Directional I lot- Wire A nevnometer. 



657 



curves in a previous paper *, indicating the more complete 

 elimination of the effect of the free convection current in 

 the directional type of instrument, It has likewise been 

 shown f that in the type of hot-wire anemometer devised 

 by Morris, the calibration curves obtained by using various 

 heating currents in the bridge intersect in the region of* 

 velocities of the air- flow, where the velocity of the free 

 convection current is comparable with the impressed 

 velocity of the air-stream. The curves shown in fig. 4 

 are characterized by abserice of such points of intersection. 



200 250 300 



Deflection 



Fig. 6 shows the form of the calibration curves in the 

 region of low impressed velocities, employing the galvano- 

 meter at its maximum sensitivity. It is clearly seen that 

 any intersections of the respective curves, if occurring at 

 all, must occur for values of the impressed velocity of the 

 air-stream less than about 0*75 cm. per second. 



The respective values of the impressed velocities of the 



* Proc. Phys. Soc. vol. xxxii. Part 3, pp. 196-207 (1920). 



t Phil. Mag-, vol. xxxix. pp. 515 516, pi. x. Ho-, (j (May 1920). 



