Solenna

Meet Solenna, a coil of wire who turns current into a strong, even magnetic field.

A current in a wire makes a magnetic field that circles the wire. Wind the wire into a long, tightly packed coil, a solenoid, and the fields of all the turns add up inside. Ampère's law shows that the field inside a long solenoid is nearly uniform, with strength B = μ₀nI, where n is the number of turns per metre and I is the current. Outside, the field is much weaker.

Take 1000 turns per metre carrying 2 A. Then B = 1.2566 × 10⁻⁶ × 1000 × 2 ≈ 2.5 × 10⁻³ T, about 2.5 millitesla, roughly fifty times Earth's surface field.

Solenoids are in door locks, relays, valves and loudspeakers. Large superconducting solenoids produce the steady fields of MRI scanners.

The formula is for an ideal, very long coil with an air core. Near the ends the field is weaker, and an iron core can multiply the field many times until the iron saturates.