A pentode-operated frequency meter of the condenser-charging type
E.L.E. Wheatcroft, G. A. Haley · Journal of Scientific Instruments · 1937
A frequency meter is described having ranges of 0-150, 0-1500, 0-15000 cycles per second. The meter is of the condenser-charging type employing two pentode valves, and is "mains operated". The indication of frequency is correct within 1 per cent for any input voltage greater than 0.3 V. r.m.s.