514 Mr. J. S. G. Thomas on 



These results, together with similar results obtained, 

 forming a series in which bridge-currents equal to 1*1, 1*2, 1*3, 

 1*4, and 1*5 amp., are shown in the accompanying diagram 

 (PI. X. fig. 5). The abscissae in the figure denote galvano- 

 meter deflexions, and the ordinates the number of cubic feet 

 of air (at 0° 0. and 760 mm.) passing per hour. These 

 latter are readily converted to the corresponding values of 

 the mean velocity of the air in the tube, in cms. per sec, 

 by multiplying by 2*374. The relative sensitiveness of the 

 anemometer employing various currents is seen from the 

 diagram. In general, a more open scale of deflexions is 

 secured at the higher velocities by employing a larger 

 current. In all the curves, the existence of a point of 

 inflexion is clearly seen. The existence of these points 

 of inflexion is, of course, connected with the existence of 

 what is termed the " free convection " current from the 

 heated wire. The total 'cooling; effect to which the wire 

 is subjected is the resultant of the cooling effect due to 

 the air current passing down the tube, and that due to a 

 convection current of heated air rising from the wire. 

 A mathematical investigation of the magnitude of the 

 convection cooling effect has been made by King *, who has 

 interpreted the experimental work of Langmuir t on the 

 subject in the light of the results obtained. In the present 

 experiments, the heated wire being fixed horizontally, the 

 air-current and the free convection current are at right 

 anodes to one another, and the resultant cooling current 

 to which the heated wire is subjected is equal to 

 V V a + v c = "V, say, where V« denotes the mean value of the 

 velocity of the horizontal air stream, and v c that of the 

 " free convection ■" current. The value of v c diminishes as the 

 impressed velocity Y a is increased, owing to the lowering of 

 temperature of the wire produced thereby. Assuming that 

 the diminution of v c is proportional to v a or some positive 

 power thereof, it follows that the free convection current 

 will influence the form of the calibration curve at a higher 

 impressed velocity, the higher the initial temperature to which 

 the wire is raised, i. e. the larger the electric current employed. 

 With a view to the investigation of the effect of the free 

 convection current upon the form of the calibration curve, 

 a series of experiments was carried out using currents 0'9, 

 1*1, 1*3, 1'4, and 1*5 amp., respectively, in conjunction 

 using a reflecting galvanometer of resistance 60 ohms by 



* King, Phil. Trans, loc. cit. p. 425. 



t Langmuir, Phys. Rev. 1912, xxxiv. 415. 



