Accurate measurement of capacitance and resistance has been interested from the past till now and many efforts were done to approach these goals. For instance, some bridges were proposed to measure the capacitances and resistances precisely such as Wheatstone bridge (measuring resistances), Schering’s bridge (Measuring capacitances with series loss), Maxwell bridge (Measuring capacitances with parallel loss), Hay bridge (measuring inductances with series loss) and Wien-Robinson bridge (measuring frequency). The dissipation factor of a capacitor is the ratio of its resistance to its capacitive reactance. The Schering Bridge is basically a four-arm alternating-current (AC) bridge circuit whose measurement depends on balancing the loads on its arms. The above figure shows a diagram of the Schering Bridge. In the Schering Bridge above, the resistance values of resistors R1 and R2 are known, while the resistance value of resistor R3 (Series resistance of unknown capacitor) is unknown. The capacitance values of C1 and C2 are also known, while the capacitance of C3 is the value being measured. To measure R3 and C3, the values of C2 and R2 are fixed, while the values of R1 and C1 are adjusted until the current through the ammeter between points A and B becomes zero. This happens when the voltages at points A and B are equal, in which case the bridge is said to be 'balanced'. In this state, one can obtain these relations: R3 = C1R2 / C2 and C3 = R1C2 / R2. Note that the balancing of a Schering Bridge is independent of frequency. There are some volumes switches that one can change the values of resistance and capacitance by turning them. After the balancing condition, the indicator will show zero. But because of the inductance of the resistances, we forced to put a capacitance in parallel with the resistances to compensate their inductances. Also for accurate measurement in high voltage tests, shielding of the circuit components is of crucial importance. |