Current of Air flowing past heated Platinum Wires, 249 



effect experienced by the second wire is determined by the 

 relative magnitudes of these opposing tendencies. The 

 effect of an impressed flow of air past the wire is, as shown 

 in fig. 5, to deflect the convection currents in such manner 

 to cause an approach to B of the hot ascending from A, and 

 to cause the cold convection current ascending towards B 

 to approach nearer to A. In fig. 5, BM is drawn at 

 right angles to B/3. It will be readily seen that with 

 increase of the velocity of the impressed stream of air, Aa 

 and B/3 rotate in a counter clockwise direction, and the 

 triangle ABM goes through a series of values corresponding 

 to the variation of the thermal effect experienced by the 

 second wire under these circumstances *. If, in like manner, 

 AO be drawn at right angles to B/3', a decrease in AG is to 

 be interpreted as an increase in the cooling effect experienced 

 by the wire A, owing to the impressed air-flow, and vice 

 versa. It has been pointed out elsewheref , and the result 

 will be clearly seen, from a comparison of the initial portions 

 of curves (1) and (2) in fig. 3, that for small velocities of 

 the impressed air stream the heat gained by the second 

 wire is considerably larger than that lost by the first. For 

 this, and other reasons, the broken lines in figs. 4 and 5 are 

 shown of shorter length than the full lines in the upper 

 region of the diagram. It is clear that the thermal effects 

 experienced by the respective wires A and B, due to a small 

 impressed flow of air, can be roughly represented by the 

 area of the triangle AMB and the reciprocal of AO. The 

 introduction of a third wire may now be readily seen by 

 drawing BN perpendicular to C7'. It is clear that, owing 

 to the impressed flow of air, the cold convection current 

 rising towards approaches B (i. e. BN<BC). We con- 

 clude, therefore, that by the introduction of a third wire C, 

 the initial rise of temperature of the wire B produced by a 

 small impressed air-flow is less than when the wire C is 

 absent. It follows, therefore, that the initial sensitivity of 

 a directional hot-wire anemometer constituted by the wires 

 A and B for small impressed velocities is greater in the 

 absence than in the presence of the third heated wire 

 (see fig. 1, later). Attention has been previously directed 

 to the significance of this phenomenon in connexion with 

 increased sensitivity of the directional type of hot-wire 

 anemometer (see Proc. Phys. Soc. Inc. cit. p. 204). The 

 effect of the respective wires D, E, F, etc. subsequent 10 C 

 upon the temperature rise experienced by B is clearly to 



* Proc. Phvs. Soc. he. cit. p. 202. 



t Phil. Mag. vol. xl. pp. 652,653 (1920). 



