Take three sinusoidal voltages of the same amplitude and frequency and dephase them by 120 degrees to create a balanced three-phase system. Such systems, easily generated by an alternator, distribute power in such a way that there is little loss along power lines.

Three phase systems, used all over the world, have other advantages. Running these dephased currents through coils geometrically shifted 120 degrees from each other creates a magnetic field. Remarkably, this magnetic field is "rotating". Thus a rotating magnetic field can be created from a fixed armature winding.

This opens the door to "electric" rotating machines.

Pedagogical overview of the magnetostatic theory.

The disk is charged. A force appears only when the disk rotates. As with the Oersted experiment, electrostatic theory is unable to explain the deviation of the compass. Charges are involved, but only the introduction of magnetostatic theory is able to explain this experiment.

Motion of a charged particle in a magnetic field.

The Earth is a magnetized planet. The Earth's magnetic field is like that produced by a large bar magnet. The south magnetic pole coincides with the north geographic pole.

Note how a compass remains tangential to the field lines at every point.

The orientation of the field lines is determined by the right hand rule.

The magnetic field of an infinite, current carrying wire is determined for any point in space, by applying the principle of superposition of magnetic fields.

Symbolic representation of Bainbridge's mass spectrometer (1933). Used to evaluate the masses of ions of the same charge.
Ejected from a collimator not shown here, the ions are first selected by their speed (v=E/B) then separated by mass. The device can also be used to separate different isotopes of the same element.