At first sight, the sleek grey dial of the LANGE 31 in the 45.9-millimetre white-gold case does not reveal any sign of the power hidden inside its manually wound calibre. Only the white 31 at the upper end of the large power-reserve indicator and the small inscription beneath the Lange outsize date modestly refer to the fact that when fully wound, the movement will run for an entire month. It is a formidable technical challenge to endow a mechanical watch with a power reserve of 744 hours – and so is assuring superb rate accuracy from the first to the last day. This is because the twin mainspring barrel with its two 185- centimetre-long springs delivers an enormous amount of torque at the beginning when fully wound, which then gradually declines toward the end of the power-reserve period. Thus, a separate technical device is needed to guarantee constant force delivery in the interest of maximised rate accuracy. The device is referred to as the constant-force escapement. In the LANGE 31, this device between the twin mainspring barrel and the going train delivers a constant amount of energy to the escapement every ten seconds, so consequently, the balance always oscillates with the same ideal amplitude. The result is superior precision from the first to the thirty-first day when a limiter mechanism stops the movement.

The function of the constant-force escapement is readily observable. A look through the sapphire-crystal caseback reveals the triangular cam attached to the fourth-wheel arbor. It controls the mechanism with an ingeniously designed pivoting lever to recharge the remontoir spring with fresh energy every ten seconds. The powerful mainsprings are tensioned with a separate key that inserts in a square socket in a recess of the sapphire- crystal back. Its technical features include an integrated backstop ratchet for smooth and easy winding as well as a torque limiter that prevents unintentional overwinding of the springs. The artisanal finissage of the horological power pack lives up to all of the expectations that connoisseurs around the world associate with the A. Lange & Söhne brand.