The Wheatstone bridge, invented in 1833, measures impedance using two fixed resistors and one variable resistor. Modern impedance bridges have multiple circuits and can test various electrical devices. AC circuits are more complex due to capacitor leakage, which can be solved with a Wagner ground loop.
A modern impedance bridge is a device used to measure impedance, capacitance or inductance. The grandfather of the generalized impedance bridge is the Wheatstone bridge, an electrical circuit conceived by Samuel Christie in 1833 and popularized a decade later by Sir Charles Wheatstone. Wheatstone bridges, with their characteristic diamond circuit patterns, use two fixed impedance resistors and one variable resistor to measure the unknown impedance of a fourth resistor.
The Wheatstone bridge is a direct current (DC) device. A modern device can be switched from DC to alternating current (AC) and vice versa. Both the Wheatstone bridge and its modern descendant, the generalized impedance bridge, work on the same principle, which is the balancing of an electrical circuit. The basic idea is that if a circuit contains two resistors of known value, a variable resistor and a resistor of unknown value can be used to determine the value of the unknown resistor, and a measuring device known as a potentiometer is used to ascertain when the variable resistor has reached the equilibrium point for the circuit. In equation form, R1/R2 = R(u)/ Rvar where R1 and R2 are resistors of known value, R(u) is the resistor with unknown impedance and Rvar is the variable resistor with a display to show its impedance in any point in time.
Modern impedance bridges have several circuits. One circuit is likely to be the original test circuit, with a reading in Ohms, also known as an ohmmeter. Other circuits may include different combinations of resistors, capacitors, inductors, and perhaps a signal generator and power source. With these circuits, the modern impedance bridge can test many electrical circuits and devices, from resistors and capacitors to tuning an antenna. AC circuits are a little more complicated than DC circuits because the circuit doesn’t balance until the phase of the AC is the same on both sides of the bridge.
A potential problem for sensitive AC bridge circuits is that capacitors tend to “leak” current. Stray current from capacitor leakage will generate erroneous readings on the detector or meter. One solution to the problem is to add what is known as a Wagner ground loop. The Wagner ground loop is a grounded voltage divider designed to have the same voltage ratio and phase shift as each side of the bridge. Often there is a two position switch to allow the user to confirm that the circuit has been set up correctly and when the potentiometer registers zero in both switch positions an error free reading is guaranteed.
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