The interphone jackbox is a broad type of switchbox, used to switch between interphone channels in an aircraft containing 2 or more crew members, who need to be able to talk to each other through a microphone and headphones. They have a mic and phones jack, with different diameters to prevent plugging the wrong device into the wrong jack. The mic plug is around 3/16" diameter, while the phones jack is a standard mono 1/4" jack.
Typically, the box has a large diamond shapped switch on the front, allowing the box to talk on "COMP.", "V.H.F. LIAISON", "COMMAND", "INTER" and "CALL."
The "COMP" position connects the box to the radio compass output. Whatever the pilot or navigator have the set tuned to, will be heard through the headphones. This switch position does not use the microphone, because the radio compass set does not transmit.
The "V.H.F. LIAISON" position connects the box to the VHF Liaison set, which is controlled by the radio operator. This set communicates to the ground stations. Not all boxes in the aircraft have the mic connected to the set, so that only authorized and necessary crew members can speak over it. All stations are permitted to listen, however.
The "COMMAND" position connects the box to the Command radio set. The transmitters are set before flight, but the receivers can be controlled by the pilot or copilot. Any crew member can listen, but the mic is reserved for use by only the pilot and copilot.
The "INTER" position connects the box to the interphone, the main communications link between any crew members. It does not connect to any radio sets.
The "CALL" position is spring loaded, and connects the mic at any station, to the phones of all other stations regardless of their switch position. It's use is typically reserved for the pilot/copilot making announcements, rolecall, or emergencies.
This is a great example of how these jackboxes were wired and mounted in the aircraft. The jackboxes do not come with any mounting holes or holes for wires to exit. This particular example shows where the installer drilled holes for mounting, and cut a slot for the wires to exit on the side. Each box would have to be machined for each individual installation. It is rare to see the rear housing removed when an aircraft is scrapped. Usually, just the front is removed.
This also shows how easy the jackboxes are to disassemble to wire or replace. They have an 11-set banana plug system, where the front simply plugs into the back. For wiring, the socket can be removed from the rear via three screws.
The rubber grommet protects the wiring from being cut in the jackbox walls. A thin phenolic sheet insulated the connectors from the metal back.
There are many variations of production. Two fronts are shown here side-by-side, so that it is easy to see differences. They are electrically the same.