**Introduction:**

In electrical engineering, voltmeter is an instrument which measures the electrical potential difference between two points in an electric circuit. The spring controlled, electrodynamic voltmeter is one type of voltmeter which operates on the principle of electromagnetic induction.

**Working Principle:**

The spring controlled, electrodynamic voltmeter works on the principle of electromagnetic induction. When an unknown voltage is applied across the terminals of the voltmeter, it induces a current in the coil which is proportional to the applied voltage. The direction of the current is determined by the direction of the magnetic field produced by the coil.

The magnetic field produced by the coil interacts with the magnetic field produced by the permanent magnet. This interaction causes the coil to rotate, which is opposed by the spring. The deflection of the coil is proportional to the applied voltage.

**Specifications:**

- Voltage range: 50V
- Full scale deflection: 90°
- Current for full scale deflection: 0.05A DC
- Scale response: Square low scale response

**Explanation:**

The voltmeter has a voltage range of 50V, which means it can measure voltages up to 50V. The full scale deflection of the voltmeter is 90°, which means that when the maximum voltage is applied to the voltmeter, the deflection of the coil will be 90°.

The voltmeter has a square low scale response, which means that the scale is marked in such a way that the deflection is proportional to the square of the applied voltage.

The voltmeter takes a current of 0.05A DC for full scale deflection. This means that when the maximum voltage is applied to the voltmeter, the current flowing through the coil will be 0.05A DC.

**Conclusion:**

The spring controlled, electrodynamic voltmeter is an instrument which operates on the principle of electromagnetic induction. It is used to measure the electrical potential difference between two points in an electric circuit. The specifications of the voltmeter include voltage range, full scale deflection, current for full scale deflection and scale response.