Abstract: An apparatus and method for measuring the atomic mass of ion species selected by the analyzing magnet (14) of an ion implanter (10). A signal proportional to the magnetic field of the analyzing magnet is obtained by means of a rotating coil (36), and this signal is used to calculate the atomic mass by means of the equation: Atomic Mass=KB.sup.2 /V where B is the magnetic field, V is the ion beam energy at the analyzing magnet, and K is a proportionality constant. The rotating coil (36) is driven by a synchronous motor (38) powered by A.C. line voltage, and the output voltage is converted to a frequency signal proportional to the magnetic field and the line frequency. The frequency signal is counted over a time corresponding to the period of the line frequency to cancel out the line frequency dependence of the frequency signal. In accordance with one embodiment of the invention, a microprocessor is utilized to evaluate the B and V signals and to calculate the atomic mass.