The problems of measuring transmission (i.e. gain and phase shift) at frequencies in the range 10−3−102c/s are briefly reviewed, and an equipment which overcomes most of them is described. The oscillator is novel, and has the important property that its amplitude is immediately self-stabilizing. The phase-shifter is also novel, and is of a resistive-potentiometer type having a set of ganged brushes. It gives an adjustable phase shift with very little change in amplitude. The conditions in which a non-sinusoidal waveform lacks a specified set of harmonics are discussed in an Appendix.