Schwingkreis
The absorption wavemeter is a simple system consisting of a single tunable LC circuit and a separate voltmeter or ammeter. The user tunes the LC circuit, typically through a variable capacitor, until the voltage in the circuit suddenly drops. This indicates that the local LC circuit is tuned to a frequency equal to that of the tested signal, at which point it begins to absorb the energy of the signal and cause the circuit's current to drop.