How do magnets work? Poles, fields, domains, and what a U-magnet does

By Eugenio TommasiUpdated September 20267 min read

A magnet works because the electrons in iron, nickel, and cobalt carry tiny magnetic moments that line up in regions called domains. When enough domains point the same way, the piece has a north pole, a south pole, and a field around it. Every magnet has both poles. Currents in wires make the same kind of field.

Figure 1 — U-magnet, open and closed. A: U-magnet ① with north pole ② and south pole ③, field lines ⑤ through the air. B: soft-iron keeper ④ across the poles; the flux path ⑥ closes inside the iron.

This is the physics reference behind our guides to Edward Leedskalnin's pamphlets, which start at Coral Castle: what is documented and what he claimed. Textbook magnetism only, one source per number.

What is a magnetic field?

A magnetic field is the region around a magnet or a current where a moving charge feels a sideways force. HyperPhysics defines the field B "in terms of force on moving charge in the Lorentz force law," and the force is perpendicular to both velocity and field.

The SI unit is the tesla; the gauss, still used on magnet packaging, is 10,000 times smaller (HyperPhysics). Earth's field at the surface is roughly 25,000 to 65,000 nanotesla, or 0.25 to 0.65 gauss (NOAA NCEI).

Fields come from moving charge: "macroscopic currents in wires" and "microscopic currents" in atoms (HyperPhysics). The first is the electromagnet; the second is the permanent magnet, whose currents are the orbital motion and spin of electrons (HyperPhysics).

Why can't a north pole be separated from a south pole?

Because a magnetic source is a dipole, not two charges glued together. HyperPhysics states it directly: "Magnetic sources are inherently dipole sources - you can't isolate North or South 'monopoles'" (HyperPhysics).

The bench test is simple. Break a bar magnet in half and each half is a complete magnet with a north and a south pole (Wikipedia). Break the halves again and you have four magnets. The poles are not substances stored at the ends. They are the two directions of one field that closes on itself.

Wikipedia adds that "the concept of poles should not be taken literally." A north pole attracts a south pole and repels another north pole. That rule, plus the dipole rule, covers every hanging-magnet experiment in the 1945 pamphlet.

What are magnetic domains, and which materials are ferromagnetic?

Domains are small regions of a ferromagnetic material in which the electron spins already point the same way. HyperPhysics describes an ordering that "causes the unpaired electron spins to line up parallel with each other in a region called a domain," 0.1 mm to several millimeters across (HyperPhysics).

In an unmagnetized bar the domains point in random directions, so it shows no net field outside. An external field turns them one way and the bar becomes a magnet. Hard alloys keep that alignment when the field is removed; the retained magnetization is called remanence (HyperPhysics). Soft iron loses most of it as soon as the field is gone.

The ferromagnetic elements are iron, nickel, cobalt, gadolinium, and dysprosium. Above the Curie temperature, about 1043 K (770 °C, 1418 °F) for iron, the domains dissolve (HyperPhysics; Wikipedia).

What does a U-magnet do, and why does a keeper "hold"?

A U-magnet, or horseshoe magnet, is a bar magnet bent so both poles face the same way (Figure 1, A ①②③⑤). With the poles close together, the flux takes "a more direct path between the poles," which "concentrates the magnetic field" (Wikipedia).

The keeper ④ is a bar of soft iron laid across the pole faces. With it on, the flux ⑥ runs through iron all the way around: pole, keeper, pole, magnet body. Wikipedia gives the reason keepers ship with horseshoe magnets: a field "holds its strength best when the entire magnetic field is given the ability to loop through a ferromagnetic substance instead of air." Pulling the keeper off takes force because it opens that circuit; nothing is stored in the keeper.

Leedskalnin's perpetual motion holder is this circuit built on an electromagnet. In Magnetic Current (1945) he writes: "Now the electric magnet holds perpetual motion. If not disturbed it will last indefinitely." Build notes and the standard explanation are in the Perpetual Motion Holder guide.

How does an electromagnet work, and what is Faraday's law?

An electromagnet is a coil of wire around an iron core. Current in the coil makes a field along its axis, and the core multiplies it. HyperPhysics gives the field inside a solenoid as B = μ0 μr n I, with n the turns per unit length and I the current (HyperPhysics). The core's domains align with that field and raise it "by factors of tens to even thousands" (HyperPhysics).

Figure 2 — coil on a nail. Iron nail ① with magnet wire ②, battery ③, momentary contact ④, compass ⑤ near the point. Arrow ⑥ marks the current; curled arrow ⑦ the right-hand rule and the north end.

Faraday's law is the reverse process. HyperPhysics states it in one line: "The induced emf in a coil is equal to the negative of the rate of change of magnetic flux times the number of turns in the coil" (HyperPhysics). Move a magnet or the coil and a voltage appears; a fast pull gives a bigger pulse than a slow one.

Leedskalnin's version, side by side

Four of Leedskalnin's claims in his words, beside the textbook account and its source.

The disagreement is about what the observations mean.