Lord Rayleigb : Acoustical Notes. 31? 



method, and I now propose to give a brief account of the 

 results, which were not available in time for incorporation in 

 the paper just cited. 



The object of the experimental arrangements is the separate 

 presentation to the two ears of pure tones, practically in 

 unison, in such a manner as to allow the effect of a variation 

 in the phase-relationship to be appreciated. When the 

 sounds proceed from tuning-forks vibrating independently, 

 the phase-difference passes cyclically through all degrees. 

 and if the beat be slow enough, there is good opportunity for 

 observation. But it is not possible to stop anywhere, nor in 

 some uses of the method to bring into juxtaposition phase- 

 relationships which differ finitely. I thought that it would 

 be of interest to observe under conditions which would allow 

 any particular phase-relation to be maintained at pleasure, 

 and to this the revolving magnet method naturally lends itself. 



The propulsion is by means of wind (under about two 

 inches water pressure) from a well regulated bellows. The 

 blade forming the magnet mav be bent wind-mill fashion and 

 receive the wind directly, but in the present experiments it 

 was combined with a diminutive turbine, the whole revolving 

 about a vertical axis. The sj:>eed was about 190 per second, 

 giving in the telephones a note of pitch g. Two inductor- 

 coils'* were used, the circuit of each being completed through 

 a telephone. The planes of the coils were vertical, their 

 centres being at the same level as the magnet. One was 

 fixed and the other was so mounted that it could revolve 

 about an axis coincident with that of the magnet and turbine. 

 The angle between the planes represents of course the phase- 

 difference of the periodic electromotive forces, subject it may 

 be to an ambiguity of half a period, dependent on the way 

 the connexions are made. If the circuits are similar, as 

 is believed, the phase-difference of the currents and of the 

 electromotive forces is the same. The telephone-discs were 

 loaded, but not so heavily as to bring them into tune with the 

 sounds employed. The circuit of one telephone included a 

 commutator bv which the current through the instrument 

 could be reversed, corresponding to a phase-change of 180°. 



In commencing observations the first step is to adjust to 

 equality the sounds heard from the two telephones. This 

 can be effected bv varying the distances between the magnet 

 and the inductor-coils. The telephones are then brought into 

 simultaneous action at the two ears, and the effect is observed. 



* They were also employed in the 1894 experiments and are there 

 spoken of as " wooden coils " from the fact that the wire is wound upon 

 wood. 



