Mainsprings lose roughly the same amount of torque per turn as they unwind. So if, for example, a mainspring loses 10% of its torque per turn as it unwinds, then a fusee mounted on the same arbor as the mainspring (a reverse fusee) must lose 10% off its radius at each turn. So the fusee would be 10/10 of its full radius at the outside, 9/10 at the first turn, 8/10 at the second turn, and so on. This is linear, so the slope on the fusee is linear.

In a "regular" fusee arrangement -- with the fusee on one arbor and the mainspring on another arbor -- that 10% power decrease per turn power would have to be spread over a variable number of turns of the fusee, depending on its radius at that point. At its small end, the fusee has to make more turns to wind a single turn off the mainspring than it does at its large end, so the curve on the small end must be flatter. Which is why the curve on a "regular" fusee is a complex curve, not linear.

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