Before using tangent galvanometer for the measurement of current, why is the plane of coil of tangent galvanometer set in the magnetic meridian?

Options

A. to avoid the magnetic effect of the earth's field
B. to produce intense magnetic field at the centre of the coil
C. to produce a field at right angle to the earth's field
D. to avoid error due to parallax

Answer

To understand why the plane of the coil of a tangent galvanometer is set in the magnetic meridian before measuring current, we can break down the explanation into several steps:

### Step-by-Step Solution:

1. **Understanding the Tangent Galvanometer**:
- A tangent galvanometer consists of a coil of wire through which current is passed. When current flows through the coil, it produces a magnetic field at its center.

2. **Principle of Operation**:
- The operation of the tangent galvanometer is based on the tangent law of magnetism. According to this law, the tangent of the angle (θ) that the magnetic needle makes with the magnetic meridian is proportional to the ratio of the magnetic field produced by the coil (B) to the horizontal component of the Earth's magnetic field (Bh).

3. **Setting the Coil in the Magnetic Meridian**:
- The magnetic meridian is the direction of the Earth's magnetic field. By setting the plane of the coil in the magnetic meridian, we ensure that the magnetic field produced by the coil is perpendicular to the horizontal component of the Earth's magnetic field.

4. **Condition for Accurate Measurement**:
- For accurate measurements, the magnetic field generated by the coil (B) must be at right angles to the horizontal component of the Earth's magnetic field (Bh). This condition allows us to use the relationship B = Bh tan(θ) effectively.

5. **Conclusion**:
- Therefore, the plane of the coil is set in the magnetic meridian to produce a magnetic field at right angles to the Earth's magnetic field. This alignment is crucial for the accurate measurement of current using the tangent galvanometer.

### Final Answer:
The plane of the coil of the tangent galvanometer is set in the magnetic meridian to produce a magnetic field at right angles to the Earth's magnetic field.