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Exhibits In Extreme Close Up: Spinning Blackboard

Exhibits In Extreme Close Up: Spinning Blackboard

Exhibits In Extreme Close Up is a series in which we examine pieces of our exhibits using a scanning electron microscope.

At Spinning Blackboard, our museum visitors encounter a large slowly-spinning disk covered in sand. They can draw in the sand using their fingers or other props at the exhibit. The combined motion of their hands and the rotating disk produces circles, spirals, stars, cardiods, and a surprising variety of other shapes.

This exhibit first appeared back in the 80s when exhibit developer Alan Wilson was doing work with an old grinding tool known as a lapping wheel. Alan noticed the interesting geometric patterns that were left behind as he ran his fingers through the gritty lapping compound on the wheel's surface. So he covered the wheel with particle board and sand and put it on the Exploratorium floor, allowing our visitors to explore these patterns too. This had the secondary advantage of saving space in our crowded fabrication shop. By building an interactive shell we were able to disguise this tool as an exhibit and "store" it on the floor when we weren't using it.

Exploratorium exhibits are never finished. We are always watching how exhibits are used, thinking of improvements, making small tweaks or sometimes rebuilding an entire exhibit from scratch. One of the problems we noticed with Spinning Blackboard was that the people using it interfered with one another. With many hands drawing on the wheel, it's difficult for any one person to do an in-depth exploration of these patterns.

In 2000 we began trying to redesign the exhibit to fix this problem. Through a process of trial and error we gradually arrived at a new version of the exhibit with three individual wheels for our visitors to draw on. Did changing the exhibit in this way improve our visitors' experience? How could we tell?

We set up cameras and took hours of footage of our visitors using both versions of the exhibit (we put up signs so that users knew they were being taped). Analyzing the video we found that, on average, a visitor would use the single wheel version of the exhibit for about 3.4 minutes. For the three wheel version the average was 7.7 minutes, more than double the hold time.

This investigation was done by our Visitor Research and Evaluation department, a group of learning researchers who use the museum floor as a kind of laboratory, studying what people do at the exhibits and how well those exhibits seem to be working. In addition to measuring hold time, VRE analyzed audio recordings at both versions of the exhibit and found that the three wheel version stimulated much richer conversations. They also developed a scheme for rating the complexity of patterns drawn on the wheel. Again, the three wheel version supported greater variety and intricacy. By giving each user their own domain of control this new version of Spinning Blackboard allowed visitors to discuss and compare their drawings, leading to deeper exploration of these patterns.


Another discovery we made while working on this exhibit was that distraction is a big problem on our floor. With so many exhibits to play with, a person will often leave an exhibit not because they're done with it but because they get dragged to a different exhibit by another member of their group. The three wheel version of Spinning Blackboard allowed groups and families to work together at the exhibit and this was a big contributor to the increase in hold time. This discovery has led to a general principle for exhibit design: whenever possible we now try to make our exhibits multi-station.

Here's what sand from Spinning Blackboard looks like at 40x magnification:

These grains are hard and sharp (unlike the soft round magnetite in Black Sand). In fact this kind of sand, called garnet, is found in certain kinds of sand paper. Because of this, exhibits like Spinning Blackboard are known as "bad neighbor" exhibits. Garnet sticks to our visitor's hands and then gets rubbed off on nearby exhibits, destroying paint finishes and causing parts to wear out quickly. We try to place Spinning Blackboard near exhibits that are sturdy enough to withstand a gradual sand blasting. 

We use garnet sand in several of our exhibits, pouring it through funnels on pendulums to draw sine waves or spilling it on vibrating plates to make geometric patterns. These days we seem to go through about 189 pounds of this stuff every month.

As we get down to 3,000x magnification, we can observe the uniqueness of each grain of sand. Here we see what looks like a mountain range with river valleys that are only as wide as a strand of spider silk.