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The electro-pneumatic action is a control system for pipe organs, whereby air pressure, controlled by an electric current and operated by the keys of an organ console, opens and closes valves within wind chests, allowing the pipes to speak. This system also allows the console to be physically detached from the organ itself. The only connection was via an electrical cable from the console to the relay, with some early organ consoles utilizing a separate wind supply to operate combination pistons.

Although early experiments with Barker lever, tubular-pneumatic and electro-pneumatic actions date as far back as the 1850s, credit for a feasible design is generally given to the English organist and inventor, Robert Hope-Jones. He overcame the difficulties inherent in earlier designs by including a rotating centrifugal air blower and replacing banks of batteries with a DC generator, which provided electrical power to the organ and drove the blower. This allowed the construction of new pipe organs without any physical linkages whatsoever. Previous organs all used tracker action, which requires a mechanical linkage between the console and the organ.

When the organist selects a stop and depresses a key, an electric circuit is completed, causing a low-voltage current to flow from depressed key, through the stop-tab switch, and on through the cable to the electro-pneumatic relay. The relay interprets the command from the console and sends an electric current to the appropriate solenoid. The solenoid is energized, causing the pipe valve connected to it to open, which emits compressed air into the pipe, allowing the pipe to speak.

A significant drawback to the system was in the large amount of wiring required for operation. With each stop tab and key being wired, the transmission cable could easily contain several hundred wires. The great number of wires required between the relay and the organ itself, with each solenoid requiring its own signal wire, made the situation worse, especially if a wire was broken (this was particularly true with consoles located on lifts and/or turntables), which made tracing the break very difficult.

These problems were even greater in larger organs, and it would not be unusual for a particular organ to have over a hundred miles of wiring in it. In fact, the largest pipe organ in the world, the Boardwalk Hall Auditorium Organ, is said to contain more than 137,500 miles of wire.

Another drawback is that tracker action organs allow a greater sense of control to the player. Electro-pneumatic action organs have two modes, the pipes are either "on" and off". An organist can be more precise as to the exact timing of the speech of the pipes with a mechanical action.

Even with these drawbacks, the action was functional, and many organists pronounced electro-pneumatic organs easier to play than tracker organs and faster and more responsive than tubular-pneumatic organs, and more reliable, as tubular-pneumatic systems were very susceptible to leakage.

In the years after the advent of the transistor, and later, integrated circuits and microprocessors, miles of wiring and electro-pneumatic relays have given way to electronic and computerized control and relay systems, which have made the control of pipe organs much more efficient. But for its time, the electro-pneumatic action was considered a great success, and even today modernized versions of this action are used in many new pipe organs.

• A Brief History of Electronics in Pipe Organs
• Foort, Reginald, "The Cinema Organ," Second Edition, New York: The Vestral Press, 1970, pp 74-78.