CHARACTERISTICS OF TONES
When a fork is driven by this method, the prong is inter¬
mittently urged forward by the magnetic pull. The prong
itself is always a very little behind the pull, that is, it lags
more or less ; this forcing of the vibration causes the period
to be slightly different from that of the same fork vibrating
freely.11
A fork retains its pitch with great constancy; ordinary
careless handling causes little change, and even rust, as it
slowly proceeds over a period of years, produces but slight
effect, rarely exceeding one vibration in two hundred and
fifty; the change usually flattens the pitch, since rust near
the yoke affects the fork more than that near the end of
the prong. The ordinary wear on a fork is usually greater
at the ends which are unprotected, and this causes the pitch
to sharpen; rust and wear, thenj in some degree produce
opposite effects and tend to maintain the original pitch.
An account of the tone quality of the tuning fork is given
in Lecture VI, while many illustrations of its usefulness will
be found throughout the lectures.
Determination of Pitch by the Method of Beats
A simple comparison by the ear will enable one who is
musically trained to tune certain intervals, such as unisons,
octaves, thirds, fourths, and fifths. Two tones nearly in
unison produce heats, the number of which per second is
equal to the difference in pitch (see page 183). Beats often
occur between the overtones of sounds which are not simple,
and under other conditions which need not be considered
here. Comparison by ear, based on the method of beats, is
the principal means employed in tuning pianos and organs
and such stringed instruments as the violin and the guitar.
The comparison of a standard tuning fork with an un-
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