This phenomena can now be used to make a ferrous metal detector. Two such bottles filled with distilled water are spaced about six feet apart. The longitudinal axes of the bottles lie east-west. The coils wound round the bottles are connected in series and a current passed through them. After three seconds, the current is cut off and an amplifier connected across the coils. If the
intensity of the earth’s field is the same at each bottle, the precession frequency at each will be the same, and the signals from both coils of equal frequency.

To understand the principles involved is easy if the cobwebs and dust are shaken off the school books and memories of atomic particles. Remember that old friend, the hydrogen atom, first in the atomic table, with just one proton and one orbiting electron, as simple a thing as any alchemist could wish. The orbiting electron acts just like electric current in a coil of wire and sets up a magnetic field about the atom as seen in Fig. 1. The proton, the main mass of the atom, is also in motion, spinning about its centre, so that the whole atom looks like a magnetic gyroscope, whose magnetic poles are on its spin axis.

The proton magnetometer, also known as the proton precession magnetometer (PPM), uses the principle of Earth's field nuclear magnetic resonance (EFNMR) to measure 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 principle of operation is outlined in the magnetometer article.