Current makes
circles in the air
A straight current-carrying wire wraps space in closed loops. There is no start and no end — nothing to leave from, nothing to land on. Drag the wires together and the loops merge into shapes neither one has alone.
whatCurrent running up a wire, and the closed loops of magnetic field it wraps around itself. Loops are traced at five heights so you can see the tube of field, not just a slice.
lawB = μ₀I/2πr, circling the wire by the right-hand rule. Field lines close on themselves: there is no magnetic charge for them to start or end on.
trySwitch on the second wire, then flip its current.
noticeOpposed currents pinch the loops into a figure-eight with a neutral point between the wires; aligned currents merge them into one wide loop enclosing both.
in lifeClamp meters read the current in a cable by measuring the loops around it, without breaking the circuit.
found byØrsted found the effect by accident in 1820; Biot, Savart and Ampère had it quantified within months.