Can you make a potato cook faster by driving a nail through the center?
The nail trick is one of the oldest in the book. Problem is, it doesn't work.
To test the theory, the ever inquisitive AmazingRibs.com science advisor, Prof. Greg Blonder, bought a commercial stainless steel spud nail kit advertised to reduce cooking time by up to 50%. Did it work? "Well, if you think 10% counts as 'up to 50%', it does speed up cooking. But hardly worth the effort."
It seems that steel is just not conductive enough to make a big difference. Keep in mind, a potato consists mostly of water, and the mass of cold water in the spud can counterract the small amount of heat coming through the skinny nail and cool the nail pretty effectively. On the other hand, a copper nail does a better job because copper conducts heat faster than steel. That's why the better stainless steel pots and pans have a copper core.
To demonstrate the point, Blonder broke out his computer simulator and ran a copper nail against a stainless nail as well as a glass rod. In the illustration, blue is cold temperature, red is hot. But don't go out and buy a copper rod. Copper reacts with water chemically and can discolor the potato and make it unsafe to eat. The experiment shows that even a copper nail doesn't significantly increase the heat and cut down the cooking time for a baked potato. This myth is busted. Click here for more details on Blonder's experiments.