HANDBOOK OF ACOUSTICS - online book

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

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ON THE PITCH OF MUSICAL SOUNDS.            37
complete vibration of the fork. A more convenient arrangement is to fasten the lamp-blacked paper round a rotating cylinder, which is also made to travel slowly from right to left by means of a screw on the axis, so as to prevent the tracing from overlapping itself. If this cylinder be kept lotating for a certain number of seconds, the number of sinuosities traced in that time can be counted; we have then only to divide this number by the number of seconds, to obtain the vibration number of the fork.
This method has been brought to a very high degree of perfection by Professor Mayer, of New Jersey. He uses lamp-blacked paper, wrapped round a rotating metallic cylinder or drum, as above described. The wave curve is traced on this by an aluminium style, attached to the end of one of the prongs of the tuning fork under examination. A pendulum (a, fig. 25), beating seconds, has a platinum wedge fastened to it below, which, at every swing, makes contact with a small basin containing mercury. This basin is in communication with one pole of a battery (b), a wire from the other pole being attached to the primary coil (p) of an inductorium, from which again a wire proceeds to the top of the pendulum. The wire from one end of the secondary coil (s) is attached to the stem of the tuning fork (t), and that from the other end to the revolving cylinder (d). "When the apparatus is at work, it is obvious that a spark will pierce the smoked paper at every contact of the pen­dulum with the mercury, leaving a minute perforation. The number of complete sinuosities between two consecutive perforations, will consequently be the vibration number of the fork.
A new instrument for counting rapid vibrations, called by its in­ventors, the Cycloscope, was de­vised a few years ago by McLeod and Clarke. A diagrammatic re­presentation of the essential parts of the apparatus is shown in fig. 26. A is a drum, rotating on a horizontal axis, and capable of being revolved at any definite rate.