What is HARMONIC in an electrical circiuts?

Question

How can we reduce Harmonics?

Answer 1

Use capacitors  

Answer 2

Harmonic distortion, which I think you are asking about, is the following:

Apply a pure sine wave to the input of a circuit or amplifier. Adjust the frequency, gain, and amplitude to those specified. At the output, use filters to separate the fundamental from it's harmonics, where the fundamental is the frequency of the input signal. The harmonics are everything left over after the fundamental is removed.

Measure those and compare it to the signal that was removed, and that is the harmonic distortion. (Actually harmonic distortion plus noise).

Harmonic distortion is unwanted signals, frequencies, that come about due to non-linearities in the amplifier. Every amplifier has non-linearities, due, for examples, to non-linear CE curves of a transistor.

They can be reduced by negative feedback or by careful design, or by both.

.

Answer 3

Harmonics are multiples of the FUNDAMENTAL frequency.  For power in the US, the fundamental frequency is 60HZ. 120 Hz would the the second harmonic.  Harmonics come from non-linear loads, or elements, in the circuit; which deform the sine wave.  These deformations can be reduced to a combination of the fundamental, plus harmonic frequencies.