mathematical procedure for describing and analyzing phenomena of a periodically recurrent nature. Many complex problems have been reduced to manageable terms by the technique of breaking complicated mathematical curves into sums of comparatively simple components.

Many physical phenomena, such as sound waves (sound), alternating electric currents (alternating current), tides (tide), and machine motions and vibrations (vibration), may be periodic in character. Such motions can be measured at a number of successive values of the independent variable, usually the time, and these data or a curve plotted from them will represent a function of that independent variable. Generally, the mathematical expression for the function will be unknown. However, with the periodic functions found in nature, the function can be expressed as the sum of a number of sine and cosine terms. Such a sum is known as a Fourier series, after the French mathematician Joseph Fourier (Fourier, Joseph, Baron) (1768–1830), and the determination of the coefficients of these terms is called harmonic analysis. One of the terms of a Fourier series has a period equal to that of the function, f(x), and is called the fundamental. Other terms have shortened periods that are integral submultiples of the fundamental; these are called harmonics. The terminology derives from one of the earliest applications, the study of the sound waves created by a violin (see analysis: Musical origins (analysis) and Fourier analysis (analysis)).

In 1822 Fourier stated that a function y=f(x) could be expressed between the limits x=0 and x=2π by the infinite series that is now given in the form

The use of a larger number of terms will increase the accuracy of the approximation, and the large amounts of calculations needed are best done by machines called harmonic (or spectrum) analyzers; these measure the relative amplitudes of sinusoidal components of a periodically recurrent function. The first such instrument was invented by the British mathematician and physicist William Thomson (later Baron Kelvin (Kelvin, William Thomson, Baron)) in 1873. This machine, used for the harmonic analysis of tidal observations, embodied 11 sets of mechanical integrators (integrator), one for each harmonic to be measured. A still more complicated machine, handling up to 80 coefficients, was designed in 1898 by the American physicists Albert Abraham Michelson (Michelson, A.A.) and Samuel W. Stratton.

Early machines and methods made use of an experimentally determined curve or set of data. In the case of electric currents or voltages, an entirely different method is possible. Instead of making an oscillographic record of the voltage or current and analyzing it mathematically, the analysis is performed directly on the electric quantity by recording the response as the natural frequency of a tuned circuit is varied through a wide range. Thus, harmonic analyzers and synthesizers of the 20th century tended to be electromechanical rather than purely mechanical devices. Modern analyzers display the frequency-modulated signals visually by means of a cathode-ray tube, and digital or analog computer principles are used to carry out the Fourier analysis automatically, thereby achieving approximations of great accuracy.