• The key relay, consisting of a set of multi-way relays, one for every note of every manual (and pedal) on the organ. These are operated electrically by the keys/pedals – if a note is played, the corresponding key-relay goes ‘on’ until the note is released. The key relays have a separate contact for every stop on the manual/pedal concerned (so if there are 13 Pedal stops, then 13 contacts, or if there are 25 Solo stops, then 25 contacts). The contacts are wired to the stop-switches.
• The stop-switches. These are multi-contact switches, one per stop, with as many contact wires as there are notes in a stop (61 for a full-compass manual stop, or 30 (Christie) or 32 (Wurlitzer or Compton) for a pedal stop). The contact wires, which are fed from the key relays, are mounted on a swivelling rail; when the stop is ‘on’ then the rail is swivelled and the 61 contact wires touch a row of 61 separate contact strips. These strips are connected to the chest magnets.
For a note to sound, the sequence of operations can be summarised as follows:
1. Stop key ‘on’ – this causes the stop-switch to swivel and the 61 contact wires to touch the 61 contact strips.
2. Note played – this causes the key relay to go ‘on’; the key relay contact wires are energized and current flows to the stop-switch contact wires; these are already touching the contact strips, so current flows through the strips to the chest magnets, the chest action is energised and wind flows to the pipe and the note sounds.
To reduce wiring runs, the key relays are usually mounted in large airtight boxes with distinctive glass panels in the front to enable the action to be inspected. On top of these are mounted boards with the stop-switches and stop-switch action.
Since a number of stop-switches can operate the same pipe (for example, middle C of the 8′ Accompaniment Violin, middle C of the 8′ Solo Violin and tenor C of 4′ Solo Violin all sound the same pipe), the stop-switches for a particular rank of pipes are grouped together horizontally with the contact strips (which are common to all these stops) running vertically. Such an arrangement is sometimes referred to as a Switch Stack, for obvious reasons.
Usually, this type of relay is electro-pneumatic in action.