A Wheatstone bridge is a type of electrical circuit known as a bridge circuit and is used to determine the resistance of a circuit element. This can be used to test the resistance of various components such as resistors, sections of wire, and any other electrical conductor. A typical bridge circuit of the Wheatstone type uses four resistors split into two legs. By balancing one leg of the circuit with a fixed, known resistance against the one with the unknown resistor, it is possible to calculate the resistance of the component being tested.
Like many inventions and techniques, this type of testing circuit is not named for the person who first invented it but for the person who developed and perfected it. It was first developed in the early 19th century by a man named Samuel Christie, but it was made popular by Sir Charles Wheatstone, by whose name it is now known. The principles by which a Wheatstone bridge circuit operates and the methods for which it is used have not changed significantly since its invention, although many refinements and variations have been developed.
In its most basic form, a Wheatstone bridge consists of an electric circuit with two legs, an electric input source with a constant voltage, four resistors, including two with a fixed, known resistance, one with a variable resistance that can be monitored as it changes, and the resistor to be tested. A voltage gauge is also necessary. This basic configuration allows for any conductive material to be tested to determine its resistance to a very high degree of precision.
The circuit is designed to split the incoming current into two legs, which are bridged together. Each leg has two resistor elements. One resistor of each leg has a fixed, known resistance. On one leg, this resistor is paired with a variable resistor, and there must be a means of monitoring the resistance as it changes. On the other leg, the resistor with the known resistance is paired with the component to be tested. When the current is introduced, the variable resistor is manipulated until the voltage gauge registers a zero current flow, meaning that the resistances of each leg are exactly identical.
By checking the resistance setting of the variable resistor, it is then possible to easily calculate the resistance of the test component. It is possible to construct a Wheatstone bridge using three resistors with fixed resistances as long as all the values are known. The calculations involved in finding the resistance of the test component are more complex when using a Wheatstone bridge of this type, but the basic principles involved are the same.