Slip Ring History Series: Brush Technology In 1916

No one seems to know who invented the sliding contact slip ring. The idea emerged from mists of the past like many ingenious things we take for granted…fire, the wheel, beer, domestication of animals and…slip rings. We know about Edison, the Wright brothers, (and their nemesis Curtiss,) Walter Reed and others whose inventions established whole industries. But what about that unsung hero who first passed current through a rotating surface? With these things in mind I set out on a Google search to see what I could find. Of course the journey proved more interesting than the objective.

I soon found myself reading an article written in 1916 by Charles H. Smith, who at the time served as “Engineer, Executive Department,” of the Westinghouse Electric and Manufacturing Company of Pittsburgh, PA. The article was published in The Electric Journal, and the article’s title was, “Brushes For Commutators and Slip Rings, Their Selection, Application and Care.” The advanced technology of the day was carbon brushes, and Smith quoted brush manufacturers who said, “Carbon brushes were brought to the attention of the electrical profession in 1889, however, it was not until 1893 that this then radical departure in brush design met with any degree of success.” Recall this was an era when the country was rapidly adopting electricity and one can imagine the demand for power station generator equipment was strong. In 1916 Charles Smith was riding that wave.

A second quote Mr. Smith chose to introduce his article on brushes could have been made by an Electro-Miniatures design engineer today. Smith wrote:

The second [quote], not without justification, relegates excessive theory to the scrap heap in the following statement: “Abstract knowledge based on theoretical considerations offers little assistance in the solution of brush problems.”

The article offered an extensive discussion of the different brush materials available, along with their advantages and disadvantages. Lubrication was discussed along with the importance of cleaning up the debris they created. Smith pointed out the importance of proper brush alignment.

In a section on slip ring brushes Smith was critical of a new development of his time, the metal-graphite brush. He wrote:

In view of the doubtful virtues of the metal graphite brush, the apparently logical solution of the slip ring brush problem is the development of a graphite, or related, brush suitable for the purpose.”

Of course we now use very capable silver-graphite brushes with hard silver rings in many demanding applications. Smith seemed to be working with all copper rings in his day, and the ‘metal graphite’ brushes cut grooves in the copper. It was tough to envision the future back then, too!

Smith concluded his article with a detailed list of specifications to provide a manufacturer when ordering brushes. He ended with two maxims that wise slip ring customers should adopt today:

  • “From the above it must be concluded that if what is wanted is expected it must be made clear what is wanted.”
  • “… requirements are in no sense stable. It is wise, therefore, to encourage a receptive disposition and to be ready for such advances in the art as may seem worth while

In a future Note I will review highlights of Charles H. Smith’s career, from his first job with Westinghouse at $.16 an hour to his trip to Cuba as an Army Officer in the Spanish American War.

Here is a link to Smith’s full 1916 article on brushes: