The Anti-Tank Mine, General Service, Mark III (or "Mark III mine") was a British anti-tank mine used during World War II. The mine had a cylindrical tin lower body with a steel pressure plate which sits on top of a shear-wire restrained spring-loaded striker. Sufficient pressure (350 lb (160 kg)) on the cover shears the restraining wire, allowing the striker spring to push the striker into a 0.11 grams (1.7 gr) percussion cap. The flash from the percussion cap is transferred to the No.27 detonator which sits in the centre of the mine, detonating the device.
References
NAVORD OP 1665, British Explosive Ordnance, Naval Ordnance Systems Command (Updated 1970)