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  Wheatstone Bridge

The Wheatstone bridge is a network of four resistors connected as shown in Figure 38-5, with a battery or direct-current voltage source (electromotive force) connected between A and C and a voltmeter (V) connected between B and D. The bridge is said to be “balanced” when the voltmeter reads zero potential difference between points B and D. From Ohm's law (V = IR), it is easy to show that balance occurs when Rx = Rs (R2/R1). If Rs is an adjustable standard resistor, and R1 and R2 are fixed known resistors, the balanced bridge provides a very precise means of determining Rx, the unknown resistance. This principle has many applications in biomedical engineering, including strain gauge pressure transducer measurements. In this case, the transducer itself is the unknown resistance Rx.