To resume the explanation of why it is desirable to make the staff and
all parts near the axis of the balance as light as possible, we would
say it is the moving portion of the balance which controls the
regularity of the intervals of vibration. To illustrate, suppose we have
a balance only 3/8" in diameter, but of the same weight as one in an
ordinary eighteen-size movement. We can readily see that such a balance
would require but a very light hairspring to cause it to give the usual
18,000 vibrations to the hour. We can also understand, after a little
thought, that such a balance would exert as much breaking force on its
pivots as a balance of the same weight, but 3/4" in diameter acting
against a very much stronger hairspring. There is another factor in the
balance problem which deserves our attention, which factor is
atmospheric resistance. This increases rapidly in proportion to the
velocity.
HOW BAROMETRIC PRESSURE AFFECTS A WATCH.
The most careful investigators in horological mechanics have decided
that a balance much above 75/100" in diameter, making 18,000 vibrations
per hour, is not desirable, because of the varying atmospheric
disturbances as indicated by barometric pressure.
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