Electromechanical relays are used to switch telephone calls, intercom calls and computer printer circuits. Early relays were based upon a wire pulling from one terminal of the relay to another or from one coil terminal to another causing opening or closing of a contact circuit. These were sometimes found in telephone exchanges as early as the late 19th century. These electromechanical relay systems required a high voltage power supply which was not practical for telephones but was common in industrial control applications using either alternating current (AC) or direct current (DC).