A magnetometer which includes an elongate reactor in which a sample can be secured in a sample support zone and which is located within a magnetic field space of a magnetic field generator and one or more signal pickup coils. Movement generating means is provided for generating relative movement in a generally linear direction between the reactor and at least one of the magnetic field and pickup coil, preferably by moving the reactor in its length. The magnetometer is characterised in that the reactor is a metal tube having a length which permits its ends to remain external of the signal pickup device during the relative movement.