A number of methods have been used for monitoring electron beams. In some ways the most attractive methods are those that monitor the whole beam, such as secondary emission monitors, induction monitors and transmission ionization chambers. These, however, suffer from certain disadvantages; (1) a change in beam position or angle may alter the fluence at the point where the beam is being used, without altering the response of the monitor; (2) if a scattering foil is used, a change in energy will alter the relation between the monitor reading and the fluence on the beam axis remote from the foil (Karzmark 1964); (3) transmission ionization chambers suffer from lack of saturation at high beam intensities.