Illustrated Physics
physics, drawn
Concept

Gyroscope — where it appears

A spinning body whose angular momentum holds its axis fixed in space unless a torque acts on it. Gyroscopes steer ships and spacecraft, and superconducting ones in orbit measured the drift of their axes predicted by general relativity.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

A gyroscope swinging into the meridian. The direction of a gyrocompass's axis, in degrees east of true north, against time in hours, released 20° east of north at latitude 50°. The Earth's rotation tilts the horizon under the spinning axis, a pendulous weight turns the tilt into a torque, and the torque precesses the axis towards the meridian. Undamped (dashed) it swings through north and back with a period of 84.4 minutes, the compass having been built to that period. With damping it settles: after 8 hours it points within 0.03° of north. Nothing in the instrument knows where north is. It finds the direction about which the Earth turns, because only when its axis lies in the meridian does the turning horizon stop tilting it.

The compass that finds the axis the Earth turns about

A spinning wheel held so that its axis stays level will, on a turning Earth, swing its axis round to point at true north and stay there — not magnetic north, the geographic pole. Nothing in the instrument knows where north is; it finds the one direction about which the Earth's turning stops tilting it. Built to be used at sea, such a compass has to ignore the ship's lurches, and that forces its swing to have one particular period: 84.4 minutes, the period of a pendulum as long as the Earth's radius, of a satellite skimming the ground, and of a stone dropped through a tunnel to the far side of the planet.

mechanics · Rotation Rotation is a magnetic field, and a superconductor cancels it. Magnetic fields along the spin axis inside a spinning metal, in units of 2mω/e, as seen by its electrons in the frame turning with it. In that frame the Coriolis force on an electron is exactly the force a magnetic field of 2mω/e would exert, pointing against the spin for a negative charge (Larmor's theorem). A normal metal spun in no field has no real field inside, and its electrons feel the rotation's equivalent field in full. A superconductor will not let its carriers feel any field in their interior — that is the Meissner effect — so it generates a real field of exactly the same size pointing the other way, and the sum vanishes. The real field is the London moment.

The field a spinning superconductor makes of itself

Spin a lump of superconductor with no magnet anywhere near it, and a magnetic field appears inside it, pointing along the axis of spin: about eleven trillionths of a tesla for every radian per second, the same in lead, niobium or a copper-oxide ceramic. The field is not left over from anything. In a turning frame the Coriolis force on a moving charge is indistinguishable from a magnetic force, a superconductor will not let its carriers feel any field inside it, and so it makes a real field to cancel the one rotation imitates. The size depends on nothing but the carriers' mass and charge, which makes the field a balance for weighing them — and the one time it was read to a part in a hundred thousand, it gave an answer theory has not matched.

electromagnetism · Superconductivity

Named alongside it

The objects these essays reach for when they reach for this one.

Rotating frameAngular momentumCooper-pairCoriolis forceEarth rotationInertial navigationLondon momentMagnetic fieldMeissner effectPendulumPenetration depthPrecession

All concepts