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This paper describes the construction, theory, and performance of various types of fixed field secondary emission multipliers. Detailed consideration is given of multiplier phototubes employing crossed electrostatic and magnetic fields and of electron multipliers using electrostatic focusing alone, to serve as coupling and amplifying units for cathode-ray tubes such as the "Iconoscope." It is shown that while the power required for the operation of the secondary emission multiplier is about the same as that for the conventional amplifier, it is superior to the latter from the standpoint of noise. In the case of the multiplier phototube the signal-to-noise ratio is essentially determined by the shot noise of the photoemission, and is therefore sixty to one hundred times greater than that for a thermionic amplifier and phototube under conditions of low light intensity. Multiplier phctotubes have been built with an amplification factor of several millions and serve to replace the conventional phototube and accompanying amplifier system. Their low "noise" level, together with their excellent frequency response and extreme simplicity, make these electron multipliers a very satisfactory form of amplifier.