When current is passed through a tangent galvanometer there is no deflection of the needle . If current is reversed , the rotates by `180^(@)`. Then the plane of the coil will be .

Options

A. parallel to magnetic meridian
B. at `90^(@)` to the magnetic meridian
C. at `60^(@)` to the magnetic meridian
D. at `45^(@)` to the magnetic meridian

Explanation

To solve the problem, let's analyze the situation step by step:

### Step 1: Understanding the Tangent Galvanometer
A tangent galvanometer consists of a circular coil of wire through which current can flow. When current passes through the coil, it generates a magnetic field (B) at the center of the coil. The magnetic needle of the galvanometer aligns itself with the resultant magnetic field, which is the vector sum of the magnetic field due to the coil (B) and the horizontal component of the Earth's magnetic field (Bh).

**Hint:** Recall that the tangent galvanometer relies on the interaction between the magnetic field produced by the coil and the Earth's magnetic field.

### Step 2: No Deflection of the Needle
In the given scenario, when current is passed through the tangent galvanometer, there is no deflection of the needle. This indicates that the magnetic field (B) produced by the coil is either zero or perfectly aligned with the horizontal component of the Earth's magnetic field (Bh). Since current is flowing, B cannot be zero, so we conclude that B is parallel to Bh.

**Hint:** Think about the conditions under which the needle would not deflect. What does it imply about the relationship between B and Bh?

### Step 3: Reversing the Current
When the current is reversed, the direction of the magnetic field (B) produced by the coil also reverses. Since the needle rotates by 180 degrees, this indicates that the magnetic field B has changed direction but remains parallel to Bh.

**Hint:** Consider how reversing the current affects the direction of the magnetic field and the resultant direction of the needle.

### Step 4: Determining the Plane of the Coil
Since B is parallel to Bh, the plane of the coil must be perpendicular to Bh. This means that the coil is oriented such that the magnetic field it produces is aligned with the horizontal component of the Earth's magnetic field.

**Hint:** Visualize the orientation of the coil in relation to the Earth's magnetic field. What does perpendicularity imply about the coil's plane?

### Step 5: Conclusion
Thus, the plane of the coil is perpendicular to the horizontal component of the Earth's magnetic field (Bh), which is also known as the magnetic meridian. Therefore, the answer to the question is that the plane of the coil will be perpendicular to the horizontal component of the Earth's magnetic field.

**Final Answer:** The plane of the coil will be perpendicular to the horizontal component of the Earth's magnetic field (Bh).