322 Lord Kayleigk : Acoustical Jfotes. 



The performance of this instrument is very satisfactory. 

 The seventh and eighth components are a little weak, but the 

 others, and especially the twelfth and higher third, are loudly 

 heard. The experimenter should bear in mind that when 

 working in-doors much depends upon the precise position. 

 The room is intersected with nodes and loops of approximately 

 stationary vibrations, whose position varies from one tone to 

 another. If a particular harmonic is ill heard, it may only 

 be that the situation of the resonator is unfavourable. A 

 motion of a few inches will often make a great difference. 

 Usually the effects are best when the resonator is held pretty 

 close to the reed in action. 



In general a harmonium note is the most convenient for 

 experiment and demonstration, but other instruments are of 

 course available. A man's voice singing the proper note 

 (B as above) gives excellent results. 



Tuning-Forks with slight Mutual Influence. 



Two forks giving 128 vibrations per second are inde- 

 pendently maintained, each making and breaking its own 

 contacts at a mercury cup. If mutual influence be altogether 

 excluded, the " beat " may be made as slow as we please. 

 But although the electric circuits may be entirely distinct, 

 if the forks stand on the same table there may be enough 

 mutual influence to bring about absolute unison. The best 

 method of observation is by Lissajous' figures. The per- 

 manence of the ellipse is a sign that there is mutual control 

 and that absolute unison is established ; otherwise the ellipse 

 undergoes more or less slowly the usual transformations. 



A series of observations on this subject were made in 1901. 

 Mutual influence may arise from both forks being connected 

 with the same battery. If the electric circuits are in a series 

 which includes two Grove cells, the forks keep together inde- 

 finitely; but this arrangement is rather akin to the familiar 

 one in which a single interrupter-fork drives another, the 

 second having no break of its own. Even when the fork 

 circuits are in parallel and are fed from two Grove cells*, 

 there is, or may be, sufficient reaction to maintain absolute 

 unison. The feebler the reaction, the more nearly must the 

 natural frequencies approach to identity. When the reaction 

 is just insufficient for control, it is interesting to watch the 

 cycle of the beat, as revealed by Lissajous' figure. At one 

 part of the cycle the changes are very slow and at the 

 opposite part relatively very quick. 



In another set of experiments the electric circuits were 

 quite distinct, each fork being driven by a separate Grove 

 * The internal resistance of the cells comes into play here. 



