You know who Léon Foucault is if you have been to a science museum in the last 150 years because you saw a Foucault pendulum - a simple way of observing the Earth's rotation.
But you rarely see them in outside museums because they are huge. Argentinean researcher Horacio Salva and colleagues report success in what they considered a fun side project - building two pendulums precise enough to make measurements of the spinning Earth yet compact enough to fit in a lobby of a science building, though still too large to be in a typical science class.
A Foucault pendulum is essentially a metal orb is suspended by a wire and hung from a considerable height - dozens of feet. The orb is pulled back and released and, as it swings back and forth over the course of a day, it appears to slowly rotate in a circle. What is actually observed is the Earth moving underneath the pendulum, which swings back and forth in a fixed plane, like a gyroscope. But pendulums swing in a mostly fixed plane - if you push a kid in a swing, you know it's tough to keep a pendulum swinging in a straight line, due to friction and other forces which make a pendulum start to travel in an ellipse, an effect that can easily distort evidence of the Earth's rotation. The heavier the suspended orb and the longer the wire, the more limited the elliptical drift, like older children on taller swings fly straighter than younger children in the shorter swings. For context, the 80-year-old Foucault pendulum on display at Philadelphia's Franklin Institute is a 180-pound-orb which hangs from a wire 85 feet long and swings back and forth once every 10 seconds.
By contrast, the pendulums in the device built by Salva are tiny; a 27-pound weight swings back and forth on a 16-foot-long piano wire once every 4 and a half seconds while second pendulum uses the same weight and an even shorter wire. Using a copper ring underneath each orb to damp down the drift, Salva was able to easily observe and measure precession, the technical name for the movement of the Earth relative to the fixed swinging of the pendulums. Indeed his jiggering of the pendulums was able to tune out all but one percent of the elliptical "noise," at least in the case of his longer pendulum.
This won't have the precision necessary to match modern measurements but it is accurate enough to be a useful tool for teaching basic physics concepts to physics students and the general public.
Pointing to one possible application, the paper notes that the device was able to detect earthquakes of medium intensity that took place as far away as 765 km. "Some earthquakes can be seen, because the seismic wave moves the support of the pendulum increasing the ellipse of the moment and changing the precession speed," said Salva. "There's obviously no pressure to do work like this. It's mostly for fun, though I think it may well help students in the future, too."
Citation: Horacio R. Salva, Rubén E. Benavides, Julio C. Perez, and Diego J. Cuscueta, 'A Foucault’s pendulum design', Rev. Sci. Instrum. 81, 115102 (2010) doi:10.1063/1.3494611