﻿Air Velocity hy meant of Hot Wire Anemometer. 5C& 



to* will show that even at very low velocities and high 

 temperatures the radiation-loss is only a small fraction of 

 the total loss, and may therefore be neglected in studying- 

 the effect of small variations of atmospheric conditions on 

 the heat-loss. The constants 7 and ft have small temperature 

 coefficients given by 



y =y o [l + (M)O114(0--0o)] and p=0p[l + O'OOOO8(0-6 o y], 



. . • (7) 



which may also he neglected in discussing the problem in 

 hand. 



According to the theoretical formula developed by the 

 writer, and shown to be in substantial agreement with the 

 results of experiments on the heat-loss from a cylindrical 

 wire of radius a. we have 



7 = «- and /3 = 2^/7rs cr /c a , ... (8) 



Kq being the thermal conductivity of air, s Q its specific heat 

 per unit mass (at constant volume), and <r its density ; the 

 suffix refers to the values of these constants under atmo- 

 spheric conditions and at temperature #,,. 



Comparing the approximate theoretical formula 



H = rP = ( 7o + /3 o */V)(0-0„) .... (9) 



with the calibration formula 



r = io 2 vlv/V, (10) 



we have 



W 2 = 7,(0-6,) and rk = j3 o (0-6 o ). . . (11) 



An anemometer-wire is calibrated under given atmospheric 

 conditions which do not affect the mnnganin resistance, 

 and therefore also leave unaffected the resistance r and the 

 temperature 6 to which the wire is raised : it is required to 

 determine the small correction SY which must be added to 

 the velocity V corresponding to the observed current i when 

 the atmospheric conditions are slightly altered. These are 

 separately discussed under the headings of pressure, humidity, 

 and temperature. 



(a) Effect of Variations of Atmospheric Pressure. — 

 The study of the variation of the convection constants 

 with pressure has recently been undertaken by Ivennelly 



* Reference (1). Tables III. & IV.. pp. lit; & 41s. 



