# 1911 Encyclopædia Britannica/Harmonic

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**HARMONIC.** In acoustics, a harmonic is a secondary tone which accompanies the fundamental or primary tone of a vibrating string, reed, &c.; the more important are the 3rd, 5th, 7th, and octave (see Sound; Harmony). A harmonic proportion in arithmetic and algebra is such that the reciprocals of the proportionals are in arithmetical proportion; thus, if *a*, *b*, *c* be in harmonic proportion then 1/*a*, 1/*b*, 1/*c* are in arithmetical proportion; this leads to the relation 2/*b* = *ac*/(*a*+*c*). A harmonic progression or series consists of terms whose reciprocals form an arithmetical progression; the simplest example is: 1 + ½ + ⅓ + ¼ + ... (see Algebra and Arithmetic). The occurrence of a similar proportion between segments of lines is the foundation of such phrases as harmonic section, harmonic ratio, harmonic conjugates, &c. (see Geometry: II. *Projective*). The connexion between acoustical and mathematical harmonicals is most probably to be found in the Pythagorean discovery that a vibrating string when stopped at ½ and ⅔ of its length yielded the octave and 5th of the original tone, the numbers, 1 ⅔, ½ being said to be, probably first by Archytas, in harmonic proportion. The mathematical investigation of the form of a vibrating string led to such phrases as harmonic curve, harmonic motion, harmonic function, harmonic analysis, &c. (see Mechanics and Spherical Harmonics).