Fig. 22.
English watches are met with having a cylindrical or round ruby pin. Such a pin should never be put into a watch. The law of the parallelogram of forces is completely ignored by using such a pin; the friction during the unlocking and impulse actions is too severe, as it is, without the addition of so unmechanical an arrangement. [Fig. 21] illustrates the action of a round ruby pin;
ii
is the path of the ruby pin;
cc
that of the acting length of the fork. It is shown at the moment the impulse is transmitted. It will be seen that the impact takes place below the center of the ruby pin, whereas it should take place at the center, as the motion of the fork is upwards and that of the ruby pin downwards until the line of the centers has been reached. The same rule applies to the flat-faced pin and it is important that the right quantity be ground off. We find that 3⁄7 is approximately the amount which should be ground away. [Fig. 22] illustrates the fork standing against the bank. The ruby pin touches the side of the slot but has not as yet begun to act;
ri
is the real impulse circle for which we allow 1¼° of freedom at the acting edge of the fork; the face of the ruby pin is therefore on this line. The next thing to do is to find the center of the pin. From the side
n
of the slot we construct the right angle