Forced Convection of Heat from Pair of heated Wires. 277 

 But fi = 0i-#i + 0i ; . 



^ 2 =0 1 — (/)] -|-(/)/ + «/>o — $2'- 



In the figure <p 2 and </> 2 ' on ly arc negative, so the general 

 formula when the symbols carry direction signs becomes 



-^r <2 = — (f) l -\-^^ — (^ 2 -\-j) 2 . 



When the incident light is a plane wave-front at normal 

 incidence, the formula is simply 



^2 = — </>i + </>/ — 4>2 + 4>2' 



" Westward," Tankerville Terrace, 1 



Newcastle-upon-T}me. 



XXVII. The Forced Convection of Heat from a Pair of 

 fine lieated Wires. By J. S. G. Thomas, F).Sc. (Lond,), 

 'B.Sc. (Wales), A.R.C.S., A.I.C., Senior Physicist, South 

 Metropolitan Gas Company, London *. 



Introduction. 



IN a recent communication f, attention was directed to 

 certain characteristics of the forced convection of heat 

 from a pair of heated wires immersed in a slow stream of air, 

 and constituting a bifilar anemometer of the directional type. 

 It was remarked % that the disposition of the wires could be 

 such that the temperature of the second wire of the pair was 

 unaffected by the stream. In the course of an investigation 

 of the velocity of the air-stream at successive cross sections 

 in a flow tube, employing the hot-wire anemometer, the 

 importance of this consideration became very evident, and 

 the present communication details some of the characteristics 

 of the forced convection of heat from a fine heated wire due to 

 the passage of a stream of air moving with low velocity and 

 heated by prior passage over a similar wire immersed in the 

 stream. The dependence of this thermal effect upon the 

 distance apart of the wires is, more especially, the subject of 

 the present communication. 



* Communicated by the Author. 



t Phil. Mas-, vol. xl. pp. 640-665 (1920V 



t Ibid. p. 652. 



