Today, in exploring the effects of centrifugal force, we see no fewer than seven gadgets demonstrated. Our curator does a lot of cranking.
1. As a spinning glass globe rotates faster, colored liquid within it rises to form a belt or ring around its middle.
2. An array of pendulums starts out hanging vertically, but the pendulums spread out as rotation speeds up.
3. A spring-loaded sphere, arranged to slide on a horizontal bar, moves outward as the bar spins, compressing the spring.
4. Two flexible strips of metal form two circular hoops, outlining an imaginary sphere. As they are spun, they are centrifugally deformed. We see that the shape they outline becomes less like a sphere and more like a pumpkin.
This demonstration makes a point of interest to planetary scientists: Many planets—Earth and Jupiter are good examples—bulge slightly at their equators, and are flattened at their poles, because of their rotation. The effect is more subtle than the dramatic bulging we see in the spinning hoops, but perhaps witnessing this apparatus helped students visualize an important piece of geophysics.
5. As glass vessel rotates, the flat surface of colored liquid within it changes in shape. The liquid’s surface becomes a bowl shape, low in the middle, high at the edges of the glass—a paraboloid.
Since a parabolic shape is ideal for applications in acoustics, radio, and optics, such as making the mirror of a reflecting telescope, and an accurate parabolic shape is not easy to make, many an engineer has contemplated the surface of a spinning liquid and daydreamed.
In the 1980s at the University of Arizona’s Steward Observatory, J. Roger P. Angel found a practical way to build a huge spinning furnace. He cast large telescope mirrors from molten glass. The speed of the furnace’s rotation and the diameter of the mold determined the shape of the resulting paraboloid. Then the furnace kept spinning as the glass was allowed to cool. The glass solidified into the desired shape, ready for testing and polishing.
By this means, in 1985 Professor Angel’s team produced a mirror 1.8 meters in diameter. This became the primary mirror of the Vatican Advanced Technology Telescope (VATT) on Mount Graham in Arizona. So astronomers of the Vatican Observatory may have a special reason to smile when they see the surface of a spinning liquid. Angel’s group went on to cast mirrors for many more telescopes, including four 8.4 meter behemoths for the Giant Magellan Telescope array.
6. As it rotates, a dangling metal ring responds to forces that leave it spinning in a horizontal plane.
7. With an apparatus similar to example 6, a loop of chain is spun. Centrifugal effects force the chain into the shape of a circle, and also bring its motion into a horizontal plane.
These seven demonstrations flash by in less than four minutes, but they establish that the Cabinet of Physics was well-equipped to present students with the phenomena of centrifugal motion… as long as the person doing the demonstration had a sturdy cranking arm.
The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments. The Foundation’s homepage may be found here, and its Youtube channel, florencefst, here.
