Gilmore “Tilly” Tilbrook and the “Rythmeter"

The “Rythmeter” ranks as one of the most intriguing devices for calculating the time of ovulation and hence the fertile and “safe” periods of the menstrual cycle. Gilmore “Tilly” Tilbrook, a graduate of Carnegie Institute of Technology (1915), military aviator in World War I, and consulting engineer, took on the challenge of making such a calculator.

Tilbrook recalled its genesis years later:

While living in Europe, I had many opportunities to visit the best medical colleges and hospitals, with American doctor friends. It was in 1930, on such a visit at Graz, Austria, that Dr. Knaus explained his conclusions regarding the exact time of ovulation of a human female. [I was] convinced of the importance of this theory - - especially to those of the Catholic Faith. A doctor friend exclaimed: “Tilbrook, you are an engineer, and have devised aviation calculators to determine engine and plane performance, etc. - - Why can’t you work out a simple, foolproof calculator for the accurate application of the Rhythm?"


Tillbrook conferred with leading medical proponents of “Nature’s Rhythm Method,” including Leo J. Latz, Thurston S. Welton, C. G. Hartman, Robert Latou Dickinson, and many others in the field of reproductive health. Tilbrook ultimately patented two variants of the "rhythmeter," (mis-spelled to give it a catchy name suitable for snappy marketing, no doubt): US patent 2343592 March 7, 1944 and US patent 2418207 April 1, 1947, "Rythmeter for determining sterility and fertility." The “Rythmeter” comprised an ingenious calculator, but Tilbrook cautioned not to use his "Rythmeter" without a record of at least nine months past menstrual cycles.

Does this device look or seem user friendly? Decidedly not. But Gilmore nevertheless labored to sell it vigorously, even advertising it in his alma mater's literature in 1950:



I don't think the "Rythmeter" lasted much beyond its inventor's boosterism, however. It had the precision and complication only an engineer could love....

But I like to think that Gilmore developed the device out of altruistic sentiments, and hoped it would do good. In later life he shared the fruits of his successful engineering career by endowing the Gilmore and Charlotte Tilbrook fund for scolarships in engineering, science, or industrial management at his alma mater, today's Carnegie Mellon University. And on a yet more curious note, Gilmore's wife, Charlotte, was for a time personal secretary to aviation pioneer and reclusive entrepreneur Howard Hughes.

Jim Edmonson

I wish especially to thank Jennie Benford, University and Heinz Archivist, Carnegie Mellon University, for her help in finding material about Gilmore Tilbrook, and providing accompanying images.