O/i the Directional Hoi- Wire Anemometer. 641 



stability of zero — over those due respectively to Morris * and 

 to King f, it appeared desirable to examine the behaviour 

 of the instrument further. Experiments were accordingly 

 made to determine (1) the relative sensitivity of the 

 directional hot-wire anemometer with the wires mounted 

 vertically compared with the sensitivity when the wires 

 were mounted horizontally; and (2) how the impressed 

 velocity of the gas stream at which the maximum galvano- 

 meter deflexion occurred — which may be termed the critical 

 velocity — was dependent upon (a) the heating current 

 employed, and (b) the diameter of the wires. 



Experimental. 

 The general procedure in all the experiments was as 

 follows : — The two anemometer wires were cut from the 

 same sample of pure platinum wire supplied by Messrs. 

 Johnson & Matthey. In experiments connected more 

 particularly with the dependence of the sensitivity of the 

 instrument upon the inclination of the wires, it is desirable 

 that the resistances of the wires should be as nearly as 

 possible the same, in the absence of flow. This adjustment 

 to equality in situ is a matter of some difficulty and, in 

 general, was only achieved after a. number of trials. The 

 following procedure was finally adopted. The sample of 

 wire was aged by the passage of a current sufficient to 

 raise it to a bright red heat for about two hours. Two 

 portions, as free as possible from flaws and pittings, and 

 of about the same length, were cut from the sample, 

 and attached to copper leads of diameter 0'82 mm. This 

 was done by filing from the end of the copper lead a 

 quadrantal segment about 3 mm. long. One end of the 

 platinum wire was inserted into the segment and joined 

 to the copper lead by means of the minimum amount of 

 silver solder affording a sound junction. The remaining 

 lead similarly treated was set up at a distance equal to 

 the diameter of the flow r -tube from the end of the first 

 lead to which the platinum wire was soldered, and the 

 platinum wire affixed thereto under a tension sufficient 

 to maintain it quite straight. A second wire was prepared 

 in an exactly similar manner, and the two wires inserted 

 into the flow-tube as previously described. The resistances 

 of the wires were compared. Employing a small current 

 for this purpose, they were found usually to be equal to 

 within 0*3 per cent. When the current employed was 

 such that the wires were raised to a bright red heat, 

 their resistances were found to be, in general, within 0'5 per 

 cent, of one another. Occasionally agreement to within 

 * Morris, Erjg. Pat. 25,923/13. t King, Eng. Pat.* 18,563/14. 



