﻿5G8 Prof:. L. Vessot Kino- on Precision Measurement o] 



hold in position the wire and potential terminals', the- 

 disturbing effect at the point o£ measurement is reduced to a 

 minimum : moreover, the velocity actually measured is that 

 over the interval between the potential terminals whose 

 disturbing effect is practically nil, so that a "guard-ring" 

 effect is obtained. The disturbing effect of the anemometer- 

 wire itself has already been discussed in the preceding section,, 

 and is seen to be extremely small. As the wire is calibrated 

 in a uniform stream in which the velocity is constant over 

 the interval between potential terminals, care must be taken 

 in using the instrument that the flow is also uniform over 

 the measuring wire; this condition is easily judged if the 

 wire be employed at a high temperature so that it glows 

 dull red, as in this case small variations of A'elocity can be 

 detected with great accuracy by the unequal brightness of 

 the wire. If the changes of velocity are too rapid to allow 

 of an approximately uniform flow over the wire being brought 

 about, the distance between potential terminals should be 

 shortened until this condition is realized. In precision work 

 care should also be taken that the conduction losses in the- 

 wire are the same under conditions of calibration as in actual 

 use. This condition may be secured by so disposing the 

 potential terminals that they occupy the same position 

 relatively to the direction of flow under conditions of 

 calibration as in the distribution of flow which it is required 

 to investigate. 



(ii.) Effect of Variations of Atmospheric Conditions. 



It has been shown experimentally by the writer * that the 

 heat-loss per unit length from a wire in a current of air of 

 velocity V is given by a formula of the form 



H = ^=E + ( 7 + /3^Y)(0-.0 u ). • • • (5) 



r being the resistance of the wire per unit length when 

 heated by a current i to a temperature — above that of 

 the atmosphere 0„. E represents the radiation-loss in watts 

 per unit length, calculated according to the formula f 



E = iir-ix 0-511 (0/1OOO) 5 ' 2 , .... (6) 



being the absolute temperature. An inspection of the- 

 tabulated beat-losses in the experiments previously referred 



* Reference (I), pp. 399—101. 



t The constants of this formula are derived from the results of 

 Lummer and Kurlbaum fcr polished platinum {Yerjt. Dent. Fhys. Gcs. r 

 Berlin, xvii. p. 106. 1898). 



