The escape wheel, 55/100" in diameter and recessed so as to be
as light as possible, should have sufficient strength to perform its
functions properly. The thickness or, more properly, the face extent of
the tooth, measured in the direction of the axis of the escape wheel,
should be about 1/20". The recessing should extend half way up the
radial back of the tooth at _t_. The curvature of the back of the teeth
is produced with the same radii as the impulse roller. To locate the
center from which the arc which defines the back of the teeth is swept,
we halve the space between the teeth _A^2_ and _a^4_ and establish
the point _n_, Fig. 141, and with our dividers set to sweep the circle
representing the impulse roller, we sweep an arc passing the point of
the tooth _A^3_ and _u_, thus locating the center _w_. From the center
_k_ of the escape wheel we sweep a complete circle, a portion of which
is represented by the arc _w v_. For delineating other teeth we set one
leg of our dividers to agree with the point of the tooth and the other
leg on the circle _w v_ and produce an arc like _z u_.
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