The displacement is measured using strain gauges, and the number of strain gauges required is four. This can be explained based on the working principle of strain gauges and the need for accurate displacement measurement.

1. Working Principle of Strain Gauges:
Strain gauges are devices that measure the strain or deformation of an object when subjected to external forces. They work on the principle that when an object undergoes deformation, its resistance changes proportionally. Strain gauges consist of a thin wire or foil that is attached to the surface of the object. As the object deforms, the wire or foil also deforms, leading to a change in resistance.

2. Measurement of Displacement using Strain Gauges:
To measure displacement using strain gauges, they are attached to the object in a specific arrangement. When the object undergoes displacement, the strain gauges experience strain, which causes a change in their resistance. By measuring this change in resistance, the displacement can be determined.

3. Bridge Configuration:
To obtain accurate displacement measurements, strain gauges are usually connected in a bridge configuration called a Wheatstone bridge. A Wheatstone bridge consists of four strain gauges connected in a specific arrangement. The strain gauges are arranged in two pairs, with each pair connected in a series and the two pairs connected in parallel.

4. Balancing the Bridge:
In the Wheatstone bridge configuration, the objective is to balance the bridge by adjusting the resistance of one of the strain gauges. This is done by applying a known reference voltage to the bridge and adjusting the resistance until the bridge is balanced. When the bridge is balanced, the output voltage is zero, indicating no displacement.

5. Measurement of Displacement:
When the object undergoes displacement, the strain gauges experience strain, causing a change in their resistance. This unbalances the Wheatstone bridge, resulting in a non-zero output voltage. By measuring this output voltage, the displacement can be determined.

6. Four Strain Gauges:
To accurately measure displacement using strain gauges, four strain gauges are required. The use of four strain gauges allows for better compensation of temperature effects, improves accuracy, and provides redundancy in case one of the strain gauges fails.

In conclusion, when measuring displacement using strain gauges, four strain gauges are normally required. This allows for accurate and reliable measurement by using a Wheatstone bridge configuration and compensating for temperature effects.