It is well known that certain factors, notably flicker noise and zero drift, determine the smallest signal voltage that can be satisfactorily amplified by a thermionic d.c. amplifier. Better results can often be obtained by using a suitable modulator to convert the direct current to alternating current and following this by an a.c. amplifier and rectifier.The paper discusses the advantages of the second-harmonic type of magnetic modulator for this purpose, and presents a theoretical analysis of an idealized modulator of this type, with particular reference to the influence of various controllable parameters on the signal/noise ratio and zero error.The paper also describes experimental work, which, with allowance for the idealizations in the theoretical analysis, is considered to provide a satisfactory qualitative verification of the latter. Particular emphasis is placed on the need for great care in the design of the various circuits, toeliminate additional sources of noise and zero error, and it is believed that the noise output of the apparatus described is due mainly to Barkhausen effect in the modulator cores, and is equivalent to a signal input of about 10−19watt for a bandwidth of 1 c/s; the zero drift is, however, considerably greater than this, and is equivalent to a signal input of about 3 × 10−18watt over a two-hour period.