The counting rate meter, or vacuum tube speedometer, is an integrating and averaging instrument with an electrical, exponentially decaying memory. It is designed to replace message‐register equipment in all particle counting apparatus, and is ideally suited to continuous photographic recording. The average current to the output meter is proportional to the counting rate for constant sources or for decaying sources whose mean life is significantly greater than the time constant RC of the output tank circuit. This RC is analogous to a radioactive mean life and may be regarded as the mean memory time. While the output current depends only on the output resistance R, and not on the output capacitance C (except for very rapidly decaying sources), the statistical fluctuations in the output current depend upon RC, and are equivalent to the fluctuation expected in a time interval of 2RC. Expressions are derived for the condenser charge Q, or output current Q/RC, for constant and decaying sources of radiation, as well as for the expected fractional statistical fluctuations in these cases.