A tangent galvanometer shows no deflection when a current is passed through it. But when the current is reversed, it gives a delfection of 180°, then the plane of the coil should have been oriented:.

Options

A. in the magnetic meridian
B. normal to magnetic meridian
C. at 45° to the magnetic meridian
D. at angle 60° the magnetic meridian

Answer

To solve the problem, we need to analyze the behavior of the tangent galvanometer when a current is passed through its coil and how it interacts with the Earth's magnetic field.

### Step-by-Step Solution:

1. **Understanding the Tangent Galvanometer**:
A tangent galvanometer consists of a coil of wire through which current flows, creating a magnetic field. The device is used to measure the strength of electric currents by observing the deflection of a magnetic needle.

**Hint**: Recall the basic principle of how a tangent galvanometer operates, particularly the relationship between the magnetic field produced by the coil and the Earth's magnetic field.

2. **Initial Condition (No Deflection)**:
The problem states that the galvanometer shows no deflection when current is passed through it. This indicates that the magnetic field produced by the coil is perfectly balanced by the horizontal component of the Earth's magnetic field.

**Hint**: Consider what it means for the needle to show no deflection. What does this imply about the relationship between the coil's magnetic field and the Earth's magnetic field?

3. **Possible Situations**:
There are two scenarios that could lead to no deflection:
- The magnetic field due to the coil is zero (not possible since current is flowing).
- The coil is oriented such that its magnetic field is perpendicular to the horizontal component of the Earth's magnetic field.

**Hint**: Think about the orientation of the coil and how it affects the resultant magnetic field.

4. **Conclusion from No Deflection**:
Since the first situation is not possible, we conclude that the plane of the coil must be oriented perpendicular to the horizontal component of the Earth's magnetic field. This means the coil is in a position where the magnetic field it generates does not cause any deflection of the needle.

**Hint**: Visualize the setup. If the coil is perpendicular to the horizontal magnetic field, what will happen to the resultant magnetic field?

5. **Reversing the Current**:
When the current is reversed, the direction of the magnetic field produced by the coil also reverses. Since the horizontal component of the Earth's magnetic field remains unchanged, the needle will now deflect 180 degrees.

**Hint**: Consider how reversing the current affects the direction of the magnetic field produced by the coil.

6. **Final Orientation of the Coil**:
The problem asks for the orientation of the plane of the coil. Since the coil must be oriented such that its normal is perpendicular to the magnetic meridian (the direction of the horizontal component of the Earth's magnetic field), we conclude that the plane of the coil is normal to the magnetic meridian.

**Hint**: Reflect on the definition of the magnetic meridian and how the orientation of the coil relates to it.

### Final Answer:
The plane of the coil should have been oriented **normal to the magnetic meridian**.