Standard for the Wireless Set No.18 and similar. The operator puts his respirator on, plugs the mike in, but continues to control the transmit/receive with the hand microphone (since that switches the valve filament supply). The microphones are connected in parallel on most sets, with the possible exception of the WS18 Mk.IIIT
The other respirator microphone is for switchboard and similar applications (some remote control units, perhaps), and has a two pin plug moulded into the unit which matches the "Transmitter, Breast" connection. In use, you unplug the harness cable from the breast set and fit it to the respirator microphone.
The inserts are identical for both respirator microphones, and are a gas-tight version of the standard telephone/hand microphone insert. A rubber sealing ring is supplied with them to ensure an airtight seal between the front of the insert and the front portion of the housing that connects to the facepiece.
Chris.
***** Important Warning from the Elves who drink Safe Tea *****
Do Not, under any circumstances, put on any respirator made prior to approximately 1956.
Everything before that time used asbestos in the filter elements. The "Service" respirator with hose and flattened canister contains crocidolite (Blue Asbestos). The "Civilian Duty" (ARP and switchboard operators, etc.) and civilian respirators with the round canisters (no hose) contain chrysotile (white asbestos), and the upgrade "green can" Contex filter that can be fitted to the front of those (intended to protect against Arsine) also contains blue asbestos. Modern respirator filters are made from spun glass fibres and are fairly safe.
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