Frequency meters are instruments that provide analog outputs as frequencies that vary as a function of the applied input. They display values on a dial, usually with a needle or moving pointer. Frequency meters measure repetitions per unit of time, usually per second, of a complete electromagnetic waveform. Most devices are deflection meters that balance two opposing forces and indicate changes in balance. There are two basic types of defection meters: moving-coil meters and ratio meters. Moving-coil meters are electrically resonant circuits that consist of two coils tuned to different frequencies and connected at right angles to one another. Frequencies in the middle of the range equalize the currents in the two coils and allow the needle or pointer to indicate the midpoint of the scale. Changes in frequency create an imbalance between these currents, causing both the coils and the needle or pointer to move. Ratio meters use two frequency inputs: a known standard and an unknown amount. Frequency ratios are often expressed as Lissajous figures, a common means of comparative frequency measurement in electronics.