British Pattern Contact Type Sea Mine Mk 17

A contact mine is set to float just below the surface of the water with a steel cable connecting it to an anchor to keep it from drifting away. Early mines had mechanical mechanisms, but these were superseded in the 1870s by the "Hertz horn" (or "chemical horn"). The mine's upper half is studded with hollow lead protuberances, each containing a glass vial filled with sulfuric acid. When a ship's hull crushes the metal horn, it cracks the vial inside it, allowing the acid to run down a tube and into a lead–acid battery which until then contained no acid electrolyte. This energizes the battery, which detonates the explosive
A mine unit comprised a mine shell containing an explosive charge case (500lb or 320lb TNT), to be fitted with a primer and detonator in the charge, connected to an electrical battery through an external ‘horn’, which was broken or actuated by contact with the hull of a ship or submarine, thus exploding the mine. The mine shell was affixed to a sinker by clamps. The sinkers were large rectangular boxes with four small wheels at each bottom corner. These fitted inside the rails so that the mine unit could roll along them when hauled aft by the winches. Inside the sinker was a drum of mooring wire, one end of which was attached to the mine case. There was also a depth mechanism, which enabled the distance of the mine below the surface to be set to any predetermined amount. On being dropped into the sea through the traps at the end of the mine rails, the mine separated from the sinker which then sank to the bottom, dragging the mine to the set distance below the surface.