Cup Anemometer 



The positive sign before the root is significant. 



l 3 



Thus if h and k are both plotted vs. d/D the points of intersection of the h and k curves for 

 corresponding cup sizes will give the proportion of d/D for that cup size which will permit the 

 instrument to operate according to the law V—V i =V li (cf. Fig. 7). 



Thus it is seen that by choosing the particular arm length given by this ratio a great simpli- 

 fication is introduced into the anemometer law whereby the simple addition of a constant to the 

 indicated velocity gives the true wind velocity. 



Fig. 8 



The error curves shown in Figure 8 are taken for the three conditions of h<k, h = k and h>k. 



As the errors are obtained by difference of two large quantities, extreme points for h<k and h>k 



are chosen. 



h > k is illustrated by the anemometer d/D = .89 



for d = .03 1 

 h < k is illustrated by the anemometer d/D = .1 



for d = . 1 27 

 h = k is illustrated by the anemometer d/D = . 1 



for d = .031. 



The theoretical error curves are obtained by employing the values of h, k and V§ for the par- 

 ticular anemometer and computing V for various assigned values of h-v. The experimental points 

 are obtained from the observed values of V and v. (Table IV.) 



