The theoretical expression for the voltage amplification of a valve followed by a transformer is developed in its simplest possible terms, and various particular cases are considered. The importance of low leakage inductance for the uniform amplification of the higher frequencies is emphasized.A practical verification of the theory is provided by actual measurements of the vector amplification ratio and comparisons with the theoretical equations, and graphical methodsare given whereby the constants of a transformer can be deduced from these measurements and the amplification ratio extrapolated to frequencies beyond the range of convenient measurement.A practical design of transformer is described in which low leakage inductance and self-capacity are ensured by a sectionalized and spaced construction, and a curve of amplification of this transformer is given.Various questions relating to intervalve transformers are briefly considered, and circuits are described for increasing the amplification of low frequencies and the effective stepup ratio of the transformer.The use of low-frequency reaction for correcting distortion is described and is illustrated by measurements.