﻿560 Prof. L. Vessot King on Precision Measurement of 



linear anemometer in precision measurements of air-velocity. 



In the following sections are set out in greater detail direc- 

 ts . " 



tions as to the most efficient method of using the instrument 

 and data now available as to the resolving power, upper and 

 lower limits of correct velocity registration, life of wires, 

 sources of error, &c, which may be of use to experimenters 

 wishing to employ the linear anemometer in aerotechnkal 

 investigations. 



Section i. On the Selection and Calibration 

 of Anemometer- Wires. 



Platinum wire 2-J or 3 mils in diameter is found to be 

 most suitable for the purposes of hot-wire anemometry. 

 The metal should be as pure as possible as judged from a 

 determination of the specific resistance and temperature 

 coefficient. The wire employed in platinum thermometry is 

 especially suited to the purpose in that its electrical constants 

 are usually specified with great accuracy. The wire should 

 be drawn down to 2-J or 3 mils, and if found to be satis- 

 factory on microscopic examination, a considerable length 

 should be reserved for the purposes of anemometry ; if 

 possible the final diamond die employed should be reserved 

 solely for drawing down anemometer-wires. The wire having 

 been mounted and annealed in the manner already described 

 and the potential terminals fused in position, the manganin 

 resistance should be set to the value previously determined 

 as that to which the anemometer must be heated by the 

 electric current in order to attain the temperature best 

 suited to the type of velocity measurement to be undertaken. 

 For most purposes a temperature which corresponds to a 

 dull red appearance of the wire is most suitable. Wires 

 may be set roughly to the same temperatures by adjusting 

 the manganin resistance so that a balance is obtained for the 

 same current when the anemometer- wire is in stagnant air, 

 carefully protected from draughts. The potential terminals 

 are generally fused to the anemometer-wire at a distance of 

 2-5 cm. apart, although for some purposes the writer has 

 worked with a distance as small as 1 cm. The distance 

 between potential terminals could be made very much less 

 if the wire is calibrated directly, though at a considerable 

 loss in galvanometer sensitivity; a sensitivity of 10" 6 ampere 

 would probably be sufficient for use with a wire of 2£ mm. 

 between potential terminals ; as the velocity measured is 

 practically that over this distance, it is seen that velocities 

 can approximately be measured at a point with a minimum 

 disturbance of flow. 



