The paper describes the principles, operation and performance of an equipment for obtaining in a rapid manner the frequency responses (amplitude and phase) of servo mechanisms, passive networks, amplifiers, etc. The frequency range covered is 0.1–100 c/s, and the phase angle is indicated on a dial for instantaneous reading, thus obviating the tedium of oscillographic methods. A useful characteristic of the equipment is its ability to discriminate in favour of the fundamental when the output signal from the system under test contains harmonics.The most novel feature of the equipment is the employment of an air-cored oil-dashpot-damped dynamometer for indicating phase shift. Preliminary investigation showed that, although air-cored, any dynamometer method would be subject to frequency errors. However, this has been overcome by driving the dynamometer with field-forcing power amplifiers of high linearity. It was also found by mathematical deduction that, provided the fixed-coil system of the dynamometer is excited with pure sinusoidal oscillations, its moving coil will indicate the phase shift of only the fundamental of any distorted signal presented to it. This appeared to be a valuable feature of such a system and has been verified by experiment.The complete equipment includes an electro-mechanical signal generator, providing either modulated carrier-frequency or sinusoidal signals, a special amplifier for coupling the signal generators to test circuits of low input impedance, and a calibrated amplifier for attenuation/frequency measurements. The individual units are designed for maximum flexibility in application, and their particular features are described.