A method is described to measure magnetic flux density in small regions, for example the stray flux density on the surface of a ferromagnetic sample. A long rectangular loop of conducting material vibrates in the magnetic field, producing a voltage at its terminals which is proportional to the flux density at the 'bow' of the loop. The results of checking measurements for the calibration factors are compared with the calculated values and are found to be in good agreement. Examples are given for the application of the instrument.