Fusee ensures that the torque coming from the cylinder is always constant. It is important to know that this mechanism was developed at the time when the springs of cylinder were out of blue steel and not out of white metal as used since the second half of the XXth century.

The blue steel springs, which the watchmaker will always be able to recognize once out of the barrel by their concentric shape and not the “S” shape that modern springs have, have a regular loss of torque throughout the seven or so revolutions of cocking that they have unlike modern springs which have much less loss of torque.

To make up for this loss of torque, a spindle is placed between the barrel and the gear-train. The barrel and the fusee are in contact either by a chain or a hose. When the spring is fully wound, the contact will be made on the largest diameter of the fusee and the more the torque will decrease the more the contact will be made on a small diameter of the fusee. Thus the torque will remain constant.

Since the end of the XXth century the fusee has made its comeback on some wristwatches that can be found on the market. It is interesting to know that in order to make this system work properly the watchmakers certainly had to integrate blue steel springs. Indeed with modern springs these watches would have problems of knock at the end of the power reserve. Also, the modification of the torque of modern springs is difficult to transpose on a fusee.

The fuses are also quite often, especially on marine chronometers, equipped with a mechanism similar to the Maltese cross or have Maltese crosses on their barrel.