Constant Current Type of Hot- Wire Anemometer. 721 



King * for the relation of the convection constant /3 to the 

 inclination of the wire. The results clearly indicate that 

 very considerable reduction in the magnitude of the effective 

 velocity of the free convection current occurs when the wire 

 is rotated from the horizontal to the vertical position. Such 

 considerable reduction is to be anticipated from a com- 

 parison of the results contained in a previous paper f, 

 showing the variation of: the resistance of a heated wire r 

 when subjected to an impressed downwardly-directed current 

 of air, and the variation of the resistance of the wire when 

 rotated from the horizontal to the vertical position given in 

 Table I. The latter variation is of the same order of magni- 

 tude as that in the previous work corresponding to complete 

 elimination of the free convection effect in the case of a 

 horizontal wire. The effective velocity of the free convection 

 current corresponding to any inclination of the wire could 

 be very accurately determined by experiments along these 

 lines carried out in a wind channel. 



It has previously been pointed out J that an inversion in 

 the respective sensitivities of an anemometer of the Mori is 

 type with horizontal wire, employing two different values 

 of the heating current, occurs as the velocity of the impressed 

 stream is gradually increased from zero, and that the velocity 

 of the impressed stream corresponding to this point of inver- 

 sion is larger, the larger the free convection current corre- 

 sponding to the greater of the two heating currents concerned. 

 A similar effect due to a decrease in the magnitude of the 

 effective velocity of the free convection current, when the 

 wire is rotated from the horizontal to the vertical position, 

 is shown in fig. 4, wherein are given the forms of the cali- 

 bration curves obtained employing currents of 1*5 and 10 

 amp. in the anemometer bridge, the bridge in each case 

 being balanced in the absence of an impressed flow of air 

 with the unshielded wire (1) horizontal and (2) vertical, 

 corresponding to the respective inclinations at which the 

 wire was subsequently used. The galvanometer shunt was 

 throughout equal to 27 ohms. It is seen that the vertical 

 disposition of the wire affords the greater sensitivity (c/. 

 curves A and C, or B and D), and that whereas in the 

 case of the horizontal wire, the larger heating current 

 affords the greater sensitivity only for velocities greater 



* Phil. Trans. A. Joe. cit. p. 425. 



t Phil. Mag-, vol. xxxix. pi. xii. tig-. 13(1920). 



X Phil. Mag. vol. xxxix. p. 51.-) (1920). 



