﻿Air Velocity by means of Hot Wire Anemometer. 563- 

 Fig. 3. 

 m*% S <?■ .5 2 / ( 2 3 *+ S *?/». 



Test of Anemometer Headings at Low Velocities: S-mil Wire No. 



A short rectangular channel of width 0*75 mm., having a plate at 

 right angle? to its length fitted flush with its upper extremity, was- 

 set up in the manner illustrated in the lower part of the figure. By 

 means of suitable connexions to a gasometer, air from the room was 

 drawn into this channel under a constant pressure-difference of 2*35 cm. 

 water. The distribution of flow at a sufficient distance from the opecing 

 of the channel is approximately that which would be set up in a perfect 

 fluid by a distribution of sinks along a line coinciding with the opening 

 of the channel into the plane mentioned. In the neighbourhood of a 

 plane bisecting this slit at right angles, where the velocity is measured 

 by the portion of the anemometer-wire between potential terminals, the 

 distribution ot velocity is approximately radial. Taking a set of axes 

 (x, y) having as their plane the diametral plane just mentioned, with 

 origin at the centre of the slit, and measuring the axis of y along the 

 direction of the ch annel, the velocity at any point {x, y) is approximately 

 given by V=V V2/ 2 /(y 2 +# 2 )j V being the maximum velocity at x=6. 

 The anemometer-wire, represented by A in the figure, was set by means 

 of a micrometer-screw in various positions in the plane y=3 mm.; the 

 observed velocity distribution thus obtained was compared with the 

 theoretical by choosing V to agree with the experimental value at 

 #=0. The figure shows that the readings of velocity are fairly accurate 

 for velocities as low as 12 cm. /sec. ; the deviations are possibly due to 

 the limitations of the simple formula employed in calculating the 

 theoretical velocity distribution. 



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