As a spring winds down, its torque decreases. The fusee ensures that the torque will always be the same. The fusee is a cone shaped barrel which is fitted alongside the mainspring barrel. A cord or chain runs from the mainspring barrel to the fusee. As the mainspring runs down it rotates the fusee. As the mainspring barrel winds down, so the cord moves from the narrower barrel diameter to the larger diameter, thus ensuring that the output torque from the fusee is the same.
A Meccano fusee has been built using stacked rods wrapped around a former, to give a cone shape. The fusee is illustrated in the photograph
.Fig. 1 Cone shaped Meccano fusee on the left and barrel on the right with Meccano chain between. The clockwork motor is providing power to the barrel