Skip to main content


Fall 2010 | Volume 25 |  Issue 3

In 1933 the 68-year-old inventor Niels Christensen finally tackled a problem that had bothered him throughout his long career in hydraulics. His elegant and simple solution—the O-ring—would become such a ubiquitous part of so many technologies that it is present by the dozens in every home and car, and applied to everything from fountain pens and soap dispensers to hydraulic presses and bomb-bay doors.

Forty years earlier, a horrible streetcar accident in Oak Park, Illinois, had turned the Danish-born inventor’s attention toward improving braking systems. The street railways of the time used the conductor’s muscle power, amplified by the electricity that ran the cars, to squeeze brake shoes against the wheels. When bigger cars appeared in the 1890s, this system proved inadequate, especially because the loss of electric power often caused the sudden braking in the first place. A means of storing energy was necessary; Christensen decided that compressed air, already used to brake railroad trains, was the most promising solution.

He created a compressor and electric motor entirely enclosed within a metal case, its moving parts surrounded by a bath of oil. (By contrast, railroad air brakes used a mechanically driven compressor rather than an electric motor.) In 1895 and 1899 Christensen secured patents for the sealed motor-compressor combination and a special triple valve to control the flow of compressed air. His first big sale came in 1897, when Frank Sprague chose Christensen braking systems for his South Side Elevated in Chicago. Soon they were being used nationwide and in Europe.

Despite his success developing hydraulic brakes, Christensen remained frustrated by the lack of a simple, reliable seal that would let a piston slide easily but block the flow of fluid. Rubber rings had been tried before, but they tended to wear out quickly. Christensen, now working for Midland Steel Products of Cleveland, decided to experiment with using different dimensions of rubber rings and explore different configurations of the grooves that they sat in.

His methods of research were not particularly refined. “He’d put a ring through a test, then look at it under a magnifying glass to see where it was scratched. No complicated analysis at all,” says his grandson Niels Owen Young.

What he finally settled on was a ring with a circular cross section, in a groove approximately one and a half times the ring’s radius. With the groove sized appropriately, the results were remarkable. “This packing ring has been tested for 2,790,000½” strokes at 600 psi and 2,790,000 return strokes at atmospheric pressure,” Christensen wrote in his notebook. “This packing ring never leaked and is still tight.”

We hope you enjoyed this essay.

Please support this 70-year tradition of trusted historical writing and the volunteers that sustain it with a donation to American Heritage.


Stay informed - subscribe to our newsletter.
The subscriber's email address.