The National MagLab is funded by the National Science Foundation and the State of Florida.

# Wheatstone Bridge

This circuit is most commonly used to determine the value of an unknown resistance to an electrical current.

The Wheatstone bridge was first described by British mathematician and scientist Samuel Hunter Christie in 1833. The circuit came to bear the name of Sir Charles Wheatstone, the English physicist who popularized it in the 1840s.

In a typical Wheatstone bridge, four resistors are positioned in a circuit designed in such a way that the current from a battery splits, flows through the sequence of resistors, then recombines into a single conductor.

Three of these resistors have known values, one of which is adjustable. The value of the fourth resistor is not known. By studying and manipulating the paths the current can take through the Wheatstone bridge grid, that fourth unknown resistance can be determined.

## Instructions

1. Note the current flowing from negative to positive through the circuit, denoted by the brown arrows.
2. See how the current splits when it reaches Point A and travels through either one of the two known resistors, R1 or R2. Resistance is measured in a unit called an ohm. Notice that when this interactive animation initializes, the resistance at R1 is 1 K ohm, while at R2 it is 5 K ohm.
3. Observe what happens after the diverging currents pass through their respective resistors (R1 or R2), and reach another fork in the road. Some or all of the current from the R1 or R2 paths will diverge down the middle path that bisects the square, which is connected to a meter measuring current in amps.
The direction of this current is determined by the value of the variable resistor (R3).
4. Adjust the slider for the variable resistor, and see how that affects the current’s direction and the reading on the amp meter.
5. Now, carefully adjust the slider so that no current flows down the middle path. When that is achieved, the amp meter reads zero and the bridge is balanced.
6. If you were successful in balancing the bridge, you will notice an equation appearing beneath the tutorial. If you don't see it, press the solve bridge button.
7. The equation shows how the value of the variable resistor in the balanced bridge can be used to calculate the unknown resistance at R4:

R1/R2 = R3/R4

or

R4 = (R2 * R3) /R1

You can experiment with different values and see the corresponding calculations by hitting the reset button and balancing the bridge or clicking solve bridge.