Naval Spherical Mine Mk III H (England)

Spherical sea mine, with four Herz electrical contact horns on the upper casing and two horns on the lower hemisphere. The explosive charge filling for the mine was introduced through a circular mouthpiece on the upper hemsphere, which was secured by a series of bolts. The mine is painted gloss light grey.

The MK III was the standard British offensive sea mine at the commencement of the First World War. The mine was originally designed to be actuated with an inertia firing mechanism, which operated when a ship bumped the mine and turned an horizontal bar mounted on the upper surface of the mine.

The Germans, by contrast, used Herz horns to activate their mines. The horn contained an electrolyte contained in a sealed capsule. When a ship bumbed the mine, the capsule was crushed, and the electrolyte flowed into a battery, completing a circuit and triggering the main charge. Following captures of German mines, the Royal Navy gradually came to see intertia devices as less effective than electrically operated 'Herz' horn devices, and eventually most British sea mines came to rely on the Herz device.