English: A resonant-reed frequency meter from 1921. The reed frequency meter, an obsolete technology, was used to measure the frequency of alternating currents below 1000 Hz. It was used to check the frequency of alternators supplying local power grids and the higher frequency AC used for traction motors on streetcars and ships. It consists of a small electromagnet placed next to a steel strip cut into reeds of graduated lengths. When the alternating current to be measured is applied to the electromagnet it vibrates the strip. The reed which is resonant at the applied frequency will absorb energy and vibrate with large amplitudes while the other reeds will hardly vibrate at all. The ends of the reeds project through the face of the instrument next to a graduated frequency scale.