﻿574 Prof. L. Vessot King- on Precision Measurement of 



occupies about an hour's time. The necessary corrections- 

 for ageing are easily applied by dividing the observations 

 into groups of 100 and applying proportional velocity- 

 corrections to each group. In the case of wire No. 17. 

 given in Table II., it was found that the correction for 

 " ageing " corresponding to 100 readings was a little less 

 than the change of velocity just detectable by the in- 

 strument. 



The life of the anemometer-wires depends to a large ex- 

 tent on conditions of service. In the measurement of steep 

 gradients of velocity the risk of burning out is considerable y 

 and during the course of the writer's experiments was the 

 cause of failure of most of the wires. It is seen from 

 Table II. that the life usually ranges from 500 to 1000 

 velocity observations. 



Table II. 









Life in num- 





Wire 



JS T o. 



Diameter. 



Maximum 



velocity. 



ber of velocity 

 determina- 

 tions. 



Calibration formulae 

 for Wire 17. 



7 



3 mil 



2930 cm ./sec. 



500 



(1) V=(/ 2 - 0-5G4) 2 X 276-4 



(2) V =(i-- 0-545)- X 286-3 



8 



o 



2500 



540 



(3)V=(/ 2 -0 535) 3 x236-3 



14 



2-5 



1100 



900 



Formulas (1) and (2) were 

 separated by 1060 obser- 



16 



2-5 



1000 



700 



vations and sbow the 

 effect of ageing. Formula 



17 



i 



25 



1150 



1700 



(3) was derived from ob- 

 servations immediately 

 following (2), and shows 

 the agreement obtained 

 by calculating the con- 

 stants from the line of 

 closest fit. 



For purposes of continuous recording it is necessary to 

 employ a wire at a somewhat lower temperature : the result- 

 ing error due to changes of atmospheric temperature can be 

 eliminated by the use of the compensating ratio-coils already 

 discussed. In this way the life of the wires should be 

 considerably lengthened and the ageing diminished ; the 

 margin of galvanometer-sensitivity is ample to meet the 

 resulting diminution of sensitiveness. The writer has found 

 in many cases that the use of a wire at a dull red tempera- 

 ture in allowing conditions of flow to be readily judged by 

 inspection, is an advantage which compensates in large 

 measure the disadvantage of being obliged to recalibrate the 

 wire at intervals to determine the correction due to ageing. 



