APPENDIX 3: Wheatstone Bridge

The Wheatstone bridge is a network of four resistors connected as shown in Figure 30-36 , 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 R_x = R_s × (R₂ /R₁ ). If R_s is an adjustable standard resistor and R₁ and R₂ are fixed known resistors, the balanced bridge provides a very precise means of determining R_x , 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 R_x .