Wires of Circular Section exposed to Air. 175 



varied in frequency between 1000 and 4000 per sec. at the 

 speeds used, and were thus well above the hum of the motor 

 and easily distinguishable. The exact frequency was ob- 

 tained by comparison with a standard tuning-fork of fre- 

 quency 500, the method being to "tune" the wire by altering 

 the motor speed until the note heard made a convenient 

 interval with the fork, usually a complete number of octaves 

 or a number of octaves plus a perfect fifth. The motor 

 speed was then observed, giving simultaneous observations of 

 frequency and velocity. 



From dimensional theory applied to the fluid flow round 

 geometrically similar bodies, it can be shown that the eddy 

 frequency is given by the equation 





second, V the 

 the kinematic 



where n is the number of eddies formed per 

 velocity, D the diameter of the wire, and v 

 viscosity of the fluid, the form of the function / beino- 



unknown. The non-dimensional quantity ==- was accordingly 



VD v 



plotted as ordinate against as abscissa. The resulting 



curve is given in fig. 3, where points indicated by a cross 



Fie-. 3. 



0-20 



015 



0-10 



0-05 



t 



x H/stcr Experiments 

 o Air Experiments. 



500 



1000 



1500 



2000 



3000 



V D 



-y 



represent experiments in water, while those denoted by a 

 circle indicate air experiments. It is seen that all the points 

 fall on a single curve within the limits of experimental 

 accuracy, and it is therefore evident that at any given value 



3500 



