510 WEBER OI'.' THE MEASUREMENT 



a musical sound, when one portion of the conducting wire, form- 

 ing as it A\ ere the inducing coil, surrounds the free vibrating end 

 of the bar, so that the direction of the vibration is at right angles 

 to the plane of the coils of the wire. All vibrations of the bar on 

 one side then produce positive currents in the wire, and all the 

 vibrations on the other side produce negative currents, which fol- 

 low each other as rapidly as the sonorous vibrations themselves. 



When the ends of the wire of the inducing coil are united to 

 the ends of that of the dynamometer, a deflection of the latter 

 during the vibration of the bar is observed, which can be ac- 

 curately measured. This deflection remains unaltered so long 

 as the intensity of the sonorous vibrations remains unaltered, 

 but speedily diminishes when the intensity of the sonorous 

 vibrations diminishes ; and when the amplitude of the sonorous 

 vibrations has fallen to a half, it then amounts to the fourth part 

 only. 



The dynamometer thus presents a means of estimating the 

 intensity of sonorous vibrations, which is of importance, be- 

 cause methods adapted to these measurements are still much re- 

 quired. 



In addition to the investigations which we have hitherto con- 

 sidered, and which are based on the use of the dynamometer, 

 there are others which will be subsequently treated of, when some 

 modifications in the construction of this instrument for special 

 objects will also be more accurately detailed. 



ox the connexion of the fundamental principle of 

 Electro-dynamics with that of Electro-statics. 



The fundamental principle of electro-statics is, that when two 

 electric (positive or negative) masses, denoted by e and e', are at 

 a distance r from each other, the amount of the force with which 

 the two masses act reciprocally upon each other is expressed by 



rr' 



and that repulsion or attraction occurs accordingly as this ex- 

 pression has a positive or negative value. 



On the other hand, the fundamental principle of electro-dyna- 

 mics is as follows : — When two elements of a current, the lengths 

 of which are « and «', and the intensities i and i', and which are 

 at the distance r from each other, so that the directions in which 



