642 Mr. J. 8. G. Thomas on the 



0*1 per cent, was found. For most purposes, equality to 

 within 1 per cent, is amply sufficient ; but when it is 

 desired to obtain a closer approximation in the adjustment 

 to equality, a current, sufficient to raise the wire o£ lower 

 resistance to a bright red heat, was passed through this 

 wire alone for the necessary period — extending to hours 

 or days — an air-stream being passed over the wires at half- 

 hourly intervals for about 10 minutes. In this manner the 

 resistances could be adjusted to equality when heated to within 

 O'l per cent., and thereafter both wires were aged by the 

 passage for about 2 hours of a current sufficient to raise 

 them to a bright red heat. The ratio of the resistances 

 remained remarkably constant for a very considerable 

 period after such treatment : thus the ratio in the case 

 of anemometer D4 showed no greater variation than O'l per 

 cent, while the anemometer was employed in determining 

 about 50 calibration curves, employing various values of the 

 heating current. In all probability, owing to the wires 

 being throughout their use subjected to practically identical 

 conditions, no calibration would be necessary until the 

 instrument had been employed in the measurement of con- 

 siderably more than the 1000 velocities recommended by 

 King* in the case of the non-directional type of instrument. 

 The wires having been inserted in the flow- tube were 

 introduced into the Wheatstone bridge arrangement as 

 described. The wires could be placed horizontally, or at 

 any inclination to the horizontal transversely to the direction 

 of flow of the air in the flow-tube which was horizontal. 

 A millivoltmeter could be introduced across the ends of the 

 second wire of the pair so that the resistance of the second 

 wire could be calculated from a knowledge of the observed 

 drop of potential and the current in the bridge, this being- 

 maintained constant by means of a rheostat inserted in the 

 battery circuit, and its value ascertained by means of a 

 Siemens and Halske milliammeter provided with suitable 

 shunts enabling current determinations to be made correct 

 to 0*001 amp. One of the two remaining arms of the 

 bridge — that connected to one end of the second anemo- 

 meter wire — was made equal to 1000 ohms and the 

 resistance of the first anemometer wire, when airy gas- 

 flow was established in the pipe, was calculated from that 

 of the second wire of the pair, by determining the resistance 

 necessary in the fourth arm of the bridge to restore the 

 balance of the bridge. The galvanometer deflexion corre- 

 sponding to the flow of air was read prior to restoring the 



* Phil. Mag. vol. xxix. p. 573 (1915). 



