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In investigating the effect of harmonic voltage in class C amplifiers, it is found that the second- or third-harmonic voltages introduced into the plate or grid circuit in the proper phase improve the performance of the tube. Harmonics of higher order are found to be undesirable in general. The most favorable path of operation is determined and it is found that this path may be easily obtained by the introduction of the third-harmonic voltage 180 degrees out of phase with the fundamental voltage into the plate circuit. Various means for doing this are devised, and the power output and over-all efficiency of the tube, acting as a class C amplifier, is increased. The method of analysis employed, when applied to frequency multipliers, shows that by the introduction of harmonic voltages into these devices in the proper phase and magnitude, substantial increases in power output and efficiency may be obtained.