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The Wheatstone Bridge

History

The circuit we now know as the Wheatstone Bridge was actually first described by Samuel Hunter Christie (1784-1865) in 1833. However, Sir Charles Wheatstone invented many uses for this circuit once he found the description in 1843. As a result, this circuit is known generally as the Wheatstone Bridge.

To this day, the Wheatstone bridge remains the most sensitive and accurate method for precisely measuring resistance values.



The Basic Bridge Circuit

The basic Wheatstone Bridge.

The fundamental concept of the Wheatstone Bridge is two voltage dividers, both fed by the same input, as shown to the right. The circuit output is taken from both voltage divider outputs, as shown here.

In its classic form, a galvanometer (a very sensitive dc current meter) is connected between the output terminals, and is used to monitor the current flowing from one voltage divider to the other. If the two voltage dividers have exactly the same ratio (R1/R2 = R3/R4), then the bridge is said to be balanced and no current flows in either direction through the galvanometer. If one of the resistors changes even a little bit in value, the bridge will become unbalanced and current will flow through the galvanometer. Thus, the galvanometer becomes a very sensitive indicator of the balance condition.



Using the Wheatstone Bridge

Using the Wheatstone Bridge.

In its basic application, a dc voltage (E) is applied to the Wheatstone Bridge, and a galvanometer (G) is used to monitor the balance condition. The values of R1 and R3 are precisely known, but do not have to be identical. R2 is a calibrated variable resistance, whose current value may be read from a dial or scale.

An unknown resistor, RX, is connected as the fourth side of the circuit, and power is applied. R2 is adjusted until the galvanometer, G, reads zero current. At this point, RX = R2×R3/R1.

This circuit is most sensitive when all four resistors have similar resistance values. However, the circuit works quite well in any event. If R2 can be varied over a 10:1 resistance range and R1 is of a similar value, we can switch decade values of R3 into and out of the circuit according to the range of value we expect from RX. Using this method, we can accurately measure any value of RX by moving one multiple-position switch and adjusting one precision potentiometer.



Applications of the Wheatstone Bridge

It is not possible to cover all of the practical variations and applications of the Wheatstone Bridge, let alone all types of bridges, in a single Web page. Sir Charles Wheatstone invented many uses himself, and others have been developed, along with many variations, since that time. One very common application in industry today is to monitor sensor devices such as strain gauges. Such devices change their internal resistance according to the specific level of strain (or pressure, temperature, etc.), and serve as the unknown resistor RX. However, instead of trying to constantly adjust R2 to balance the circuit, the galvanometer is replaced by a circuit that can be calibrated to record the degree of imbalance in the bridge as the value of strain or other condition being applied to the sensor.

In many cases, the galvanometer is replaced by an instrumentation amplifier. This assists with callibration, and also allows different scales to be used.

A second application is used by electrical power distributors to accurately locate breaks in a power line. The method is fast and accurate, and does not require a large number of field technicians.

Other applications abound in electronic circuits. We'll see a number of them in action as these pages continue to expand.


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