Wheatstone bridge is the circuit setup made up of four resistances as shown in the figure which is generally used to find out the unknown resistance. Wheatstone bridge experiment was first conducted by Samuel Hunter Christie in the year 1833.
The concept of wheatstone bridge was introduced in order to measure the unknown resistance with the help of two known and one variable resistance.
The circuit has much broad and important applications such as measuring the intensity of light and many other quantities.
Principle of Wheatstone bridge experiment
According to the wheatstone bridge principle if the ratio of the two resistance (R1/R3) on the one edge is equal to the ratio of the two resistance (R4/R2) in another edge then there will be no flow of current between the midpoints of the two edges of resistance.
Wheatstone Bridge basically has four resistances that are arranged in diamond like shape consisting a resistance in each of the edges.
As shown in figure the two conjunctions of edges are connected to the voltage supply and the midpoint of two opposite edges are connected to any current flow detecting device (it could be galvanometer, multimeter etc) in order to detect the presence of electric current between the two points.
Components of Wheatstone bridge experiment
- Resistor with an unknown resistance value.
- Two resistors (with known resistance value)
- Variable Resistor (device like Rheostat or Preset could work)
- Voltage/Power source
- Galvanometer (or any device which indicates)
- Connecting Wires
Working of Wheatstone bridge
Analysis of a wheatstone bridge circuit can be done very easily. In order to measure the unknown resistance in the circuit a variable resistance is connected in series to the unknown resistor and the two resistor with known values are connected on the other edge.
After the setup for wheatstone bridge has been done, The variable resistance is varied till there is no current detection in between the mid point of the two edges.
R1/R3 = R4/R2
Assume that R4 and R2 are the known resistors you have used, resistance value of the variable resistor is measured. The unknown resistance will be R1 and the variable resistor value would be R3 or vice-versa according to the perspective.
R1 = Unknown resistor
R3 = Variable Resistance value
R2 and R4 are known resistors.
R1/R3 = R4/R2
i.e. R1 = R3 (R4/R2)
In the wheatstone bridge experiment, one resistor should always be variable in order to obtain a balanced condition. The Circuit performs at its best when the regulated voltage source is used, instead of the current with repel characteristics,
After getting the values of the three resistors they are used in an equation above in order to find the unknown resistance value. Balanced or null condition (when there is no voltage difference or flow of current) in between the midpoints of the two edges is necessary to find the value of unknown resistance through wheatstone bridge.
Applications of Wheatstone bridge
The circuit has many application, you might have been using a device which have been using the setup. There are literally too many devices using the circuit, you can very easily spot wheatstone bridge application in daily life.
Measuring Strain with the help of Wheatstone bridge
The Wheatstone Bridge circuit is used to measure the strain.
In order to measure the strain,
The strain gauges are used in the place of variable resistance. The electrical resistance of the strain gauges varies with the increase or decrease in the strain of the device. The range of 30 ohms to 3000 ohms in the variable resistance end on the device is used in order to obtain the balance point of the circuit.
In order to measure strain the wheatstone bridge circuit is used. The setup is very helpfull when it comes to measure the fraction of changes in resistance with high accuracy.
Role of Wheatstone bridge in Light Detectors
The Wheatstone Bridge is used in a Light Detecting sensors.
In this application one edge consists of the two fixed resistances and the other edge has photoresistive sensors (LDR) and a potentiometer (Variable resistor).
The resistance or LDR does varies as the intensity of light varies, With the help of potentiometer we can define that at how much intensity of light there will be the balance point.
There are other bonus conditions as well such as before and after what amount of light the current flows in a positive as well as negative direction from one edge to another.
Other Applications of Wheatstone bridge
- The circuit is used in the light detecting devices.
- For measuring the changes in the pressure.
- For measuring the changes in the strain of the circuit.
- They are also used for sensing mechanical and electrical quantities.
- These circuits can also be used in devices like photoresistive devices.
- Wheatstone bridges are also used in the thermometers for the temperature measurements which need to be accurate.