The 400-day, or anniversary, clock is so named
because it needs winding only once a year—on the
anniversary of its last winding. These clocks became
a traditional gift for weddings and other special,
annual celebrations.
Most clocks with mechanical movements run for 8 days on a single winding. Anniversary clocks use a torsion pendulum, which runs for more than a year. Instead of swinging back and forth like a standard pendulum, a torsion pendulum rotates, twisting its long and thin suspension spring and allowing a lengthy period of vibration. Very little energy is needed to keep a torsion pendulum moving, and so anniversary clocks can run for a very long time between windings.
Philadelphian Robert Leslie invented the torsion pendulum design and received a British patent for it in 1793 while working in London. Aaron Dodd Crane received the first US patent for a torsion clock design in 1841.