Text: Oscillations to which a circuit will respond in addition to the basic or fundamental oscillations. The effect is similar to that in sound, where, for instance, the primary or fundamental tone struck by an instrument may be accompanied by notes or tones of a higher pitch. In sound these harmonics are principally the third, fifth, seventh and the octave. In radio, a circuit may respond to oscillations of a frequency either higher or lower than the fundamental frequency, as, for example, one third, one fifth or one seventh, or, again, double or treble the original frequency. As a concrete example with a fundamental wave-length of 240 meters, the harmonics might be 40 meters or 80 meters. The corresponding frequencies would be approximately 1,250,000 cycles, 7,500,000 cycles and 3,750,000 cycles.

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Source: 190