THE ELECTRIC TELEGRAPH. 



approach the latter very closely. Absolute contact is to be 

 avoided, because it is found that in that case the arm adheres to 

 the magnet with a certain force after the current ceases to flow, 

 but so long as absolute contact is prevented, it is immediately 

 brought back by the spring, s, when the current is suspended. 



148. It is evident, therefore, that the limit of the possible 

 celerity of vibration to be imparted to the lever, 'o g, by the pulsa- 

 tions of the current will depend on the nice adjustment of the 

 weight and play of the lever, and the force of the spring, s. 



The velocity of oscillation, however, which can in this way be 

 imparted to the lever, is such as can scarcely be credited with- 

 out actually witnessing its effects. When that velocity does not 

 exceed a certain limit the oscillations may be registered and 

 counted, by causing the lever to give motion to the anchor of an 

 escapement, connected with a train of wheel- work, by which a 

 hand or index, moving on a graduated dial, is governed. But 

 these oscillations are susceptible of velocities so great that it 

 would be difficult to apply this expedient for counting them. M. 

 Gustave Froment, of Paris, suggested and applied to this purpose 

 with complete success, a method of ascertaining the velocity 

 depending on the laws which govern the vibrations of musical 

 strings. 



149. It is well known that the pitch of any musical note is the 

 consequence of the rate of vibration of the string by which it is 

 produced, and that the more rapid the vibration the higher the 

 note will be in the musical scale, and the slower the vibration the 

 lower it will be. Thus the string of a pianoforte which produces 



the bass note "- t vibrates 132 times in a second, that 

 which produces the note = vibrates 66 times in a second, 



and that which produces the note BJ : vibrates 264 



t) ^&^ 



times per second. 



On a seven octave pianoforte the highest note in the treble is 



three octaves above ph nr , and the lowest note in the bass is 



four octaves below it. The number of complete vibrations corre- 

 sponding to the former must be 3520 ; and the number of vibra- 

 tions per second corresponding to the latter is 27. 



If, therefore, the lever, o g y have any rate of vibration more 

 rapid than 27| vibrations per second, and less rapid than 3520 per 

 200 



