O-rings are so ubiquitous today – at least for all of us building underwater instrumentation for the underwater technology industry – that we generally think of them as something that has always been around. But that is not the case. The other night Sequoia’s President, Ole Mikkelsen, was lying on his couch pondering our business. Somehow, he got to thinking about the O-rings we use, who invented them and when. He looked it up.

Much to his surprise, O-rings were invented by a fellow Dane – Niels Christensen – who had immigrated to the US at the age of 26 in 1891. He was a trained as a machinist and had already patented several mechanical inventions when he got the idea for the O-ring in 1933. After four years of trial and error he finally got it right and filed a US patent in 1937 (issued in 1939). He was 72 years old when he filed the patent application. He tried for years to get manufacturers interested. Nobody saw any use for his invention.

Niels Christensen – O-ring inventor

Then came WWII and with that the manufacturing of airplanes with hydraulic landing gears as part of the defense build-up (prior to Pearl Harbor). Nobody knew how to seal them properly. So, in the summer of 1940, when he was 75, Niels contacted a couple of US Army Air Corps engineers in Ohio and got an interview for a demo. His O-rings worked fantastic, and the engineers specified them as the seal of choice in virtually every application for the US military where a seal was needed.

Niels licensed the patent in April 1941 to United Aircraft for a fee of $0.15 to $2 per O-ring, depending on size. In today’s money, this amounts to $2.6 – $35 FOR EACH O-RING! Niels was about to become Bill Gates * Jeff Bezos rich in a fraction of the time.

Then Pearl Harbor happened, and a national emergency was declared. The US government seized all patents related to the military effort and made them available – royalty-free – to all manufacturers who were interested. The government paid Niels $75,000 (~$1.3 million in 2022) for use of the patent for the duration of the national emergency.

While the war itself ended in 1945 when Japan surrendered, the national emergency lasted until 1952 when the state of war with Japan officially ended with the San Francisco peace treaty. At that time, there were four years left of Niels’ O-ring patent. However, everybody had started using O-rings almost everywhere during the war, and nobody really cared about an 87-year old man trying to get them to pay him a royalty for their continued use of his invention. Niels died in October 1952.

His estate took up a lawsuit against the US government for royalty payments for the period 1952-1956. Eventually (in 1971…) the courts decided that 24,221,745 patent-infringing O-rings had been used in the US from 1952-1956, and his estate was awarded a compensation of about $100,000 ($635,000 in 2020). All told, Niels and his estate got about $2 million (2022 value) out of his O-ring invention.

Finally, we should mention that a Swedish patent from 1897 *also* describes an O-ring invention. It was, however, published in Swedish and probably not very well known. Niels’s invention and the fact it was used by the US military during WWII seems to have been what catapulted the O-ring into everyday use.

For (much) more on Niels, follow this link: https://web.archive.org/web/20070223213039/http://www.americanheritage.com/articles/magazine/it/1991/1/1991_1_58.shtml and this Wikipedia page: https://en.wikipedia.org/wiki/Niels_Christensen .