1795 - Breguet balance-spring
First description of the “sympathique clocks” in a letter to his son.
Development of perpetual date calendar, the Breguet balance-spring and the ruby cylinder.
A small spring which through its elasticity regulates the oscillations of the balance. It is attached at its inner extremity to the axis of the balance and at its outer extremity to the cock. The flat balance spring, invented by the Dutch mathematician Huygens in 1675, had established a degree of isochronism that still left something to be desired. Breguet applied himself to the problem and in 1795 solved it by drawing the outer extremity of the spiral in towards the centre, following a precisely calculated curve. Thus endowed with the ‘Breguet curb’, the balance spring henceforth became concentric in form. Watches gained in precision, and the balance staff wore less quickly. Breguet also perfected a bimetallic compensation bar in order to cancel out the effects of changes in temperature on the balance spring.
The ‘Breguet balance spring’ was adopted by all the great watchmaking firms, who continue to use it to this day for high-precision pieces. Between about 1880 and 1910, many manufacturers inscribed the words ‘spiral Breguet’ or ‘Breguet overcoil’ in large letters on the cuvettes (back covers) of their watches in order to enhance their commercial credibility. Breguet, by contrast, never resorted to this type of inscription.