282 Lord Rayleigh : Acoustical Notes, 



as that of the fork. But if the fork be so situated that the 

 motion of the point of attachment is along the length of the 

 string, the vibrations are of an entirely different character, 

 and are executed in a period the double of that of the fork. 

 The theory of vibrations of this class was discussed in a paper 

 on Maintained Vibrations * published many years ago, refer- 

 ence to which must here suffice. 



A convenient device for demonstrating the relationship of 

 periods is to illuminate the string by sparks synchronous with 

 the vibrations of the fork itself. For this purpose an induc- 

 tion-coil is included in the circuit by which the fork is driven, 

 so that every break at the fork causes a spark between the 

 secondary terminals, to which a small jar is connected in the 

 usual manner. If then the vibrations of the string be iso- 

 chronous with the fork, and therefore with the sparks, the 

 intermittent illumination exhibits what is ordinarily seen as a 

 gauzy spindle resolved into the appearance corresponding to 

 a single phase of the vibration ; that is, the string is seen appa- 

 rently fixed (in a displaced position) and single. But if, as 

 when the point of attachment moves parallel to the length of 

 the string, the vibrations are only half as fast as tbose of the 

 fork, the string is found in two (opposite) phases at the 

 moments of illumination, and is consequently seen double. 

 The effect is improved by a piece of ground glass, which may 

 be held either between the sparks and the string, or between 

 the string and the eye. In the latter case it is a shadow that 

 is seen. It is desirable to retain enough continuous light to 

 allow the form of the gauzy spindle to remain visible. In this 

 way the difference between the two kinds of vibration may be 

 exhibited to many persons at once. 



A detail of some importance relates to the use of the 

 condenser, associated as usual with the primary circuit of the 

 coil. If its poles be connected simply with the outer terminals 

 of the fork apparatus regarded as an interrupter, the secondary 

 sparks will be inferior or may fail altogether. The expla- 

 nation is to be sought in the self-induction of the magnet 

 associated with the fork, which apparently interferes with the 

 suddenness of the break. The poles of the condenser should 

 be connected as directly as possible with the two pieces of 

 metal between which the break takes place. In the apparatus 

 at the Royal Institution it makes all the difference on which 

 side of the small electromagnet the pole of the condenser is 

 attached. 



* Phil. Mao-, vol. xv. p. 229 (1883); 'Scientific Papers/ vol. ii. 

 p. 188. 



