﻿570 Prof. L. Vessot King on Precision Measurement of 



and Sanborn *. Tlie experiments were carried out over 

 a range of pressures from h to 3 atmospheres, and show 

 that that part of the heat-loss depending on the velocity X 

 varies as \Z<r V over this range of pressures. This result is 

 in agreement with what we should expect from the theoretical 

 equation (9) : and as it is well known that the thermal con- 

 ductivity of a gas is independent of the pressure, it follows 

 that the only factor in this equation which depends on the 

 -density is J3 . We thus have by differentiating (9) with 

 respect to a . the current i being given, the relation 



From (8) 8/3 a ; ^o=h^cr /o- = Sp () lp , where p refers to 

 .atmospheric pressure, we obtain finally 



SV/V=-8 Po lp, (12) 



It will thus be seen that ordinary variations of pressure 

 have a very small effect on velocity determinations ; if they 

 should be sufficiently large to affect appreciably the measure- 

 ment of velocity, the corresponding correction may lie made 

 by the application of formula (12). 



A compensating arrangement to correct for pressure 

 changes might, if necessaiy, be devised : an inspection of 

 formula (9), together with the experiments of Kennelly and 

 Sanborn, show that the anemometer measures the product 

 of the density and the velocity, that is. the mass-flow o2 a 

 gas. From many practical points of view the measurement 

 of mass-flow is the desideratum, and compensation is in those 

 cases unnecessary. 



(ft) Effect of Variations of Atmospheric Humidity. — The 

 extent to which the presence of water-vapour affects the 

 various factors involving thermal conductivity and specific 

 heat which enter into the theoretical formula (9) is diffi- 

 cult to foresee. As the proportion of water-vapour mole- 

 cules even at saturation is relatively small, their influence 



* A. E. Kennelly and H. S. Sanborn. "The Influence of Atmospheric 

 Pressure upon the Forced Thermal Convection from Small Electrically- 

 Heated Platinum Wires," Proc. American Phil. Soc. vol. viii. 1914. 

 The results of experiment are examined in the light of Boussinesq's 

 formula, R = 8 ^ (s<ric v«/7r)(e-0 o ). 



It would be interesting to examine these results in the light of the 



■emendation of Boussinesq's formula; small discrepancies at high and 

 low pressures might thus be explained. The results given in the paper 

 mentioned are not published completely enough to enable this to be 

 done ; moreover, an absolute comparison would not be satisfactory, as 

 the A-elocities are considerably affected by " swirl," which would be 

 ■difficult to allow for without a special determination. 



