The paper discusses the design of an equipment for the continuous recording of the very small r.f. noise powers received from the sun and the galaxy.It is first shown how a noise power can be described in terms of an equivalent temperature, and how cable attenuation affects the noise power measured at the end of an aerial feeder. The fundamentals of the measurement of noise power are considered; it is shown that the minimum detectable power is determined by the receiver noise and by the ratio of input and output bandwidths of the receiver. The practical difficulties inherent in the measurement have been overcome by the design of a self-balancing equipment in which a locally generated noise power is continuously adjusted to equality with the incoming power.A description of such an equipment is given; the design of the r.f. switch, the local noise source, the receiver and the control circuits for the noise source are considered in some detail.The performance of the equipment is analysed in terms of its accuracy, its response to a step function of input and the fluctuations of its output indication. Experimental determinations of the performance are found to compare reasonably well with theory.