The Brush Dynamo

by Jeffrey La Favre

When Brush was formulating his plans for an arc lighting system he realized that an efficient and economic power source was the key to success. Volta had produced the first battery in 1800 and there were improvements by others in the middle of the century. Nevertheless, Brush did not consider the battery a viable power source for arc lighting. The cost of battery power was simply prohibitive for the lighting systems he wanted to develop. The dynamo seemed to be a promising source of electricity but Brush felt that there was a need to increase the efficiency of existing machines.

Hippolyte Pixii, a French instrument maker, constructed the first direct current dynamo in 1832. The machine contained a permanent magnet which was rotated by a hand crank. The spinning magnet was positioned so that its north and south poles passed by a piece of iron wrapped with wire. Pixii found that the spinning magnet produced a pulse of current in the wire each time a pole passed the coil. Furthermore, the north and south poles of the magnet induced currents in opposite directions. By adding a commutator, Pixii was able to convert the alternating current to direct current (current that flows only in one direction).

In 1867 Cooke and Wheatstone produced a dynamo that was clearly superior to Pixii's. Their dynamo was constructed with electromagnets rather than permanent magnets. Electromagnets were found to be superior because they could develop a stronger magnetic field, thereby increasing the electric output. In addition, electromagnets provided a degree of built-in regulation because the field strength would adjust in direct proportion to the output of the dynamo.

Another innovation in dynamo design during this time period was conceived by an Italian scientist, Antonio Pacinotti. He wrapped a ring of iron with a continuous coil of wire to form an armature. The "ring armature" was set to spin between electromagnets. This design produced a fairly steady stream of current because the wire wrappings of the armature were continually passing the electromagnet poles. Pacinotti's machine was the first practical direct current dynamo because it did not produce severely isolated pulses of electric current.

Brush considered the dynamos available in 1875 to be ill-suited to his needs. "The best forms of magneto-electric apparatus at present before the public are unnecessarily bulky, heavy, and expensive, and are more or less wasteful of mechanical power." (US Patent 189 997).


Gramme dynamo, 1870



Nevertheless, Brush was aware of a commercially successful dynamo in use in Europe. This was the machine developed by Zenobe Gramme in 1870. The Gramme dynamo was similar to Pacinotti's in design, containing a ring armature. Brush started with the basic Gramme design and set out to improve the dynamo. In the end the Brush dynamo represented a marked divergence from the Gramme. The ring armature was the only element that Brush retained from the Gramme design.

End view drawing of
Gramme-style armature
with part of the coil
removed so that the
core can be seen

Brush armature


At first Brush worked out his designs on paper, considering the weak points of the Gramme design and how he might improve upon it. One major fault of the Gramme was the poor contact of armature coils with the electromagnet fields. The pole shoes of the electromagnets were formed to fit around the circumference (edge) of the ring armature. Therefore, only the outside portion of the coils would cut through the strong magnetic field. The wire on the sides and interior of the ring were outside the effective zone of the field. To improve upon this design, Brush would need to find a way to bring a larger percentage of the armature wire close to the magnetic pole shoes.

Another weakness of the Gramme was a tendency to retain heat in the armature. The continuous winding acted as an insulating layer that retarded cooling of the armature core. Excessive heating of the armature would result in a drop in the efficiency of current generation.

Brush had completed his dynamo design on paper early in 1876. His ring armature was shaped like a disc rather than the cylinder shape of the Gramme armature. The field electromagnets were positioned on the sides of the armature disc rather than around the circumference.

There were four electromagnets, two with north pole shoes and two with south pole shoes. The like poles opposed each other, one on each side of the disc armature. With this arrangement Brush was able to bring a major portion of each loop of wire on the armature very close to the field magnets, thus increasing the efficiency of the machine.

The armature was wound with several wire coils and the core was exposed between the coils, which promoted air cooling (see illustration above).


Brush's first dynamo

A. Commutator brushes
B. Electromagnet shoe pole
C. Armature
D. Field electromagnet

Case Western Reserve University
Photograph and digital image owned by CWRU Library. Permission to reproduce this image must be obtained from CWRU.


  The armature coils were connected to a commutator in such a way that they did not form a continuous coil around the circumference of the armature. The commutator was designed to remove armature coils from the current circuit when they were midway between north and south field magnets. This open coil design was more efficient because it removed coils from the circuit while they were making the transit between pole shoes, in a region where they would not generate electricity but only add unwanted resistance to the circuit.

The open coil design was unique to Brush's dynamo and it was often called an open-coil dynamo.

Brush presented his dynamo design to George Stockley, VP and general manager of the Telegraph Supply Company of Cleveland. He believed that his design would prove superior to the Gramme. Stockley had great confidence in his boyhood friend and also saw the potential profit in the dynamo for his company. Continuing under a contractual agreement, Brush would now proceed to construct a working model of the dynamo. The Telegraph Supply Company supplied the resources necessary to build the dynamo and Brush supervised the machine shop work closely. For his involvement as the inventor, Brush would receive substantial royalties for sales of all equipment manufactured as a result of his inventions.

By the summer of 1876, the parts of the model dynamo were ready for assembly. Brush returned to his old home, the Walnut Hills Farm, with the parts of the dynamo. Working in the workshop where he had experimented with electricity as a boy, he carefully wound the armature and electromagnets. Then he assembled the parts to complete the dynamo.

Brush was an unusual man of many talents. Not only did he possess the scientific and engineering skills needed to create a dynamo, he was also skilled in the mechanical arts needed to fabricate a dynamo.

Brush connected the dynamo to a horse-drawn treadmill, the only source of mechanical power available on the farm. At first it appeared that the dynamo could not generate electricity. After some thought, Brush connected the field magnets to a battery, reasoning that this would energize the electromagnets and provide the conditions necessary to initiate current generation. With the battery in place, Brush applied mechanical power to the dynamo and found that it produced a current. After the dynamo started generating electricity, the battery could be disconnected, it was only required to provide the needed starting conditions. Brush had fulfilled his boyhood dream of constructing a dynamo.

Brush returned to Cleveland with his new dynamo and continued development work at the Telegraph Supply Company. By the end of 1877 he had also developed a commercially viable arc lamp and was ready to market his lighting system. At the time Brush was a relatively unknown figure in the emerging electrical industry. He soon found the opportunity to gain recognition for his new dynamo.

The Franklin Institute of Philadelphia announced their desire to obtain a dynamo. In order to select the best model, the institute called upon manufacturers to submit their machines for evaluation. Several manufacturers declined the invitation, fearing adverse publicity if their dynamo was determined to be inferior. Brush felt that the risk of an unfavorable evaluation was offset by the potential benefits of a top evaluation. He submitted a small and large dynamo for consideration. The Wallace-Farmer Company was the only other American manufacturer to submit a machine.


Wallace-Farmer dynamo

Early commercial Brush dynamo

Charles Brush (right) and Elihu Thomson
examine early Brush dynamo -- same type
as illustration above. Image was processed to
brighten area containing dynamo to enhance its
visibility. Photo circa 1928.

Case Western Reserve University
Photograph and digital image owned by
CWRU Library. Permission to reproduce
this image must be obtained from CWRU.


The Franklin Institute was not satisfied with the number of dynamos submitted for testing and sought another machine for comparison. They were able to obtain a Gramme dynamo which had been shipped from Europe for the Centennial Exhibition of 1876.

The institute evaluation committee commenced testing of the four dynamos in the spring of 1878, using them to power arc lights for the tests. Test runs were done for 5 hours, evaluating each machine for mechanical efficiency, power, range of currents, sparking, losses due to friction and local action of heat. The tests were completed by June of 1878.

The Wallace-Farmer machine was rated inferior due to inefficiency and excessive overheating. Tests of the Brush and Gramme dynamos yielded mixed results. The Gramme was found to be the most efficient in the conversion of mechanical energy to electrical energy. However, Brush's machines were found capable of producing stronger currents at a wide range of voltages. The Brush dynamos had a simple design compared to the Gramme and the committee felt that the Brush dynamos were easier to maintain. In the end the Franklin Institute purchased the small Brush dynamo.

The testing committee at the institute included a prominent electrical scientist, Elihu Thomson, who added an element of authority to the testing results. By placing their stamp of approval on the Brush dynamo, the Franklin Institute was effectively endorsing it as the machine of choice for generating electricity. Brush used the testing results to his advantage in marketing his dynamo. Ironically, Thomson later formed a company to produce arc lighting, which became a significant competitor to the Brush Electric Company until the two merged in 1889.


A central station with numerous Brush dynamos. Note Brush arc lamps hanging from ceiling - see page on Brush arc lights for a photo of this type of lamp.

Case Western Reserve University
Photograph and digital image owned by
CWRU Library. Permission to reproduce
this image must be obtained from CWRU.


For additional information on the Brush arc lights, you might like to take a look at a web site authored by Charles Brush's great grandson, Charles F. Brush IV.

The primary sources for this article were US Patents and Charles F. Brush: Pioneer Innovator in Electrical Technology, a Ph.D. dissertation by Harry Eisenman III . The author would like to thank Charles F. Brush IV, the great grandson of Charles Brush, for helpful comments during the preparation of this article.




These pages on Charles F. Brush were authored by Jeffrey La Favre

© 1998, Jeffrey La Favre, Ph.D.