-Directional Hot-Wire Anemometer, 



663 



the second wire attains its maximum value when various 

 heating currents are employed in the bridge, may be 

 expressed in terms of the excess temperature above 

 atmospheric, to which, in the absence of flow, the wires 

 are raised by the respective currents by the empirical 

 relation V Bl ss3;951og0-5 , 14. By virtue of the relation 

 = 289O 3 " 567 existing between the excess temperature and 

 the heating current C employed, this may be alternatively 

 expressed V m = 10*14 log C + 4'58. 



Table V. shows the agreement between the experimental 

 values of Y m given in column 10, Table IV., and those 

 calculated by the above formulae. 



Table V. 



Heating 



current 



C (amps.). 



Excess 



temperature 



of wires above 



atmospheric. 



Zero flow. 



(0-) 



Velocity of air-stream at which second wire attains 



its maximum resistance. 



(Cms. per sec.) 



Experi- 

 mental. 



Calculated from 

 V =3-951og0-514 



Calculated from 

 Y m =10-141ogC+4-58. 



lo 



826 



6-40 



6-59 



6-36 



1-4 



692 



6-10 



6'07 



6-0& 



1-3 



569 



5-76 



5-74 



563 



1-2 



471 



5-56 



5-41 



538 



11 



368 



5-00 



4-99 



5-00 



10 



277 



460 



4-49 



4-58 



0-9 



210 



400 



403 



4-08 



0-8 



158 



3-40 



3-54 



360 



0-7 



113 



2-96 



1- — 



3-00 



3-01 



In the case of wires of diameter 0*202 mm. the corre- 

 sponding relations were found to be 



Y m = 3-57 log (9-4-05 = 10*8 log C-0'10. 



The values of the maximum deflexions given in the 

 12th column of Table IV. are plotted in fig. 9, as ordinates 

 against the respective excess temperatures above atmo- 

 spheric to which the wires are raised in the absence of 

 an impressed air-stream, as abscissae. Corresponding 

 results for the anemometer employing wires of diameter 



