What Is a Tank Circuit?

Also known as an inductor-capacitor circuit, an LC circuit, a resonator circuit or a tuned circuit, a tank circuit is a simple electrical circuit that uses magnetic resonance to store an electrical charge or produce an electromagnetic frequency. If you've ever wondered how your radio picks up single frequencies when you turn the dial -- instead of playing all the radio frequencies in the air at the same time -- the answer is a tank circuit.

The first study on the electromagnetic oscillations found in a tank circuit appeared in France in 1827, published by Felix Savary. Savary used a Leyden Jar, the same device used by Benjamin Franklin to "capture" electricity from his kite experiments, to document how the opposite charges on the inside and outside of the bottle caused a magnetized needle to turn back and forth. Savary's pioneering work showed how a magnetic charge oscillated between a coil and a charged plate. These oscillations would later be recognized as electromagnetic frequencies and become vital to the early radio technology developed by scientists like Guglielmo Marconi.

A tank circuit consists of a capacitor connected to a coil, an inductor, by wires. A capacitor is simply two conductive plates separated by a non-conductive material such as wax paper. When the capacitor receives an electric charge, positive and negative charges accumulate on opposite ends of the non-conductive surface. Because opposite charges attract but cannot pass through the non-conductive surface, they move through the wires to the inductor coil, electromagnetically charging the inductor.

The resonance in a tank circuit is created by the movement between the capacitor and the inductor, the same movement Savary noticed in the Leyden Jar. As electrical charge moves from the capacitor to the coil, the capacitor loses electromagnetic energy and the inductor becomes electromagnetically charged. Once the inductor is more charged than the capacitor, however, the electromagnetic cloud around the coil begins to dissipate and energy flows back through the wires to the capacitor. The process then begins again and repeats over and over until all of the original energy is lost to resistance in the circuit.

The back-and-forth of electrical energy between the capacitor and inductor emits an electromagnetic frequency that is very useful in telecommunications technology. Often used in tuning radio transmitters and receivers, tank circuits can be charged to produce a specific frequency. When you turn a radio to a specific station, you're really changing the charge on a tank circuit so that it resonates at that frequency. That specific resonance is used to filter out other frequencies and only play the station you selected. The same technology is used in all sorts of communications equipment, from radio towers to walkie-talkies.