A Magnetic proximity fuze was patented by P.J. Eliomarkakis, (United States Patent US2434551 of January 13, 1948) although similar devices had been in service for nearly a decade. It is a type of proximity fuze

Proximity fuze

A proximity fuze is a fuze that is designed to detonate an explosive device automatically when the distance to target becomes smaller than a predetermined value or when the target passes through a given plane...

 that initiates a detonator

Detonator

A detonator is a device used to trigger an explosive device. Detonators can be chemically, mechanically, or electrically initiated, the latter two being the most common....

 in a piece of ordnance

Weapon

A weapon, arm, or armament is a tool or instrument used with the aim of causing damage or harm to living beings or artificial structures or systems...

 such as a land mine

Land mine

A land mine is usually a weight-triggered explosive device which is intended to damage a target—either human or inanimate—by means of a blast and/or fragment impact....

, naval mine

Naval mine

A naval mine is a self-contained explosive device placed in water to destroy surface ships or submarines. Unlike depth charges, mines are deposited and left to wait until they are triggered by the approach of, or contact with, an enemy vessel...

, depth charge

Depth charge

A depth charge is an anti-submarine warfare weapon intended to destroy or cripple a target submarine by the shock of exploding near it. Most use explosives and a fuze set to go off at a preselected depth in the ocean. Depth charges can be dropped by either surface ships, patrol aircraft, or from...

, or shell

Shell (projectile)

A shell is a payload-carrying projectile, which, as opposed to shot, contains an explosive or other filling, though modern usage sometimes includes large solid projectiles properly termed shot . Solid shot may contain a pyrotechnic compound if a tracer or spotting charge is used...

 when the fuse's magnetic equilibrium is upset by a magnetic object such as a tank

Tank

A tank is a tracked, armoured fighting vehicle designed for front-line combat which combines operational mobility, tactical offensive, and defensive capabilities...

 or a submarine

Submarine

A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

.

Magnetic field sensors and movement sensors inside the ordnance detect changes to the terrestrial magnetic field

Earth's magnetic field

Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...

 of the ordnance caused by another ferromagnetic object. A signal processor

Signal processor

A signal processor is an electronic circuit that removes information from an analog signal as quantifiable units for further analysis. Most modern signal processors are either dedicated microprocessor integrated circuits or general-purpose programmable microprocessors. A DSP is an electronic...

 inside the ordnance receives the signals from the magnetic field sensors and movement sensors and activates the detonator which will then detonate the explosives within the ordnance. http://www.patentstorm.us/patents/5423262/fulltext.html

Examples

Examples of pieces of ordnance that employ a magnetic fuze include:

• the Chinese Chen-2 bottom mine
  Chen-2 bottom mine
  The Sinking-2 bottom mine was the first indigenous designed Chinese naval mine that could adopt a range of fuzes, such as acoustic, contact, infrasonic, ultrasonic and magnetic fuzes...
  
  
• the Egyptian T-93 mine
  T-93 mine
  The T-93 is an Egyptian scatterable anti-tank mine similar in appearance to the German AT-2 scatterable mine. It can be dispensed from launchers deployed on the ground or mounted on vehicles which can hold as many as 600 mines. The mine is cylindrical with twelve spring-loaded legs folded up...