The paper is a sequel to the paper by Williams and Noble, entitled “The fundamental limitations of the second-harmonic type of magnetic modulator as applied to the amplification of small d.c. signals”, and it describes some of the considerations involved in the design of a practical and compact d.c. amplifier based on the previous work, with particular reference to an instrument designed and built at the Telecommunications Research Establishment.The paper includes a description of the following items, which were not considered in the previous paper:(a) A 1500-c/s oscillator of relatively simple design giving less than 0.0005% second-harmonic distortion.(b) A low-pass filter to prevent power from the oscillator reaching the input d.c. signal-source.(c) A phase-sensitive rectifier of unusual design.(d) Arrangements for range switching to cover a wide range of signal inputs.Considerable attention is devoted to the problem of applying overall d.c. negative feedback to the amplifier, which necessitates careful design of several sections of the equipment from the point of view of their gain and phase characteristics.The results of tests and measurements carried out on the completed equipment are also given.