Ring Armature for DC Dynamos Honored With an IEEE Milestone

The small device paved the way for more practical and reliable electrical machines

19 December 2018

Back in the 1800s, dynamo was used to describe a generator that made DC power. Such generators are now used in car batteries, wind-driven turbines, and more.

This month the ring armature for DC dynamos—the Pacinotti ring—has been named an IEEE Milestone. Administered by the IEEE History Center and supported by donors, the Milestone program recognizes outstanding technical developments around the world.


Dynamos have three components: a stator, an armature, and a commutator. The stator is a fixed structure that energy flows through. The armature is made of coiled copper windings that rotate inside the magnetic field made by the stator. The commutator conducts the current from the windings and the brushes collect the current from the commutator.

The dynamo evolved from work done in the 1820s by Michael Faraday and Joseph Henry. While working at Princeton, Henry experimented with electromagnetic engines and converting magnetism into electricity, a process now known as induction. The unit of the rate of change of current per second was designated the henry in his honor. Both men worked separately, and their experiments eventually led to the invention of the generator by Faraday.

Instrument maker Hippolyte Pixii of France advanced the dynamo. His first model created pulses of electricity separated by no current. He then focused his experiments on generating DC electricity instead of AC—which led to the creation of the commutator in 1832.


Between the 1830s and the 1860s, batteries were used to supply electricity to generators. They were costly, cumbersome, and unsafe to handle because of their sulfuric acid, and they had to be replaced frequently. Italian inventor Antonio Pacinotti worked to solve the problem.

In 1858, while Pacinotti was attending the University of Pisa, in Italy, he experimented with creating a dynamo that could transform mechanical force into continuous electric current. After spending a year serving in the engineering corps during the Second War of Italian Independence, he returned to the university in 1860 and continued with his experiments.

He finished building a small armature in 1863. What became known as Pacinotti’s ring would transform the dynamo. The ring armature consisted of symmetrically grouped coils that closed upon themselves and connected to the bars of a commutator. Its brushes delivered a practically continuous direct current.

Pacinotti, with the help of technician Giuseppe Poggiali from the physics laboratory at the University of Pisa, started building a dynamo that would hold the ring. Pacinotti discovered the reversibility of his invention, having noted that if the machine was supplied with energy from an outside source, it would run as a motor.

His invention became a key element in the evolution from the magnetoelectric generator to the commercial self-excited dynamo built in 1873 by Zénobe Gramme, an electrical engineer from Belgium. Gramme’s machine used residual magnetism of the electromagnet in the machine—which made it capable of generating smoother and much higher voltages than the dynamos used at the time. Pacinotti’s ring contributed to the invention of Gramme’s commercial generator.

The ring armature was honored on 4 December at the University of Pisa. A plaque mounted near the entrance of the main lecture hall of the University of Pisa’s School of Engineering reads:

A dynamo with a slotted ring armature, described and built at the University of Pisa by Antonio Pacinotti, was a significant step leading to practical electrical machines for direct current. Groups of turns of the closed winding were connected to the bars of a commutator. The machine worked as a motor also.

This article was written with assistance from the IEEE History Center, which is partially funded by donations to the IEEE Foundation.

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