the Hot-wire Anemometer. 531 



An examination of the calibration curves given in fig. 17 

 shows that the deflexion corresponding to any constant flow 

 of air was consistently a little greater when the air was dry 

 than was the case when the air was saturated with water 

 vapour at the temperature of the experiment. The difference 

 in deflexion was of the order of from 0*5 to 1:2 per cent., 

 and was obtained consistently in a series of experiments, 

 two of which are shown in fig. 18. The difference is 

 not attributable to the slight difference in pressure at the 

 meter outlet in the two cases. King* was unable to detect 

 any effect of variation of humidity upon his final readings. 

 In the present series of experiments the variation of 

 humidity was much greater than that recorded by King, 

 and the result obtained shows that a variation of about 

 25 per cent, in the humidity of the atmosphere is the 

 minimum that could influence the results in King's experi- 

 ments. It is not stated whether such a variation occurred. 

 The relative sensitivities of a glass-covered hot wire in 

 air, hydrogen, and coal-gas when the same current is used in 

 the bridge is shown in Plate XIII. fig. 18. The comparative 

 insensitiveness in the cases of hydrogen and coal-gas is 

 directly attributable to the much greater thermal con- 

 ductivity of these gases compared with air. A reference 

 to fig. 16 shows that with zero flow, employing an electric 

 current equal to 1*3 amp., the resistance of the wire when 

 in hydrogen is less than that when immersed in coal-gas, 

 and very considerably less than when immersed in air. 

 On this account, the initial temperature of the wire for 

 zero flow was much less in the case of hvdrooen and coal- 

 gas than was the case with air. A fair basis of comparison 

 of the respective sensitivities is only afforded when the wire 

 immersed in the several gases has the same resistance in the 

 absence of flow. The results in Plate XIII. fig. 17 enable a 

 comparison of the thermal conductivities of the various gases 

 detailed to be made. Expressions for the heat convected 

 from the wire have been given by Boussinesq t, King t, and 

 Eayleigh§. Boussinesq's expression, H = 8(so7 t : vVtt)^,, 

 where s is the specific heat at constant volume of the gas, 

 o- its density, and £ is its thermal conductivity, V the 



* Phil. Trans., lac. cit. p. 417. 



t Journal de Mathematiques, 1. pp. 285-332 (1905). 



X Phil. Trans., loc. cit. p. 381. 



§ 'Nature,' xcv. p. 66 (1915). It may be remarked that the final 

 expression given "by Lord Payleig-h is obviously in error due to a 

 misprint, and in place of h oc l&^(bvcjk) should read h x WsJ(bvch). 



