Lever the power out, and the ratio of output to input force is given by the ratio of the distances from the fulcrum to the points of application of these forces. This is known as the "law of the lever." The mechanical advantage of a lever can be determined by considering the balance of moments or torque, "T", about the fulcrum. where F is the input force to the lever and F is the output force. The distances "a" and "b" are the perpendicular distances between the forces and the fulcrum. Since the moments of torque must be balanced, formula_3 .
Lever escapement escapement was invented by British clockmaker Thomas Mudge around 1755, and improved by Abraham-Louis Breguet (1787), Peter Litherland (1791), Edward Massey (1800), and its modern ("table roller") form was developed by George Savage in the early 1800s. Since about 1900 virtually every mechanical watch, alarm clock and other portable timepiece has used the lever escapement. The advantages of the lever are, first, that it is a "detached" escapement, it allows the balance wheel to swing completely free of the escapement during most of its oscillation, except when giving it a short impulse, improving timekeeping accuracy. Second, due to "locking" and