Starting with a functional relation between the plate current and the grid and plate voltages, the plate current variation is deduced to a sufficient degree of approximation in terms of constants of the tube and the grid voltage variation (for pure resistance in the output circuit). The requirements of amplification, modulation, and detection are then deduced from the equations obtained. Heterodyne amplification is discussed, and it is held that within the working range of the tube this amplification is the ratio of the amplitudes of the locally and remotely generated waves provided the latter is small as compared with the former. The general problem of output current variation with an impedance in the output circuit is then considered, and the production of harmonic frequencies noted.