Documentation for Long Period Vertical Seismograph

A few days later, I received a carefully wrapped reprint from Physics titled "A New Type Long Period Vertical Seismograph" from the July, 1934 issue. On the front cover was written a personal note, "With appreciation to you, Uncle Henry, ...for the many things you have done for us." Signed, "Lucien." I found a treasure trove with LaCoste's detailed description.

Armed with a picture of his instrument and the mathematical descriptions, I could proceed with build a model based on facts over my foggy memory. The spring was the main component which might present a problem. His was hand-wound using Elinvar wire (an alloy of iron, nickel, and chromium) for its temperature and magnetic characteristics. The technique he used to wind the spring is described in the article; he called it a zero-length spring. I do not have a lathe, nor Elinvar wire. However, a common door spring has similar characteristics. I went to a couple of building supply stores and bought a sample of several sizes of screen door springs. I would determine which one to use by its elongation with the proper weight attached. I found one that was close and a bit longer than I needed.

While LaCoste used what appears to be 1/4" x 1-1/4" steel bar for his frame, I used 1/8" x 1-1/4" steel bar due to its avalability and workability with hand tools.

There were other variations in my construction, but the overall system is quite close (temperature and magnetics not considerations for my model). The wire I used for the rear pivot and the main spring extensions was larger than the original model because I was concerned with the smaller wire breaking when transporting it to the ACTLab studio for presentation. (I did not want to have to restring it, although I took the tools and wire in case of a breakage.) The difference in spring material also affected the period of my seismograph, shorter than that of the LaCoste model due to its stiffness.

Although LaCoste's seismograph did not turn into a commercial device, its principals and the zero-length spring became the basis for the standard of gravity meters, those made by LaCoste and Romberg, Inc., an Austin company founded by Dr. LaCoste, by then a quantum mechanics professor, and Dr. Romberg in 1938. The 2 men took a leave of absence from the UT faculty and never returned to the formal classroom.

As a student in a physics class finding a new way to wind a spring for his project, and using the data he gathered for his seismograph, LaCoste applied it to another project that lead to a world-wide standard for gravity meters used in geophysics research and oil exploration. Perhaps Dr. Romberg's physics class was a forerunner of ACTLab.