Replicating a simple but elegant experiment

MIT Lincoln Laboratory's installation of a pendulum re-creates Foucault's seminal demonstration of Earth's rotation

On 15 December, Videh Seksaria, a senior at Lexington High School in Lexington, Mass., held a candle to the string that drew the gleaming brass bob of MIT Lincoln Laboratory's new pendulum back to an endpoint of its potential arc. When the flame burned through the thin fabric, the bob was released, launching the pendulum into its swing over a stonework base on which 60 pegs arranged in a circle awaited their turns to be toppled by the decorative finial on the bob.

The Lincoln Laboratory pendulum

Lincoln Laboratory’s pendulum awaits its inaugural "swing."
Photo: Jon Barron

The Laboratory's pendulum display daily replicates the experiment that French scientist Jean Bernard Léon Foucault performed at the Panthéon during the 1851 Paris Exhibition. Foucault's pendulum, a 62-pound ball suspended from the Panthéon dome on a 220-foot wire, swung over a ring of sand on the floor. His audience watched as the stylus attached to the ball traced a series of lines in the sand, each slightly clockwise to the previous one. Because a pendulum does not change its plane of motion, the explanation for this drawing must be that the floor was moving—that is, Earth was rotating about its axis. As with many modern pendulum exhibits, the Lincoln Laboratory pendulum reenvisions Foucault's sand tracings with the sequential tumbling of the pegs beneath it.

The installation of the pendulum was a collaborative project that enlisted a multidisciplinary team from the Laboratory and four physics students from Lexington High to develop a museum-quality display on Foucault's experiment and related science. The dual goals of this educational outreach project were to involve students in an engineering activity and to create an attraction that would engage the curiosity of the many middle- and high-school students who come to Lincoln Laboratory for outreach programs. A bonus is that the pendulum is intriguing Laboratory employees who are drawn to watch its movement.

Photo of Lexington, Mass., students who helped with teh pendulum project

At the December event to unveil the pendulum exhibit, the four Lexington High School students who helped with the project—(left to right) Jerrick Chen, May Zhou, Mia Kobs, and Videh Seksaria—were photographed with Frank Robey, technical lead for the project (far left), David Scott, manager of the Technology Office Innovation Laboratory (second from right), and Eric Evans, director, Lincoln Laboratory (far right).   Photo: Jon Barron

A Novel Pendulum Display
The final exhibit comprised not only the pendulum but also an interactive touchscreen presentation that summarized Foucault's experiment, discussed the physics of the pendulum's motion, and highlighted some current scientific explorations in which Lincoln Laboratory is involved. In addition, the exhibit included a new twist—radar tracking of the pendulum's movements, supplied by two small radar units set up at corners of the base. 

The radar unit of the pendulum displayThe tiny radar system here is one of two that provide real-time returns of the pendulum's motion. These radar measurements are displayed on one of the screens of the interactive computer interface installed nearby. Photo: Jon Barron

Lincoln Laboratory's pendulum is a unique variation on the pendulums mounted in museums and public buildings around the world. Other pendulums are kept in continual and radial motion by ceiling-mounted electromagnetic systems. However, the difficult access to the skylight ceiling of the atrium in which the Laboratory's 52-foot pendulum is installed made it impractical to put the electromagnetic system above the pendulum. Hence, the engineering team devised a customized "beneath the bob" energy source.

An electromagnetic coil in the base is centered beneath the Lincoln Laboratory pendulum to attract and repel the bob, keeping the pendulum swinging; a microcomputer recurrently measures the speed of the bob and then controls the current applied to the coil to maintain a steady motion undiminished by frictional losses (mainly resulting from the ambient air).

Meanwhile, developing a solution to the problem of the pendulum's deviating into an elliptical "swing" was a significant engineering challenge, says Frank Robey, associate leader in the RF Technology Group and the technical lead for the pendulum project. "Our question was how to tame the elliptical motion so that the precession resulting from Earth’s rotation dominates." The answer is a 6-foot-diameter eddy current ring on the base; as the bob swings over the ring, small electrical currents are generated to preferentially damp any undesired elliptical motion that could interfere with the precession. In a sense, the eddy ring applies the "brakes" to any sideways wandering of the bob.

The small radar units tracking the pendulum are also novel technology. "We had to get all the radar components into a 1-inch square unit," says Robey. "We’d never built an I/Q [in-phase and quadrature-phase] radar this tiny." The miniature radar units contain all the features of large radar systems, including I/Q signal processing and real-time display of the radar returns.

Lexington High student Jerrick Chen worked with the engineering team on the radar piece of the pendulum project, taking measurements to see if all the parts performed as desired. "I learned that by asking questions and trying to understand and contribute your own ideas, sometimes you can unearth buried opportunities," says Jerrick, who also helped staff from the Information Services Department with transitioning art and text to the HTML code that gave life to the computerized touchscreen display.

Students' Design Work
Videh and classmates Mia Kobs and May Zhou worked with David Scott, the manager of the Technology Office Innovation Laboratory (TOIL), to design and produce items for the project. "At Lincoln Lab, I got real world experience. No two days were the same—every time, there was something different—another challenge or technical fix to study," says Videh.

Videh, left, works with David Scott, TOIL manager, to develop a design for a peg that could be used in the pendulum exhibit. Videh, left, works with David Scott, TOIL manager, to develop a design for a peg that could be used in the pendulum exhibit.
Photo: Jon Barron

Scott introduced the students to the 3D printing process, including the computer-aided-design software, SolidWorks. "This software is not immediately intuitive, but they adapted to it very fast," says Scott. Mia found the work with SolidWorks one of the highlights of her experience: "I loved spending time problem-solving, tinkering, and creating in Solidworks. It was like a big puzzle that we had to figure out. It was this experience that convinced me that I wanted to be an engineer."

"One of the students' most important tasks was to design a case for the radar components," says Scott. "I stayed hands-off for that to see where they would go. A portion of their design was used in the final housings." The three students also fabricated multiple prototypes of pegs, ranging from a simple curved pin to a piece resembling a chess rook to fanciful obelisk.   

Videh, Mia, and May also were involved with the interactive display. "The students contributed valuable feedback on the graphics, storyline, and flow of the pendulum's touchscreen content. Their perspective helped shape the story and validate that the tone and language were appropriate for the primary user group—STEM [science, technology, engineering, and math] students," says Marie Dow, the artistic lead for the touchscreen boards. "They confirmed that the story we are telling is both entertaining and educational."

One of the touch screens; this one shows what forces affect the pendulum's swing. The touch-screen interactive display for the pendulum exhibit features a series of panels, such as the one above, that illustrate the history and science of the pendulum, as well as some current Lincoln Laboratory scientific investigations. (Click on image for larger view.)   Rendering: Paul Furey

"Through this project, the students saw the process of engineering. See a problem, create a solution, evaluate that, and modify if necessary," explains Scott. The students also witnessed how a project involves the collaboration and multiple skills of diverse people. "Watching everyone work together to build this showed me how each person plays a unique role to ultimately construct something enormous like the pendulum," says May, adding that "This experience also solidified my interest in becoming an engineer."

The roughly 35 individuals who made the pendulum exhibit a reality included technical staff members, technicians and support staff from the Facility Services Department, artists, editors, web developers, interns from the Wentworth Institute of Technology, and a photographer to document the history of the project. As the exhibit took shape, many of the participants volunteered beyond-the-workday hours to puzzle out solutions to technical, logistical, and artistic issues.

The pendulum exhibit is not only a tool to teach science but is a model for a successful, community outreach undertaking. "The students did a great job solving the problems involved in creating this unique pendulum. We are looking forward to starting other projects like this for local high-school students," says Lincoln Laboratory Director Eric Evans.

Posted January 2015

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