SOS Lord Rayleigh on the Relation of the 



necessary for audibility at various frequencies, support his 

 view of the question. Certainly this is their tendency. At 

 the time when these observations were made the whole 

 modus operandi of the telephone was still involved in doubt, 

 and my object in these observations was rather to elucidate 

 the action of this instrument. Even now there are points 

 which remain obscure, for example the easy audibility of 

 sounds when the iron disk is replaced by one of copper or 

 aluminium. It is to be presumed that the movements of the 

 disk then depend upon electric currents induced in it. If 

 so, they would follow different laws from those governing 

 the behaviour of a simply magnetised disk ; and in the case 

 of iron complications would ensue from the cooperation of 

 both causes. 



Again, though this is partly a matter of definition, I am of 

 opinion that the sensitiveness of the ear is best investigated 

 with the ear free. When a telephone, pressed closely up, is 

 employed, the situation is materially altered. For example, 

 the natural resonances of the ear-passage must be seriously 

 disturbed. 



The above objections do not apply to some of Wien's 

 determinations, where the ear was placed at a .distance from 

 the telephone and the vibrations of the plate were directly 

 measured ; and his conclusions must necessarily carry great 

 weight. But I could not forget that my own experiments 

 in 1894 had been carefully made, and I was desirous, if pos- 

 sible, of checking the results by some new method different 

 from those previously employed either by Wien or myself. 

 Tile difficulties of the problem are considerable ; but it 

 occurred to me that, so far as the important question of the 

 dependence of sensitiveness upon pitch is concerned, they 

 might be turned by calling to our aid the general principle 

 of dynamical similarity *. Thus if vibrations are com- 

 municated to the air from the prongs of a tuning-fork, we 

 are unable to calculate the theoretical connexion between the 

 invisible aerial vibrations and the visible amplitude of the 

 prongs. But if there are two precisely similar forks of 

 different dimensions, and each communicating vibrations, a 

 comparison may be effected. In the first place, if the material 

 is the same, the times of vibration of the forks (regarded 

 as uninfluenced by the air) are as the linear dimensions. 

 And further, what is more important for our purpose, the 



* The application of this principle to acoustical problems is discussed 

 in 'Theory of Sound;' 2nd ed. § 381. 



