Magnetism

Step 1: Magnetic field inside a solenoid.
The magnetic field inside a long solenoid is given by: where is the permeability of free space, is the number of turns per unit length, and is the current.
Step 2: Magnetic field outside a solenoid.
The magnetic field outside a solenoid decreases with distance from the solenoid. The ratio of the magnetic field at a point on the axis of the solenoid to a point from the axis is proportional to the inverse of the distance. Thus, the ratio is .
Step 3: Conclusion.
The ratio of the magnetic fields is 4.