According to a rate adjusting method, the movement of a mechanical timepiece is first assembled. The assembled movement is then disposed in a vertical position, and the rate is measured with respect to a plurality of vertical positions. On the basis of the result of this rate measurement, the magnitude and direction of the positional difference vector are calculated. On the basis of the result of this calculaton, the amount of weight to be added to or subtracted from the annular balance of the movement is calculated and the positions on the annular balance where this amount of weight is to be added to or to be subtracted from is calculated. Based on the results of these calculations, the amount of weight is added to or subtracted from the annular balance at the calculated positions.
In order particularly to put a watch into beat and adjust the isochronism, in addition to the poising screws (2a, 2b) of the balance (1) and the fine adjusting screw (7) pressing on the index tail 20a to alter the orientation of the index (20), the device includes, on the one hand, two screws (19, 29) respectively fixed in the balance cock (10) and in the index (20) to maneuver clamps by means of L-shaped rocking elements (13, 23) to fix the end of the balance spring and define its useful length, and on the other hand, a screw (9) for immobilizing the index (20) in a determined position, these three screws (9, 19, 29) enabling a fine adjustment to be carried out.
