gas streams in narrow 

 channels. Thus, as shown by the initial portions of the 

 curves H, I, and M, for small values of the impressed 

 velocity, the second wire may be more cooled when the first 

 wire is heated than when the wire is not heated. In the 

 case of curves E-H, the initial value of the resistance of 

 the second wire being determined while the wire is subjected 

 to the heating effect just referred to, and as, moreover, this 

 heating effect due to the accumulated layer of hot air clearly 

 increases as the wires approach one another, it is to be 

 anticipated that with the reduction or removal of this effect 

 by the incidence of the impressed stream of air, the maximum 

 increase of resistance of the second wire will be reduced as 

 the distance apart of the wires is decreased. A further 

 feature of the curves E to H (and the same may be said of 

 all the remaining curves except L) is that, as the distance 

 apart of the wires decreases, the resistance of the second wire 

 corresponding to a definite large value of the impressed 

 velocity of the stream increases. This arises owing to the 

 diuiinisjaed loss of heat occurring in the forced convection 

 stream during the time of' passage from one wire to the 

 other as the distance apart of the wires is diminished. The 

 third group of curves H, I, J, K is characterized by a 

 gradual increase in the maximum temperature of the second 

 wire as the distance apart of the wires diminishes. In the . 

 case of the curve K, corresponding to a distance apart of 

 the wires equal to 0*189 cm., the maximum increase of 

 resistance of the second wire occurring in the present series 

 of experiments was attained. The degree of symmetry 

 about a vertical axis through the apex shown by this curve 

 indicates the elimination or compensation of the disturbing 

 influences referred to above, and points to a disposition of 

 the wires at a distance apart equal to about 0*2 cm. as most 

 suitable for the construction of a bifilar or directional hot- 

 wire anemometer. The gradual increase in the maximum 

 temperature attained by the second wire as the distance 

 apart of the wires is decreased, contrasted with the gradual 

 decrease in the maximum in the case of the second group of 

 curves (E-H) discussed above, is to be explained as follows : — 



