The duplex escapement is a watch escapement named after its escape wheel, which carries two separate sets of teeth. It kept much better time than the cylinder escapement, though it was harder to make. Fine English pocket watches used it from roughly 1790 until 1860.
How does a duplex escapement work?
The escape wheel has two rows of teeth. Long, pointed locking teeth, also called resting teeth, stick out from its side. Short impulse teeth, trapezoid in shape, point upward from its top face. The balance staff carries a roller with a notch cut into it. While the balance swings, a resting tooth bears against this roller. When the notch comes round, the tooth drops into it and is released. The wheel then turns, and an impulse tooth gives the balance a push. That push comes in one direction of the swing only. On the return swing the balance gets no impulse.
What are its strengths and weaknesses?
It has less friction than either the cylinder escapement or the verge escapement. That suited it to the precise pocket watches made before the Swiss lever escapement arrived. Its weaknesses are practical ones. Like the chronometer escapement, it cannot start itself. A watch with a duplex can also stop if it is jolted.
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