Abstract: When an electrically conducting material is subjected to an alternating magnetic field local changes in the magnetic field adjacent to the conducting material are detected by a detector comprising a magnetoresistor or Hall crystal which generate an electrical signal when subjected to the magnetic field.The process and apparatus can be used to identify, classify and locate electrically conductive material.The detectors can be of the order of a few millimeters in size and so can be used to detect local changes in the magnetic field over correspondingly small areas of the material.The invention is particularly useful in identifying coins used in coin operated machines.