That there are advantages in so shaping the incline or
impulse face is conceded, we believe, by all recent manufacturers. The
chief benefit derived from such curved impulse faces will be evident
after a little thought and study of the situation and relation of parts
as shown in Fig. 129. It will be seen on inspection that the angular
motion imparted to the cylinder by the impulse face of the tooth when
curved as shown, is greater during the first half of the twelve degrees
of escape-wheel action than during the last half, thus giving the escape
wheel the advantage at the time the balance spring increases its
resistance to the passage of the escape-wheel tooth across the lip of
the cylinder. Or, in other words, as the ratio of resistance of the
balance spring increases, in a like ratio the curved form of the impulse
face of the tooth gives greater power to the escape-wheel action in
proportion to the angular motion of the escape wheel. Hence, in actual
service it is found that cylinder watches with curved impulse planes to
the escape-wheel teeth are less liable to set in the pocket than the
teeth having straight impulse faces.
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