Therefore Gv 
ee = 2! | EE 
/ 
tu (+t) = ae le 
Examination of this result shows that the response time is given by 
Ta encad \ peaR ee AON: 
C= Cy 2 (23) 
Thus the response time of the current meter is not a constant but is 
inversely proportional to the mean water velocity. The quantity defined by 
nae rey ir (24) 
is however a constant for the current meter and is referred to as the 
response distance, 
The response distance in air is considerably larger than in water 
and consequently more easily measured. The value obtained can be converted 
to what it should be if it were measured in water. The procedure is similar 
to that used in calibrating ocean current meters in the wind tunnel (23). 
The dimensions of each term in equation (17) are ML tT; and since the 
dimensions of ( and u are T7+ and tee respectively, the dimensions of 
the constant cy are ML. Constant rm) is necessarily of the form 
es A B Cc 
co = 2 P WA ih (25) 
