A synchro (also known as selsyn and by other brand names) is, in effect, a transformer whose primary-to-secondary coupling may be varied by physically changing the relative orientation of the two windings. Synchros are often used for measuring the angle of a rotating machine such as an antenna platform. In its general physical construction, it is much like an electric motor. The primary winding of the transformer, fixed to the rotor, is excited by an alternating current, which by electromagnetic induction, causes voltages to appear between the Y-connected secondary windings fixed at 120 degrees to each other on the stator. The voltages are measured and used to determine the angle of the rotor relative to the stator. Uses Synchro systems were first used in the control system of the Panama Canal in the early 1900s to transmit lock gate and valve stem positions, and water levels, to the control desks. Fire-control system designs developed during World War II used synchros extensively, to transmit angular information from guns and sights to an analog fire control computer, and to transmit the desired gun position back to the gun location. Early systems just moved indicator dials, but with the advent of the amplidyne, as well as motor-driven high-powered hydraulic servos, the fire control system could directly control the positions of heavy guns. Smaller synchros are still used to remotely drive indicator gauges and as rotary position sensors for aircraft control surfaces, where the reliability of these rugged devices is needed. Digital devices such as the rotary encoder have replaced synchros in most other applications. Selsyn motors were widely used in motion picture equipment to synchronize movie cameras and sound recording equipment, before the advent of crystal oscillators and microelectronics. Large synchros were used on naval warships, such as destroyers, to operate the steering gear from the wheel on the bridge. Synchro system types There are two types of synchro systems: torque systems and control systems. In a torque system, a synchro will provide a low-power mechanical output sufficient to position an indicating device, actuate a sensitive switch or move light loads without power amplification. In simpler terms, a torque synchro system is a system in which the transmitted signal does the usable work. In such a system, accuracy on the order of one degree is attainable. In a control system, a synchro will provide a voltage for conversion to torque through an amplifier