﻿Air Velocity by means of Hot Wire Anemometer. 5tJ7 



various velocities was found to agree fairly well with the 

 calculated values. If conditions are sufficiently steady, and 

 a higher resolving power than this is required, the use of a 

 Weston Laboratory Standard ammeter for which $i=0'000'5 

 ampere will give a fourfold increase in resolving power. 



It is shown theoretically * that the disturbing effect of a 

 thin wire in a uniform stream of fluid is extremely small ; 

 .at a distance of ten radii from the centre of the wire the 

 velocity is only disturbed by 1 per cent, of its value, so that 

 the velocity is practically measured at a point. 



The anemometer employed by the writer was mounted on 

 a micrometer-screw in the manner indicated in the photo- 

 graphic reproduction given in Plate X. Each division of 

 the divided head corresponded to 1/100 mm., and it was 

 found that in very steep gradients of velocity a movement of 

 the wire of this magnitude resulted in an easily detectable 

 change of velocity. The limit of the resolving power in a 

 steep gradient is easily estimated by referring to fig. 4 ; a 

 movement of the wire over a distance of 0'05 mm. resulted 

 in an increase of velocity from 1570 to 1970 cm. /sec. As 

 the resolving power V/SV of the wire employed is about 160, 

 the change of velocity just detectable at the lower velocity is 

 x\ bout 10 cm ./sec. ; so that it is easily seen that a change of 

 velocity of this amount can be detected in a distance of 

 1/800 mm. It is thus possible, by the use of the linear 

 hot-wire anemometer, to measure velocity gradients as high 

 as 80,000 cm. /sec. per cm. In extremely sharp gradients, 

 in which the velocity changes very greatly over a distance 

 comparable to the diameter of the wire, a correction for the 

 disturbance of flow and the effect of the gradient on the 

 heat-loss might be appreciable. The correction is difficult 

 to determine theoretically, but may, if necessary, be deter- 

 mined experimentally by measuring the same gradient by 

 means of anemometer-wires of different radii. The fact 

 that the total flow determined by integrating the velocity 

 distribution of fig. 4 agrees with that determined directly, 

 indicates that in gradients as high as 8 X 10 4 cm. sec. per cm. 

 the corrections mentioned are probably small. 



Section 4. Sources of Error in Hot- Wire Anemometry. 



(i.) Disturbing Effect of the Anemometer-Fork ami Wire. 



The special design of fork required to hold in position the 

 anemometer-wire will depend largely on the nature of the 

 work to be undertaken. By employing line steel needles to 

 * Reference (1), p. 400. 



