Encircla

Meet Encircla, the magnetic field that wraps in rings around every current-carrying wire.

In 1820 Hans Christian Ørsted saw a compass needle swing near a current. A long straight wire turns out to be surrounded by circular field lines. Ampère's law says that the field added up around any closed loop equals μ₀ times the current threading the loop. For a long straight wire, symmetry makes the field the same at every point on a circle of radius r, giving B = μ₀I/(2πr).

Example: 10 amperes at 1 centimetre gives B ≈ 2×10⁻⁷ × 10 / 0.01 = 2×10⁻⁴ tesla, a few times Earth's field. At 10 cm it is ten times weaker.

The direction follows the right-hand rule: point your thumb along the current and your fingers curl the way the field goes. Electromagnets and current clamps that measure current without cutting the wire both rely on this field.

The simple formula is for an infinitely long, straight wire. Near the ends of a real wire, or near other currents, the field differs.