732 Constant Current Type of Hot- Wire Anemometer. 



The research detailed herein was carried out in the 

 Physical Laboratory of the South Metropolitan Gas Co. 

 Mr. W. H. B. Hall assisted in the experimental part of the 

 work. 



The author desires to express to Dr. Charles Carpenter, 

 C.B.E., M.I.C.E , his sincerest thanks lor his unfailing 

 readiness to provide all facilities for carrying out the work, 

 and for his inspiring interest in the investigation. 



Summary. 



A form of hot-wire anemometer is described in which the 

 heating current is maintained constant, and the cooling 

 effect experienced by the exposed wire due to a small 

 impressed velocity of the gas stream, compensated by alter- 

 ation of the inclination of the wire from its initial horizontal 

 position. The limits of application of such a device are 

 shown by a series of calibration curves, in which the neces- 

 sary alteration is plotted against the value of the impressed 

 velocity, for values of the heating current ranging from 

 0*9 to 1*5 amp. The results are employed to deduce the 

 value of the effective free convection current corresponding 

 to any given inclination of the wire when heated by the 

 respective electric currents. The alteration in the effective 

 velocity of the free convection current accompanying alter- 

 ation in the inclination of the wire is utilized to determine 

 the inferior limit of the value of the impressed velocity of 

 the stream for which the effect of the free convection current 

 may be safely neglected. Empirical linear formulae are 

 obtained relating such limiting velocities to the temperature 

 of the wire. The effect of the free convection current upon 

 the form of the calibration curves is illustrated by reference 

 to the forms of the curves obtained employing two values of 

 the heating current for both the horizontal and vertical dispo- 

 sitions of the wire, and for various inclinations of the wire 

 employing a constant heating current. 



Physical Laboratory, 



South Metropolitan Gas Company, 

 709 Old Kent Koad, S.E. 

 17 Jan., 1921. 



