# HANDBOOK OF ACOUSTICS - online book

### A complete view of Acoustical Science & its bearings on music, for musicians & music students.

 78 HAND-BOOK OF ACOUSTICS. fork will continue to vibrate. Thus, if the current be intermittent, and the number of interruptions per second be the same as the vibration number of the fork, the vibra­tion of the latter will be continuous. If the electro-magnet be powerful enough, the fork will also continue in motion, though the number of interruptions per second be &c, of the vibration number of the fork. These interruptions of the current can be brought about by another fork, the vibration number of which is either the same, or &c., of the first one. Let 0 (fig. 40) represent this second fork, and A., B the poles of an electro-magnet. To the upper prong a small wire is fastened which just dips into a little mercury con Fig. 40 tained in a cup D, when the fork is at rest. In this position the current from one end of the battery passes to the cup D, thence through the fork and the wire E to the electro-magnet A B, and then by wire F back to battery. But directly the current passes, the poles A and B attract the prongs, and thus the wire attached to the upper one is lifted out of the mercury in the cup D. The current is thus broken, A and B cease to attract, and the prongs return. But in so doing the wire again comes into contact with the mercury, the current is again set up, and A and B again attract the prongs. This alternate making and breaking of the circuit will thus be kept up, and the motion of the fork is rendered continuous. If now the current from B (fig. 40) instead of passing directly back to the battery, be first led through the electro-magnet of the