A proton magnetometer measures very small variations in the earth's magnetic field allowing ferrous objects on land and at sea to be detected.
They are used in land-based archaeology to map the positions of demolished walls and buildings and at sea to locate wrecked ships for recreational diving.
The proton magnetometer consists of a coil surrounding a proton-rich material such as a cylinder of paraffin / kerosene. A measurable electric current is induced in the coil by the realignment of the protons in the paraffin as the magnetometer passes through a changing magnetic field.