In considering the action and relation of the parts of the fork and
roller, we will first define what is considered necessary to constitute
a good, sound construction where the fork vibrates through ten degrees
of angular motion and is supposed to be engaged with the roller by means
of the jewel pin for thirty degrees of angular motion of the balance.
There is no special reason why thirty degrees of roller action should be
employed, except that experience in practical construction has come to
admit this as about the right arc for watches of ordinary good, sound
construction. Manufacturers have made departures from this standard, but
in almost every instance have finally come back to pretty near these
proportions. In deciding on the length of fork and size of roller, we
first decide on the distance apart at which to place the center of the
balance and the center of the pallet staff. These two points
established, we have the length of the fork and diameter of the roller
defined at once.
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