Mk VI Wireless Electric P Tube - Drawing. Public domain image.

Diagram of British Mk VI wireless electric "P" vent-sealing tube, 1914.
Body is of solid drawn brass.
The end of the wire projecting through the cement is bared of insulation and has an iridio-platinum bridge solderded to it giving a resistance of 0.9 to 1.1 ohms. The other end of the bridge is soldered to a small pole piece attached to the copper plug.
Action :

Current passes from the battery through the striker to the contact disc and down to the insulated wire in the copper plug, through the iridio-platinum bridge to the small copper pole and the tube body and back to the battery.
The bridge becomes incandescent and fires the priming composition and the F.G. powder.
Tube body expands against the vent and prevents gas escaping out between itself and the vent. The copper sealing plug is forced upwards and the upper part of the plug fits into the small cone seating in the body and prevents gas escaping through the head of the tube. The gas-check portion expands and assists sealing.
For use with all BL guns having wireless locks, except 13.5 inch Mk V, 12 inch Mk XI - XII, 4 inch Mk VII & VIII. May also be used with QF 12 pounders, 14 pounder, 4 inch, 4.7 inch and 6 inch.