Write an expression for mutual inductance of two co-axial solenoids.

Explanation

To derive the expression for the mutual inductance of two coaxial solenoids, we will follow these steps:

### Step 1: Define the Parameters
Let:
- \( N_1 \) = Number of turns in the first solenoid
- \( N_2 \) = Number of turns in the second solenoid
- \( L_1 \) = Length of the first solenoid
- \( L_2 \) = Length of the second solenoid
...