Lever , clockmaking tool , used for adapting the fusee to the spring. See our plates [Plate XV, key MM] for Horology.

It consists of a rather long rod or arm, AB [Figure 60]; a clamp, E, with a square hole that secures it to the squared arbor of the fusee ; and a weight, P, carried on another small rod, V [1], with a square-holed part that can slide on rod AB, which must be square, at least towards the end. Two screws, V S, clamp the arm in the following way. Screw S does not enter part A of jaw A, so when it is tightened it cocks this jaw and brings end E closer to end G. The other screw, V, extends through jaw EF and screws into the other jaw, AG. This adjustment allows first clamping the jaws on the square by means of screw V; then when screw S is tightened, jaws E and G are securely clamped on the squared fusee arbor. With only screw V tightened, the clamp tends to wobble at the end, making the lever slip on the squared arbor of the fusee , often breaking the spring and the chain.

For use, the spring barrel and fusee are mounted between the plates, and the chain is fitted as if the watch were to be set going; observe that no other parts of the movement are added. Then clamp E of the lever is fitted to the square on the fusee , and it is secured firmly by the two small screws, V S, so that the lever is integral with the arbor. When thus prepared, the lever is used as a key, and as if winding the watch, it is turned until the chain has reached the top of the fusee . As we said in the article Fusee, this winds the spring exactly as many times as the chain was wrapped around the barrel. Then the lever is released to see if it stays horizontal and the action of the spring on the fusee balances weight P at the end of the lever.

If the fusee prevails, the weight is moved away from clamp E; if the lever prevails, the weight is moved closer to the clamp; plainly, these motions increase or diminish the force of the weight. Once these two forces are in equilibrium, they are measured again at all points along the fusee , from the top to the base. If they balance throughout, the fusee is perfectly equalized and will always deliver an equal power to the train, despite variations in the spring power. If there is imbalance and the spring has least power towards the base, sometimes adding a little tension to the spring will produce balance. When the spring draws much stronger on one part of the fusee than on others, the fusee is cut down. Thus, by varying the tension of the spring and by reducing the portions of the fusee where the spring is too strong, the fusee is brought into perfect balance with the spring. See Equalize, Spring, Fusee , Tension, Barrel, Worm gear, etc.

It will readily be seen that the length of rod or arm AB merely reduces the weight while retaining the same leverage, in order to reduce the friction of weight P on the pivots of the fusee , and to replicate better the conditions when the watch is going.

Previously, this tool did not have the small arm, V, so weight P was slid along the long arm, AB; but M. le Roy observed that this considerably increased the friction in the pivot to which the lever was attached. He devised the small arm, on which moving weight P farther from rod AB, brings the center of gravity of the whole mechanism between the fusee pivots, which distributes the friction equally between them.

1. Reference lacking in plate (translator's note).