Spinner magnetometers are used to measure the remanent magnetism of rock samples: that is, magnetism that was created and given direction by the magnetic field prevailing when the rock was formed.
Old rocks will have been exposed to many environmental factors, and the field itself could have changed direction (or the rock may have changed direction relative to the field) several times since the rock's creation. Therefore, different levels and orientations of magnetism might exist in the same sample. As they have strength and direction, they are vectors. A spinner magnetometer is able to measure the total natural remanent magnetism, which is the sum of all the vectors.
Usually a magnetometer takes its reading by being held against or close to a stationary sample. However, a spinner magnetometer rotates the sample around a fixed axis inside an annular-shaped magnetometer. Variations of the sample's magnetic field are picked up by the magnetometer and converted into voltage variations which provide output.