Current of Air flowing past heated Platinum Wires. 257 



of wires Nos. 1 and 2 alone, results in what is tantamount to 

 the abolition of the end effect, as the end effect in this case 

 is the whole effect, there being only two wires. The effect 

 of the subsequent heated wires upon the temperature rise 

 experienced by No. 2 wire owing to an impressed air-stream 

 moving with slow velocity has already been discussed. The 

 case of wire No. 5 being employed in conjunction with wire 

 No. 1 can be readily discussed as follows : — When wires 

 subsequent to No. 5 in the sequence are not heated, wire 

 No. 5 is one of the end pair of wires, and is subject to what 

 has been termed the " end effect." Its initial temperature 

 and that of No. 4 wire are both less than that of No. 3 wire. 

 When wires subsequent to No. 5 in the sequence are heated, 

 wires 4 and 5 no longer experience the end effect. Their 

 temperatures are considerably higher than was previously 

 the case. With the establishment of a slow impressed 

 stream of air, wire No. 5 now experiences a thermal effect, 

 due principally to the approach towards it of the hot convec- 

 tion current arising from No. 4 wire. The convection 

 current being warmer than was previously the case, the rise 

 of temperature of wire No. 5 is materially greater, and the 

 sensitivity of the anemometer device employing wires Nos. 

 1 and 5 consequently initially greater when wires subsequent 

 to No. 5 are heated than is the case when these are not 

 heated electrically. Possibly the matter may be made 

 clearer by reference to fig. 3. There it will be seen that 

 initially the temperature rise experienced by wire No. 11 

 the last of the sequence, is very much less than that ex- 

 perienced by the adjacent wire No. 10. In like manner, it- 

 is to be anticipated that when No. 5 wire is the last of the 

 sequence of heated wires, its rise of temperature due to an 

 impressed air-stream will be inirially small compared with 

 what it would be if wire No. 6 were heated, and similarly 

 for subsequent wires of the whole sequence. In fig. 2 it 

 will also be seen that the last wire of the sequence ultimately 

 attains the greatest rise of temperature. Similarly, it is to 

 be anticipated that when wire No. 5 is the last heated 

 wire of the sequence, with increase in the velocity of the 

 impressed stream, its temperature rise will ultimately be 

 greater than when subsequent wires of the sequence are 

 heated. Und< j r these circumstances it is to be expected 

 that ultimately the sensitivity of the anemometer device 

 employing wires Nos. 1 and 5 will be greater if subsequent 

 wires in the series are not heated than would be the case if 

 these latter were heated. 



In conclusion, it may be remarked that the sequence o\' 

 wires illustrated in fig. 1 may be used, after the manner 



Phil. Mag. S. 6. Vol. 41. No*. 242. Feb. 1921. S 



