Here's a great project and site from Adam Davis that shares details about the Thermal Grill Illusion and projects he has built to demonstrate it. Ever wanted to build a replica of the Dune Pain Box? Now you can! via the MAKE blog.
The Thermal Grill Illusion uses alternating warm and cool bars to stimulate the temperature receptors in skin and confuse them. Most people feel a burning cold or burning hot sensation, even though the warm bars are not as hot as some people shower in, and the cool bars are just a few degrees below room temperature.
Not everyone is sensitive to the illusion. Some merely feel the warm and cool bars individually, others report that it feels strange but not uncomfortable. Many report that it's very uncomfortable, and some report that it's painful.
I experienced this illusion for the first time over a decade ago in a science museum. It looked like it had been put together from a small refrigerator, and was as noisy and large as one. The effect was amazing to me, and at the time I theorized that it could be made using Peltier devices. These are semiconductor devices that move heat from one surface to the other, and can be found in small electric coolers and beverage coolers/warmers. They were expensive and fragile at the time, though, so I would occasionally consider how I would make it using one Peltier device, and move the heat around so as to present alternating cool and warm metal surfaces to the user.
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