The jewels all have to be set by hand after they are made, and the distance to which the impulse jewel protrudes beyond the periphery of the impulse roller is entirely a matter for hand and eye, but should never exceed 2/1000”. After the locking jewel c is set, we can set the foot F of the detent D forward or back, to perfect and correct the engagement of the escape-wheel teeth with the impulse roller B. If we set this too far forward, the tooth A^3 will encounter the roller while the tooth A^2 will be free.
We would beg to say here there is no escape wheel made which requires the same extreme accuracy as the chronometer, as the tooth spaces and the equal radial extent of each tooth should be only limited by our powers toward perfection. It is usual to give the detent a locking of about two degrees; that is, it requires about two degrees to open it, counting the center of fluxion of the detent spring E and five degrees of balance arc.
FITTING UP OF THE FOOT.
Several attempts have been made by chronometer makers to have the foot F adjustable; that is, so it could be moved back and forth with a screw, but we have never known of anything satisfactory being accomplished in this direction. About the best way of fitting up the foot F seems to be to provide it with two soft iron steady pins (shown at j) with corresponding holes in the chariot, said holes being conically enlarged so they (the pins) can be bent and manipulated so the detent not only stands in the proper position as regards the escape wheel, but also to give the detent spring E the proper elastic force to return in time to afford a secure locking to the arresting tooth of the escape wheel after an impulse has been given.
If these pins j are bent properly by the adjuster, whoever afterwards cleans the chronometer needs only to gently push the foot F forward so as to cause the pins j to take the correct positions as determined by the adjuster and set the screw l up to hold the foot F when all the other relations are as they should be, except such as we can control by the screw G, which prevents the locking jewel from entering too deeply into the escape wheel.
In addition to being a complete master of the technical part of his business, it is also desirable that the up-to-date workman should be familiar with the subject from a historical point of view. To aid in such an understanding of the matter we have translated from “L’Almanach de l’Horologerie et de la Bijouterie” the matter contained in the following chapter.
HISTORY OF ESCAPEMENTS.