^18 Lord Rayleigh : Acoustical Notes. 



A rotation o£ the movable coil may then be made, or the 

 current in one telephone may be reversed by means of the 

 commutator. The results were for the most part in harmony 

 with what had been expected from the experiments with 

 forks. But one anomaly must be noted, relating to the 

 neutral condition where no pronouncement can be made in 

 favour of either right or left. This should occur when the 

 phases of vibration at the ears are either the same or precisely 

 opposed ; and it had been expected that the condition would 

 be realized when the planes of the inductor-coils were strictly 

 parallel. There is no difficulty in determining .the neutral 

 position, where neither right nor left has the advantage in 

 ■either state of the commutator ; but I was surprised to find 

 that according to my own judgement the neutral position 

 deviated very appreciably, perhaps 10° and one occasion 

 -even more, from that of parallelism. At first I supposed 

 that the explanation of the anomaly was to be sought in the 

 behaviour of the telephone plates, whose vibrations may not 

 have the same relation in the two cases to the electric currents 

 actuating them. It is possible that this cause of disturbance 

 may have been operative to some extent ; but that it was not 

 a complete account of the matter became evident later when 

 it was found that in Mr. Enock's judgement the neutral 

 position did coincide sensibly with parallelism of the coils. 

 There is no doubt at all but that the judgements of the two 

 observers really differed ; each repudiated the setting of the 

 •coil satisfactory to the other. 



In the judgement of the individual observer, the neutral 

 position can be determined with considerable precision by 

 the observed absence of lateral effect, in conformity with the 

 results of the tuning-fork experiments. In using the com- 

 mutator in order to ascertain that no change in respect of 

 lateral effect accompanies reversal, a complication arises from 

 the fact that during reversal one circuit is momentarily 

 broken. The telephones may be removed from the ears 

 during commutation. If this be not done, care must be 

 taken that the judgement made relates to the permanent effect, 

 or errors may ensue due to the momentary action of the 

 sound on one ear only. 



When the neutral position of the coil is departed from, a 

 lateral sensation — say to the right — is experienced, and this 

 increases until the displacement reaches 140°. A reversal at 

 the commutator changes the right into a left sensation, having 

 the same effect as a rotation of. the coil through 1 80°. 



When the adjustment is such that the combined lateral 

 sensation (to the right) is a maximum, it is interesting to 

 observe the effect of applying the telephones to the ears 



