Current of Air flowing past heated Platinum Wires. 251 



effect of the hot ascending currents upon members or! this 

 group is the same throughout. An increase of the initial 

 rise of temperature of successive members of this group is 

 therefore to be anticipated. 



Group (d). In this group, constituted of wires K and L 

 in fig. 5, it will be seen from the figure that, whereas the 

 perpendiculars from K upon the lines representing the 

 various hot convection currents meet four full-length lines, 

 those cut by the perpendicular from L include the line of 

 shorter length drawn from K. As already explained, the 

 effect of the cold convection current rising to L upon the 

 temperature of K is small. The temperature of L, being 

 therefore principally conditioned by the relatively small 

 free convection current rising from K, while the tempera- 

 ture of the latter is determined principally by the relatively 

 larger convection currents arising from J, in accordance 

 with principles enunciated above, it may be anticipated that 

 in the presence of an impressed air-flow the initial rise of 

 temperature of the wire L will be less than that in the case 

 of wire K. 



The thermal effect experienced by the individual wires 

 consequent upon an increase in the velocity of the impressed 

 stream of air can be readily seen by suitable modification of 

 fio\ 5. Such modification is to be effected, bearino- in mind 

 that a slight increase in the impressed velocity of the stream 

 lends to cause the lines representing the respective convec- 

 tion currents to take up positions wbich are reached by a 

 small anti-clockwise rotation from their previous positions. 

 In like manner, an increase in temperature of any wire tends 

 to cause the appropriate line to take up a position reached 

 by a suitable clockwise rotation, a decrease of temperature 

 having a. contrary tendency. The resultant positions of the 

 lines representing the convection currents are to be deter- 

 mined from the joint consideration of such tendencies. The 

 magnitude of the free convection current appropriate to any 

 temperature of the wire may be found from the results given 

 previously*. 



Application of above results to the construction of sensitive 

 directional hot-wire anemometers. 



The results contained in fio-s. 2 and 3 may be utilized in a 

 variety of manners for the construction of hot-wire anemo- 

 meters of the directional type, which, as already pointed out, 

 have a special sphere of usefulness in the region of low 



* Phil. Mag. vol. xxxix. p. 523 (1920). 



