he fork has
been moved in the direction of the arrow by the ruby pin; the escapement
has been unlocked and the opposite side of the slot has just struck the
ruby pin. The exact position in which the impulse is transmitted varies
with the locking angle, the width of ruby pin, its shake in the slot,
the length of fork, its weight, and the velocity of the ruby pin, which
is determined by the vibrations of the balance and the impulse radius.

In an escapement with a total lock of 1 3/4deg. and 1 1/4 of shake in
the slot, theoretically, the impulse would be transmitted 2deg. from the
bankings. The narrow ruby pin n receives the impulse on the line _v_,
which is closer to the line of centers than the line _u_, on which the
large ruby pin receives the impulse. Here then we have an advantage of
the narrow ruby pin over a wide one; with a wider ruby pin the balance
is also more liable to rebank when it takes a long vibration. Also on
account of the greater angle at which the ruby pin stands to the slot
when the impulse takes place, the _drop_ of the fork against the jewel
will amount to more than its shake in the slot (which is measured when
standing on the line of centers). On this account some watches have
slots dovetailed in form, being wider at the bottom, others have ruby
pins of this form. They require very exact execution; we think we can do
without them by judiciously selecting a width of ruby pin between the
two extremes. We would choose a ruby pin of a width equal to half the
angular motion of the fork. There is an ingenious arrangement of fork
and roller which aims to, and partially does, overcome the difficulty of
choosing between a wide and narrow ruby pin, it is known as the Savage
pin roller escapement. We intend to describe it later.

If the face of the ruby pin were planted on the theoretical impulse
radius _ii_, Fig. 19, the impulse would end in a butting action as
shown; hence the great importance of distinguishing between the
theoretical and real impulse radius and establishing a reliable data
from which to work. We feel that these actions have never been properly
and thoroughly treated in simple language; we have tried to make them
plain so that anyone can comprehend them with a little study.

Three good forms of ruby pins are the triangular, the oval and the flat
faced; for ordinary work the latter is as good as any, but for fine work
the triangular pin with the corners slightly rounded off is preferable.