Fusee. In a watch, the conical part around which the chain is wound, relaying its action to the wheel train. See our Horology → plates .

It is very useful because with its conical shape it compensates for the unevenness of the spring; when newly wound it is stronger, and weaker when run down, making the watch gain in the first instance, and lose in the latter. The first watchmakers tried to remedy this disadvantage of the spring by means of a device they called a stackfreed. Its defects soon led them to improve it, or to replace it; it was abandoned as soon as the fusee was invented. Although this discovery is ingenious, its author is unknown. However, it is surely very old.

To understand how the fusee compensates for the unevenness of the spring, observe that in a watch that has run down, the whole chain is wrapped around the barrel or drum; rewinding simply moves it to the fusee . That is the same as pulling the chain until none remains on the barrel. This cannot be done without turning the barrel as many times as the chain was wrapped around it. The article Barrel has shown that as the parts are arranged, turning the barrel winds the spring. So it will be wound exactly as many times as the barrel revolves, three and one-half, which is the number of turns a chain usually makes around a barrel.

Once this has been understood, plainly the greatest tension of the spring, and consequently its greatest force, comes when the watch is fully wound up; the force steadily diminishes as the fusee turns, and it is the least when the watch has almost run down. To make the action of the spring on the wheel train equal despite its uneven power, the diameter of the fusee is made smaller at the top, so that when the spring is most powerful, the lever arm pulled by the chain is shortest; at any point on the fusee , the product of the lever and the power of the spring at that point is the same throughout. By this means the power applied to the movement by the spring is constant, and almost as uniform as if applied by a weight.

Finding the exact shape for a watch fusee is a problem for geometers, defining the curve around the axis that would produce the correct solid for the fusee .

M. de Varignon determined the curve, for all hypothetical tensions of the spring, on p. 198 of the Mémoires of the Académie royale des Sciences for 1702. His solution is unusual in that the base of the fusee is determined and has a certain size instead of widening to infinity, as would seem proper so that the spring might pull evenly as its power grew infinitely small. In more geometrical terms, the curve he found, which in revolving around the axis would give the shape of the fusee , has only one asymptote, whereas it should have two; on the one hand it must come closer and closer to the axis and never touch it, and on the other hand move farther and farther from it, to infinity. Determining this curve would not be very helpful in ← horology because however perfect the spring, its elasticity will never be uniform enough, given both the nature of steel and faults in production, for use with a fusee shaped to a curve defined by some hypothetical spring tension. Horologists have found a more reliable way to shape the fusee , using a tool called a lever ( see Lever ) , with which they verify at every point on the fusee that the power from the spring is the same, by measuring it with the same weight.

Clockmakers in England use fusees in spring-driven clocks, but here they are not used. 1) The spring is made a little longer, and only the most uniform turns are used, and 2) The escapement can always be made so that despite the decreasing power as the spring unwinds, the clock will always go with the same accuracy. See Clock, Escapement, Spring, etc.

Having discussed the shape the fusee must have, we shall now explain its construction. It comprises an arbor (Plate X, key CC) and is integral with it. The arbor has two pivots, C and P, at the ends [1]; pivot P must be heavy and long enough to extend a little through the dial and have a square for the winding key. When building the watch, pivot C must be much smaller because the radius of the fusee is much shorter at the top than at the base, which greatly increases the friction on this pivot; this disadvantage is partly remedied by making the pivot small. Among the many advantages French-style watches have over English ones, this is not one of the least; in the latter, the square is at the top of the fusee and requires a very large pivot, greatly increasing the friction, which is already considerable because of the short radius on this end and the greater force of the spring.

Also at this end is stop C ( see Plate [X, key CC, Figure 9, G]), which prevents overwinding the watch. See Chain stop.

At the base it has a slight projection with teeth of triangular shape; this is called the ratchet, and its purpose will be explained below.

The great or first wheel, on the fusee arbor, fits against the base. It revolves on the arbor, which is round. To maintain it against the base of the fusee , it is retained by collet C, which is pressed onto the arbor and fits into a recess in the wheel. See Great wheel, Collet, etc., the plates, and their legends.

When the fusee and the great wheel are assembled, pawl c of the great wheel engages the teeth of the ratchet, in such a way that when the fusee is being driven by the chain as the watch is going, the teeth bear against the ratchet, and fusee and great wheel turn together in the same direction. When the fusee is turned in the opposite direction, it turns independently of the great wheel because the ratchet does not block its motion. This mechanism is necessary so that when the watch is wound the fusee turns free of the great wheel; a base is necessary, but if the great wheel turned with the fusee , it would be impossible to wind the watch.

There are fusees designed in such a way that turning the key in either direction winds the watch. Watches with this kind of fusee are called "tipsy"; since it is seldom used, we shall not discuss it, especially since these kinds of fusee are quite useless. See Watch.

1. Some of these references are lacking from the plate (translator's note).