﻿Air Velocity by means of Hot Wire Anemometer. 571 



may be judged to be small on the heat loss from the heated 

 anemometer-wire. In fact this point was specially tested 

 in the experiments of Kennelly and Sanborn referred * to 

 with the result that no appreciable effect on the heat-loss 

 •due to this cause could be detected, although a small effect 

 was thouulit possible. No disturbing effect on velocity 

 determinations due to this cause has been noticed by the 

 writer : it is hoped, however, to definitely settle this point 

 by a special series of experiments. 



(7) Effect of Variations of Atmospheric Temperature. — 

 Differentiating equation (2) with respect to , the current 

 i being given, we easily obtain by making use of (fO) 

 and (11) 



8V/Y = 2S0 o /(0~6 o ).(l-i o 2 li 2 )-\ , . . (13) 



In the case of a 2^-mil wire employed at 1000° C, i () 2 = 0'5 

 -approximately, while for a. range of velocity from 50 to 

 2800 cm./sec. i varies between 1 and 2 amperes, so that the 

 factor (1— v/i 2 ) -1 varies from 2'00 to I'll. It will be seen 

 that by employing a wire in the neighbourhood of 1000° 0. 

 variations of room temperature of ±2°0. give rise to errors 

 of velocity determinations which at most are less than one 

 per cent. If fluctuations of room temperature exceed this 

 amount, the corresponding correction can easily be made by 

 use of formula (13). 



In the experiments carried out by the writer, the room 

 temperature rarely fluctuated more than by ± 2° C, and 

 as the wire was employed at a high temperature, it was not 

 thought necessary to adopt temperature-compensating de- 

 vices. The Kelvin Bridge connexions lend themselves 

 extremely well to a compensating arrangement, which we pro- 

 ceed to describe. The fundamental relation A/B = a;b-=a//3 

 will remain valid at all temperatures if the resistances 

 (B, b, fi) are constructed of manganin, while (A, a, a) are 

 of platinum or of a wire or combination of wires having the 

 same equivalent temperature coefficient e as the platinum 

 anemometer-wire A. The coils (a. ol) are so disposed that 

 they can readily attain the temperature of the air-stream 

 whose velocity it is required to measure. Under these cir- 

 cumstances if (a , « ) refer to 0°C, while for convenience 

 temperatures are measured on the platinum scale, we have 



A=A o (l + e0 y ) and « = a o (l + e0 b > . (14) 



"When a balance is obtained on the bridge we have A=B«/$ 



* Loo. cit. p. C9. 



