﻿572 Prof. L. Vessot King on Precision Measurement of 



so that the temperature attained by the anemometer-wire 

 A is given by 



A o (l + *0 p ) = (B//3).<* o (l + €0„ p ), 



or more conveniently by 



*,-V=<W£).[VAo-0/BKl+«V • ( 15 ) 



To the order of approximation employed in (9) the law 

 expressed by this formula will not be greatly altered if the 

 temperatures are measured on the platinum scale, the effect 

 being to alter the convection constants y and /3 to slightly 

 different values j ' and $/ : hence if / denote the length of 

 the anemometer-wire between potential terminals, we may 

 write {\)) in the form 



which gives, on making use of (14) and (15), 



(B/ ( 3)« (1 (L + e^);- = /(B//3) . [« /A„-/3/B]( 7o '+/3 VV)(l + <#v) ; 

 or finally, 



«„; a =%„'+j8„VV)[«„/A -/3/B]. . . (16) 



This formula indicates that to the first order of small 

 corrections the determinations of velocity will be independent 

 of room temperature. 



In the practical realization of this system of compensation 

 the ratio-coils (a, a) may be made of platinum or of some 

 metal having the same temperature coefficient as platinum, 

 but more conveniently by combining two resistances, either 

 in series or in parallel, so that the equivalent resistance is of 

 the required value and the equivalent temperature coefficient 

 that of the anemometer-wire. It is easily proved that by 

 employing wires whose temperature coefficients are respec- 

 tively less and greater than that of platinum, the required 

 combination can always be obtained and predetermined. 

 The parallel combination is to be preferred, as its greater 

 current-carrying capacity would materially lessen the danger 

 of heating from this source. This point should be carefully 

 tested for before these coils are inserted in the Kelvin 

 Bridge. The current in the coils (a, j3) and (a, b) are 

 easily calculated in terms of the current through the anemo- 

 meter-wire*, and these coils should be so wound as to dissi- 

 pate heat as readily as possible in order that the change of 

 resistance due to current-heating be entirely neglible for the 



* Reference (1), p. 385. 



