the Hot-wire Anemometer. 511 



the compensating device employed. There is still a possible 

 outsta tiding source of error due to the fact that the passage 

 of heat from the wire to the stream of fluid is determined 

 by the difference of temperature of the two. The bridge 

 being of the constant current type, for ordinary variations 

 of atmospheric temperature, this difference will be practically 

 independent of the latter temperature, so that the correction 

 necessary for variation in atmospheric temperature is, in 

 any case, very small. In fig. 17 (PI. XIII.) are plotted 

 separated observations for two calibration curves, the mean 

 temperature of the air stream in the one case being 20°' 2 C. 

 and in the other 17°*9 0. These are seen to be practically 

 coincident. With regard to the vibration of the heated wire, 

 the magnitude of the necessary correction can be deduced 

 by the formula developed by King*. As the wire employed 

 in the present experiments was not heated to a very elevated 

 temperature, the maximum amplitude of vibration was small. 

 of the order of O01 cm., and as, moreover, the experiments 

 were confined to measurements at comparatively low velo- 

 cities, the correction due to this cause was small. The 

 correction due to conduction along the leads is reduced by 

 the consideration that the distribution of velocities across the 

 cross-section of the tube is such that the wire is cooled most 

 at the centre and least at the wall of the tube. The investi- 

 gations of Kingt have shown how the necessary correction 

 can in any case be calculated, and he directs attention to the 

 fact that no correction is necessary if the anemometer is 

 employed under the same conditions of air velocity in the 

 neighbourhood of the terminals of the wire as those ruling 

 during calibration. 



A preliminary series of experiments showed that the ratio 

 of the resistance of the unprotected wire to that of the 

 shielded wire was not constant for different values of the 

 current in the bridge. The variation in the ratio is shown in 

 Table I. It is seen that in the case of zero air-flow the 

 ratio of the resistance of the unprotected wire to that of the 

 protected wire increases with increasing current. This is 

 no doubt partly due to the fact that the cooling of the pro- 

 tected wire is facilitated by the presence of the protecting 

 sheath, an effect analogous to that described by Porter J. 

 This point will be returned to later. It will be observed 

 from Table II. that the resistance of the protected wire was 



* Phil. Trans. Roy. Soc. A. 520, pp. 214, 398 (1914). 



t Loc. cit. p. -397. 



X Phil. Mag-., Sept. 1910, pp. 511-522. 



