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Action of the Escapement

In Fig. 61 the tooth c is at rest upon the locking stone d. The dotted circle e represents the unlocking roller which is mounted upon the balance staff beneath the impulse roller k. The unlocking spring g is rigidly held at the back end by a screw, while the end next to the balance is free to move towards the escape wheel, but when pressed in the opposite direction it will carry the detent with it by striking against the horn of the detent h, thus accomplishing the unlocking of the escape wheel for the succeeding impulse. Upon the detent arbor there is mounted a small spiral spring, not shown, of the same form as the balance spring only very much smaller and having four or five coils for the purpose of returning the detent to the locking position. This spring is mounted by means of a collet and is held by a stud similar to the balance spring. As shown the unlocking pallet is in position ready to unlock the escape wheel, thus allowing the tooth i to fall upon the impulse pallet j, which imparts the impulse to the balance through the impulse roller k, which is mounted upon the balance staff in a similar manner to that of the roller in a lever escapement. Having received an impulse the balance continues its excursion around to the left, until the momentum of the balance has been absorbed by the balance spring. The balance is immediately started upon its return vibration by the tension of the balance spring produced by the impulse and the momentum of the balance. The unlocking pallet in returning strikes upon the back of the unlocking spring, but as it is free to bend in this direction no other action takes place. When the balance returns again the unlocking pallet strikes against the unlocking spring, which is resting against the horn of the detent, thus unlocking the escape wheel, ready to deliver another impulse. There is a banking screw in the marine chronometer, shown in Fig. 63, against which the detent strikes and the amount of lock is adjusted. This screw is usually located about the center of the locking stone, but in all cases it should be located at the point where the detent will come to rest with the least jar or trembling, which would be very detrimental to accurate timing. The escape wheel makes a revolution in the same time as that of the lever escapement, because each tooth delivers but one impulse, and the entire space between two teeth passes every time the detent is unlocked, while in the lever each tooth gives two impulses. While the balance in this escapement is more nearly free from errors of the train which makes it so accurate as a timepiece, it is nevertheless at the same time very sensitive to external shocks, and for this reason is not suitable for carrying in the pocket except in rare cases when carried by a careful person. Another reason why it is not suitable for ordinary wear is its liability to set, that is the motion of the balance is counteracted to a greater or less extent, which may be sufficient to entirely stop the watch, and as this escapement has no power to start itself, even in the highest grades, the watch would stand still until again started. The escapement shown in Fig. 61 is taken from one of Jules Jurgensen's pocket chronometers, and in the drawing is shown enlarged ten times. These measurements were very carefully taken and transferred to the drawing, so that it may act as a reliable guide to those who may wish to construct one.