the Hot-wire Anemometer, 523 



currents being calculated on the assumption that the tem- 

 perature of the ascending convection current is the same as 

 that of the wire. 





Table 



V. 





Keating 

 Current. 



Temperature 

 of Wire. 





Velocity of free 

 Convection Current 



amp. 



°C. 





cms. per sec. 



0-6 



135 





31 



0-7 



191 





40 



0-8 



245 





5-4 



0-9 



315 





7-0 



10 



420 





9-1 



1-1 



535 





12-2 



1-2 



677 





15-9 



From a. comparison of these values with the values of the 

 free convection current as calculated by King * for wires of 

 diameters 0*00691 cm. and 0*01262 cm., on the basis of 

 LangmmVs t observations, it is seen that the present experi- 

 mentally determined values for the velocities agree within 

 the limits of estimated experimental error with the calculated 

 values. 



The Double Exposed Wire Anemometer. 



A calibration curve was obtained employing two flow- 

 tubes set up in a vertical position adjacent to one another. 

 An exposed anemometer wire was inserted as already 

 explained in each flow-tube. Owing to the close juxta- 

 position of the tubes, the employment of subsidiary shielded 

 temperature-correcting wires was unnecessary. The two 

 exposed anemometer wires were made from the same sample 

 of wire, and were adjusted as nearly as possible to the same 

 resistance. They constituted two arms of a Wheatstone 

 bridge, the other two arms of which were formed of re- 

 sistances unplugged from the resistance-box. One of these 

 latter was made equal to 2000 ohms throughout the experi- 

 ments, and the other adjusted so that balance of the bridge 

 was obtained when no air-flow was established in the flow- 

 tubes^ the bridge-current being adjusted to 1*0 amp. The 

 flow-tubes were connected in series, and the air-current 

 passed upward through one tube and downward through the 

 other. Now, with this arrangement the effective velocity 



* Phil. Trans., lac. cit. p. 424. 

 f Langmuir, loc. cit. p. 415. 



2 M 2 



