﻿•566 Prof. L. Vessot King on Precision Measurement of 



sensitivity and resolving power, while in the use of stouter 

 wire the working currents increase rapidly and the apparatus 

 Tias to be specially designed to meet the resulting heating 



'effects. 



Section 3. Ox the Resolving Power of the Hot-Wire 

 Anemometer. 



By differentiating formula (3), we obtain 



V/SV = i(t/KXl-W) (*) 



We can conveniently define the " resolving power " of the 

 anemometer by the ratio V/SV, where SV is the increment 

 oE velocity just observable at the velocity V. Formula (4) 

 then expresses the resolving power of the anemometer in 

 terms of that of the ammeter for current. In the ammeter 

 employed by the writer the scale was uniformly graduated, 

 and could be read to Si = 0"002 ampere ; on this basis 

 the resolving powers of typical 2J- and 3-mil wires are 

 calculated for convenience of reference in Table I. 



Table I. 

 Tiesolving Powers of 2|- and 3-mil Anemometer- Wires. 



Y/8Y 



0-564) 2 x 276*4 

 =0-002 amp. 

 i/5i(l -0-564,72). 



3-mil wire Xo. 8. 

 Y = (p- 0-81 )-X 116-6. 



c/=0-002 amp. 

 V/8V=ii/Si(l-0-81/P 



i. 



V. 



V/8V. 



SY. 



cm. /sec. 

 0-48 



i. 



V. 



v., 



SY. 



amperes. 

 090 



cm. /sec. 

 16-7 



34-2 



amperes. 

 1-10 



cm. /sec. 

 18 



45-4 



cm. /.sec. 

 0-40 



1-00 



52-4 



54 - 5 



096 



1-20 



46 



65 - 5 



0-70 



1-50 



785 



141 



5-6 



1-80 



684 



170 



400 



200 



3260 



215 



15-1 



i 2-45 

 1 



3150 



264 



12-0 



In most applications the steadiness of velocity conditions 

 is not sufficiently great to warrant making use of the full 

 resolving power of the instrument ; in experiments carried 

 out by the writer on the flow of air between parallel planes 

 an extremely good pressure regulation was maintained by 

 means of the Topler tilting manometer shown in Plate X.*, 

 and it was found possible to make use of the full resolving 

 power of the instrument. The observed resolving power at 



* For a description of this instrument, see a paper by A. Topler, Ann. 

 d. rhys. lvi. p. 610, 1895 ; also Muller-Pouillet, Lehrbuch der Physik, 

 190G, vol. i. p. 462. 



