An integrator has been constructed which is capable of computing the yield of nuclear reactions produced by radiation of intensity that varies arbitrarily with time. By combining the appropriate number of such machines the yield resulting from a series of consecutive reactions may be found. While the emphasis in this paper is on applications involving the yield of radioactive isotopes produced by a pile or other source of radiation, the instrument may be used to calculate temperatures in thermodynamic problems, voltages and currents in electrical networks, and so on, wherever the quantity (x) to be determined is governed by the differential equation dx/dt=−λx+f(t),f(t) being a given function of the time, and λ a known constant.