280 



Lord Rayleigh's Acoustical Observations. 



the two new ones differing twice as much as before from 

 that of the primary sound. 



Other cases might be treated in the same way ; but my object 

 at present is to describe a modified form of the experiment 

 which I planned some years ago, but first carried out last 

 summer. Fig. 1. 





The desired forces were obtained electromagnetically. A 

 fork interrupter of frequency 128 gave a periodic current, by 

 the passage of which through an electromagnet a second fork of 

 like pitch could be excited. The action of this current on 

 the second fork could be rendered intermittent by short-cir- 

 cuiting the electromagnet. This was effected by another in- 

 terrupter of frequency 4, worked by an independent current 

 from a Smee cell. To excite the main current a Grove cell 

 was employed. When the contact of the second interrupter 

 was permanently broken, so that the main current passed con- 

 tinuously through the electromagnet, the fork was, of course, 

 most powerfully affected when tuned to 128. Scarcely any 

 response was observable when the pitch was changed to 124 

 or 132. But if the second interrupter was allowed to operate, 

 so as to render the periodic current through the electromagnet 

 intermittent, then the fork would respond powerfully when 

 tuned to 124 or 132 as well as when tuned to 128, but not 

 when tuned to intermediate pitches, such as 126 or 130. 



The operation of the intermittence in producing a sensitive- 

 ness which would not otherwise exist, is easily understood. 

 When a fork of frequency 124 starts from rest under the in- 

 fluence of a force of frequency 128, the impulses cooperate at 

 first, but after ^ of a second the new impulses begin to oppose 

 the earlier ones. After J of a second, another series of im- 

 pulses begins whose effect agrees with that of the first, and so 

 Thus if all these impulses are allowed to act, the resul- 



on 



tant effect is trifling ; but if every alternate series is stopped 

 off, a large vibration accumulates. 



A new form of Siren. 

 Some years ago*, I observed that a light pivoted blade is 

 * " On the Resistance of Fluids," Phil. Mag. Dec. 1876. 



