Sound emitted by Wires of Circular Section. 173 



may be wanted. This can be overcome by so weighting the 

 apparatus that the position of the axis is outside the range of 

 probable barometric heights. 



For accurate work the adjustment will be in error i£ the 

 mercury rises or falls below the level for which the adjust- 

 ment has been calculated, not only on account of the change 

 in the position of the mercury within the tube, but also by 

 reason of the motion of the sliding sight, which is made to 

 follow 7 the mercury in the operation of reading the barometer. 

 It should be possible to arrange a second sliding w r eight, which 

 should, be fixed at predetermined positions on a scale for given 

 barometric heights, and which would permit the relationship 

 I — 2d to be fulfilled on all occasions. 



We desire to express our thanks to the Department of 

 Scientific and Industrial Research for a grant which has 

 made this investigation possible. 



University College, Reading. 

 April 1921. 



XVII. On the Sound emitted by Wires of Circular Section wlien 

 exposed to an Air-Current. By E. F. Relf, A.R.C.Sc* 



THE object of the experiments described below was to 

 investigate the cause of the musical note which is heard 

 when a wire of circular section is moved through the air 

 at considerable speed. It was suspected that the frequency 

 of the note was that of the periodic eddy formation behind 

 the wire, and that the note was consequently independent 

 of the tension in the wire and of its elastic constants. 



Experiments were made in a small water channel of cross- 

 section 5 inches by 3 inches, in which steady speeds up to a 

 little over an inch per second could be obtained. Cylinders 

 of various diameters were introduced into the stream, and 

 the eddy period was measured by a very simple and effective 

 device. A small piece of paper (about 3/16 inch square) 

 was hung behind the cylinder by means of a fine wire bent 

 into a hook at its upper end, and resting on a strip of metal 

 placed across the water channel as shown in fig. 1. The 

 piece of paper was thus free to swing as a pendulum, and 

 also to rotate through a considerable angle about a vertical 

 axis. It actually performed a combination of the two move- 

 ments, and it was found very easy to obtain the eddy frequency 

 by observing the periodic motion of the paper. By intro- 

 ducing red ink into the water channel, and so rendering the 

 * Communicated by the Superintendent of the Aerodynamics De- 

 partment, National Physical Laboratory. 



