724 Dr. J. S. G. Thomas on a JSull-JJe flexion 



wire approaching vertically are seen to he practically 

 perpendicular to the axis of velocities. These extremely 

 steep portions of the calibration curves represent a condition 

 of affairs where the velocity of the impressed stream has 

 attained such a value that the temperature of the resultant 

 convection current from the wire differs very little from 

 atmospheric, so that very little temperature change occurs 

 in the wire on rotation. Moreover the fact that the con- 

 vection current from the wire is of approximately uniform 

 temperature over a region considerably wider than the 

 diameter of the wire *, is a factor likewise operative in 

 accentuating the steepness of the calibration curves in this 

 region compared with their smaller inclination in the region 

 of the origin. 



The curves in fig. 3. show that, employing a current of 

 1*5 amp., the type of anemometer described may be usefully 

 employed for the determination of velocities up to about 

 4*5 cm. per sec. The range of application diminishes with 

 decrease in the bridge current employed. For a current 

 of 0*9 amp. the maximum velocity measurable with 

 accuracy is about 2 cm. per sec. 



Determination of the effective velocity of the free convection 

 current arising from the tvire ivhen inclined to the hori- 

 zontal. 



As already remarked, the cooling effect, due to the free 

 convection current, experienced by the heated unshielded 

 wire in a position inclined to the horizontal is less than when 

 the wire is horizontal. Such diminished cooling effect 

 arises from the greater thermal shielding influence afforded 

 by the free convection current in the case of the inclined 

 wire, and may for purposes of calculation be ascribed 

 to a diminution in the velocity of the free convection 

 current. The approximate values of such effective velocities 

 under the conditions of the present experiments can be 

 readily determined from the results represented in tig. 3. 

 In the case of any one of the curves shown therein, the total 

 heat-loss from the wire is the same for any point on the 

 curve, as the points represent a balanced condition of the 

 bridge. If, then, Noneffective velocity of the free convection 

 current with the ware horizontal, r a = the effective velocity 

 of the same when the wire is inclined at an angle a to the 



* Proc. Phys. Soc. vol. xxxii. Part 5, p. 301 (1920). 



