The Q-factor is called as Quality factor or storage factor. It is defined as ration of power stored in the element to the power dissipated in the element it is magnification provided by the circuit. It is defined as ratio of reactance to resistance of the reactive element. Thus for inductive reactance it is ratio of XL to R and for capacitive reactance it is XC to R. Q-meter is an instrument used to measure a electrical properties of the coil and capacitors.

Working:

The working of Q-meter is based on the characteristics of series resonant circuit. The series resonant circuit has a characteristic that the applied voltage across coil and capacitor is equal to the applied voltage time the Q-factor of the circuit.

The fixed voltage is applied to the circuit and voltage across capacitor can be calibrated to read Q-value.

In series resonant circuit when it is in resonance then XC = XL

$Q =\dfrac {X_L}{R} =\dfrac {X_C}{R} =\dfrac {E_C}{E}$

From above equation we can see that if the applied voltage E is maintained constant and at known level then a voltmeter across the capacitor can be calibrated directly interns of the circuit Q.