This tourbillon system was registered on March, 27, 1930 by the Ecole d'horlogerie de la Vallée, Le Sentier, and a license was granted to Le Coultre & Cie. It became one of the most successful tourbillon designs.A virtually identical watch (Breguet, No. 3046) is described and illustrated in "Le Tourbillon" by Reinhard Meis, Editions de l'Amateur, Paris, 1990, p. 187.Anibal (acier au nickel pour balanciers), an alloy invented by Dr. Charles Edouard Guillaume, exhibits unusual properties, both in terms of thermal expansion and changes in elasticity. These properties are very different from those of two other famous alloys invented by Guillaume, Invar and Elinvar. At the end of the 1800's, Guillaume attempted to eliminate the so-called Middle Temperature Error caused by the fact that the change of rate in a timekeeper with a steel-brass bimetallic balance is aproximately a linear function of temperature, while the modifications caused by change in elasticity of a balance spring are approximately a quadratic function. Thus, it equals zero at only two temperatures, causing secondary error. Countless attempts were made to eliminate Middle Temperature Error, usually by means of auxiliary compensation devices. In 1899, Guillaume noticed that steel with an addition of 44.4% nickel had a negative square coefficient of thermal expansion. This, combined withrass in bimetallic lamina, makes its expansion close to quadratic. Balances with bimetallic rims made of anibal and brass are usually called Guillaume balances, or, as their inventor called them, integral balances. When combined with special balance springs, they exhibit remarkable temperature stability, on occasion not exceeding 1/50 second per day at 1oC.