Video Overview
To observe a chain fountain, a chain consisting of many linked spherical beads
is used.
The chain is placed inside a glass, filling it to the brim.
The end of the chain at the brim of the glass is pulled slightly away from the rim of the
glass and released.
The surprising thing that happens is that once the chain starts to flow out of the glass, the
beads of the chain first travel a height upward before they fall back
downward.
As the chain flows out of the glass, gravity pulls the part of the chain outside the glass
downward. The tension in the chain pulls the end of the chain that is inside the glass upward.
If the only force that acts on a bead as it is leaving the glass is the tension in the chain
produced by gravity, the chain should only rise to the height of the rim of the glass. For the
chain to rise a significant distance above the rim of the glass, some force other than this
tension must act on the beads.
The first explanation for what the additional force on the beads might be was that a group of
consecutive beads would act like a thin rod. When the bead at the end of the rod was pulled
upward, the rod would rotate, pushing downward on the surface below the opposite end of the
rod to the end being pulled upward.
This explanation has however been shown to be incorrect. A chain made up of actual rods
rather than of spheres will produce a fountain in this way, but a chain of spheres will only
do so if the chain is resting on an uneven surface. The shape of an uneven surface that a
chain of spheres must initially rest on for it to produce a fountain includes the kinds of
shapes that would naturally exist at the top of a glass containing a chain of spheres that was
placed into a glass in an uncontrolled way that allowed the chain to become tangled.
Current models and experiments involving chain fountains are not able to explain why an
uneven surface enables a chain of spheres to produce a fountain.
The scientific method for understanding phenomena ideally involves trying to measure the
changes in some variable and comparing them to changes in a different variable that have been
given chosen values, and doing this while, as far as possible, keeping all other variables
constant. For the chain fountain phenomenon, it is not obvious what variables significantly
affect whether a chain does or does not produce a fountain. Are, for example, the following
variables significant?
- The size of the spheres
- The spacing distance between the spheres
- The material from which the spheres are made
- The total length of the chain
- The shape of the chain inside the glass
Each variable could be significant, and each variable could affect other variables. For
example, the shape of the chain inside the glass affects the shape of the surface at the top
of the glass.
Also, each variable could be insignificant alone, but a combination of variables could be
significant; perhaps neither just changing the size of the spheres nor just changing the
spacing distance between the spheres affects whether a fountain is produced, but changing
both the size of the spheres and the spacing distance between them does affect
whether a fountain is produced.
The chain fountain is a good example of how the scientific method needs to be used in subtle
ways to understand some phenomenon; while simple explanations may be incorrect, they might still be
able to help in finding better explanations.
