Experiment: Push the drawing board from the side so that it is moving in more or less a straight line, left to right from the perspective of where we normally stand as explainers (we’ll call this the y direction). Take the cap off and watch the pattern evolve.
Results: The line will gradually become an oval that’s higher on the left side and lower on the right (again from the perspective of the explainer with the pen arm). As the oval gets smaller it becomes more and more circular and more and more tilted, taking on significantly more motion in the x direction than it started with. Even if the original swing is a bit off, the pattern always looks the same and tilts in the same direction.
Hypothesis: This is because of the Coriolis Effect.
Explanation: One of my favorite things to do at the museum recently is to be baffled about drawing board, often very verbally, and so many people have posited this hypothesis, and stuck to it vehemently despite my protests that the Coriolis Effect is not that strong, that I thought I would research it.
Sidenote: Remember hearing that the Coriolis Effect causes sinks and bathtubs to drain counterclockwise in the Northern Hemisphere and the opposite direction in the Southern Hemisphere? If you do and you haven’t tried it yet, you should go check all the sinks you can find.
Calculations: This exhibit basically behaves like a giant pendulum. The turning movement of a really big pendulum was actually the first way the spinning of the Earth was directly measured. This is called a Foucault pendulum, and the line along which it swings rotates at a rate of 360° sin(a) per day, where a is the latitude. The Exploratorium is at latitude 37° 48’14”, exactly, so a free pendulum there will rotate about 220° a day. Now if one color on drawing board takes about 2 minutes, that means the Coriolis Effect will cause the board to rotate about 18’ clockwise before friction stops it. That’s about a third of a degree, or 1/1200th of a complete circle, not even enough to observe.
New Hypothesis: Something is off in the construction of the structure. Perhaps a cord is twisted, one side of the board is slightly lower, or the hooks that hold it together cause uneven friction. Look at this picture from the Exploratorium website. Drawing board no longer makes patterns that look quite like that. What changed? Is there some way to make drawing board more precise? What if we snuck in at night, took it apart, and put everything back together again. Would that change it?
Private Note: Dear Daniel, I was right about the Coriolis Effect. You owe me a beer.