To test the correctness of a watch depth, wedge the driven wheel tightly so that it cannot turn, then try the shake of the wheel teeth between the pinion leaves. If there is no shake, or if this is scarcely perceptible, the depth is too deep; if there is a lot, it is too shallow. In a correct depth the pitch circles of the wheel and pinion must roll upon each other. In the above figure the wheel teeth consist of radial lines to form the sides up to the pitch circle; beyond that they curve to a dome-shaped point.

Similarly, the pinion leaves consist of radial lines up to the pitch circle, and beyond that are finished off with a semicircle. In a correct depth the pitch circles roll upon each other, and the curved portions of the wheel teeth act upon the straight sides of the pinion leaves.

Testing Correctness of Watch Depth.