We describe a multi‐staged thermionic amplifier, using two discharge counters, and having an output meter which reads directly the ratio of the activity of two radioactive sources. Continuous photographic recording of this ratio for two fractions of a rapidly decaying source therefore provides a direct observation of their relative activity which is invariant with time, even though a mixture of substances having a variety of half‐periods is involved. The amplifier holds advantages of greater convenience and higher statistical accuracy over the usual single counter amplifier when dealing with sources having moderate strength and half‐periods near the range of five minutes to one hour. It is particularly suited to experiments in which such radioactive substances are used as chemical or biological indicators. A portion of the circuit, consisting of three thermionic tubes, has general applicability wherever direct observations are wanted of the ratio of two d.c. currents.