An electronic device that produces an alternating output of known frequency. If the output voltage or current has the form of a sine wave with respect to time, the device is called a sinusoidal (or harmonic) oscillator. If the output voltage changes abruptly from one level to another (as in a square wave or sawtooth waveform) it is called a relaxation oscillator. A harmonic oscillator consists of a frequency-determining circuit or device, such as a resonant circuit, maintained in oscillation by a source of power that by positive feedback also makes up for the resistive losses. In some relaxation oscillators the circuit is arranged so that in each cycle energy is stored in a reactive element (a capacitor or inductor) and subsequently discharged over a different time interval. See also multivibrator.