| Here’s a new twist on the “Ball and Ring” that proves that metal expands when heated. |
| •Why does the rod fit in the gauge at room temperature, but not when heated? |
| •Demonstrate the linear and spherical expansion of metal when it is heated. |
| Why do bridges need expansion joints? Why are small gaps left between metal sleepers on railway tracks? Why are airplanes designed so that they can expand without breaking? The “Ball and Ring” demonstration has been around awhile and attempts to answer these questions. The ball fits through the ring until it is heated and expands just enough to become too big. But just when you thought this experiment was old hat, we’ve developed a new twist. Since it was invented by our late owner, engineer Ray Bell, we’d like to call it the “Bell” and Ring – but we don’t dare. This precision-machined thermal expansion demonstration is literally “two for one.” It demonstrates both linear and spherical expansion, unlike the traditional “Ball and Ring.” It consists of two pieces, both with wooden handles for safety. The first is a rod that is 7.5 cm long and 1 cm in diameter. The other is a gauge with a precision-machined 7.5 cm slot with two holes about 1 cm in diameter. (But in this case, the “about” is critical. Perform the demonstration to find out why.) |
| How it works: At room temperature, the rod fits snugly inside the slot and either end fits into either hole in the gauge. Now heat the rod with a propane torch. The metal – as do most materials – expands when heated. The rod no longer fits in the slot. It does fit in one of the holes but does not fit in the other, which gives a good inkling of how infinitessimally small the thermal expansion can be while still wreaking havoc with civil engineering projects. |
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