The need for a direct-current amplifier, suitable for industrial conditions, has been realized and a suitable design has been developed. It is entirely mains-operated and the accuracy is independent of mains fluctuations or change in valve characteristics. The amplifier is capable of an output of 3 watts and has a power gain of about 1010. It can be used either as a voltage amplifier or as a current amplifier and is extremely stable. It has been applied to high-speed temperature-recording for the measurement of liquid steel, and also to the metering of heat transport in large hot-water plants. It has also been applied to optical pyrometry, measurements of illumination by barrier-layer-type photocells, to the polarograph and to many other problems in which very small e.m.f.'s are available as a function of the quantity to be measured. The theory of the amplifier has been worked out in sufficient detail to meet practical design requirements. The departure from linearity of response is of the order of only a few parts in 10 000, and voltages of a few microvolts and currents as low as 0.01 micro-amp. can be amplified, and thus used to operate recorders and controllers.