Field coil

These include simple electromagnets through to complex lab instruments such as mass spectrometers and NMR machines.

Single-phase AC motors may follow either of these patterns: Many[note 1] rotary electrical machines require current to be conveyed to (or extracted from) a moving rotor, usually by means of sliding contacts: a commutator or slip rings.

The field coils can be mounted on either the rotor or the stator, depending on whichever method is the most cost-effective for the device design.

In a brushed DC motor the field is static but the armature current must be commutated, so as to continually rotate.

This is done by supplying the armature windings on the rotor through a commutator, a combination of rotating slip ring and switches.

Bipolar generators were universal prior to 1890 but in the years following it was replaced by the multipolar field magnets.

[1] The stepping stone between these two major types was the consequent-pole bipolar generator, with two field coils arranged in a ring around the stator.

During World War II the Manhattan project to build the first atomic bomb used electromagnetic devices known as calutrons to enrich uranium.

Thousands of tons of silver were borrowed from the U.S. Treasury reserves to build highly efficient low-resistance field coils for their magnets.

Modern (c. 2009) low-cost universal motor , from a vacuum cleaner . Field windings are dark copper colored, toward the back, on both sides. The rotor's laminated core is gray metallic, with dark slots for winding the coils. The commutator (partly hidden) has become dark from use; it's toward the front. The large brown molded-plastic piece in the foreground supports the brush guides and brushes (both sides), as well as the front motor bearing.
Salient field bipolar generator
Consequent field bipolar generator
Consequent field, four-pole, shunt-wound DC generator
Field lines of a four-pole stator passing through a Gramme ring or drum rotor.