The Relay
At the far end of the organ's main cable from the console is what is called a "relay" unit. The workings of this need not detain us long, for it can briefly be described as a series of selector switches, in computer terms acting as "AND" gates. This means that when, and only when, the appropriate stopkey and note are activated at the console the requisite pipe will sound. On Wurlitzer and Christie organs, the relays are physically separated into key or note relays ("Relay 1" and "Relay 2" in the diagram below) and stop relays ("switchboard".in the diagram)
Modern technologies now available mean that the cumbersome electro-pneumatic or electro-mechanical relays originally fitted to theatre organs can be replaced by micro-chips. This has been done in a number of cases, leading to considerable savings in cost, space and maintenance. However, there is a strong feeling amongst some, myself included, that when complete original instruments are restored, the original relay system should be retained to maintain their historical integrity.