Lord Rayleigh : Acoustical Notes. 281 



of revolution being horizontal, the deflecting action is then 

 greatest when the revolving magnet points towards the sus- 

 pended magnet. In one of these positions, say when the spot 

 is deflected to the right, a bell rings automatically. Uniform 

 rotation at any desired speed is maintained by hand with the 

 aid of gearing, diminishing the speed in the ratio of 5 : 1, and 

 of a metronome set as required. 



To illustrate propositions (1) and (2) the long magnet is 

 caused to rotate with a frequency of 8 per minute, i. e. with a fre- 

 quency somewhat less than that natural to the suspended system. 

 At first the phenomenon is complicated by the interaction of 

 natural and forced vibrations ; but the former soon die aw T ay. 

 It is then recognized that the vibrations observed upon the 

 screen are isochronous with the revolution of the magnet, 

 and that the bell rings at the moment when the spot of 

 light attains its greatest elongation towards the right. 



In the next experiment the speed of revolution is altered 

 to 12 per minute, so as to bring about the condition of things 

 contemplated in (3). After a little interval of settling down 

 the bell rings always at the moment when the spot is most 

 deflected to the left, showing that the phase has been altered 

 by half a period. 



To illustrate (4) the speed of revolution may now be 

 adjusted to 8 per minute. The arc of vibration is seen gra- 

 dually to increase until it reaches a large value, the bell now 

 ringing, not at either extreme elongation, but as the spot 

 passes from left to right through its position of equilibrium. 



Vibrations of Strings. 



At the Royal Institution it is usual to illustrate this subject 

 by experiments after the method of Melde and Tyndall. 

 The string is connected with a large tuning-fork, whose prongs 

 stand vertically, and the vibrations are maintained electrically 

 in the well-known manner. The electric contact is between 

 solids (of platinum), one attached to the prong, the other 

 forming the point of an adjustable screw carried by the 

 framework. 



The string, 10 feet long, is stretched horizontally and the 

 tension is adjusted until a vigorous vibration ensues, which 

 happens when one of the modes of vibration has a period in 

 simple relation to that of the fork. There is here an im- 

 portant distinction according as the length of the string is 

 parallel or perpendicular to the motion of the point of 

 attachment. In the latter case the vibrations are of the cha- 

 racter commonly classified as forced, and the period is the same 



