Chainfount

Chainfount, a bead-chain djinn, rises from a jar in a shining arch that climbs above the rim before it falls to the floor.

The Physics

Pile a long metal bead chain in a jar, pull one end over the rim and let it drop to the floor. The chain then pours itself out, and instead of sliding over the rim it leaps up into a self-supporting arch above the jar. It is also known as the Mould effect, after the video that made it famous. John Biggins and Mark Warner (Proc. R. Soc. A 470, 20130689, 2014) predicted and measured that the arch height is proportional to the drop height.

h₂ ∝ h₁

• h₁: height of the jar above the floor, the drop height (m)
• h₂: height of the fountain above the jar rim (m)

Biggins and Warner argued that the pile of chain itself gives each link an upward push as it is picked up, an extra reaction force from the jar on top of the pull from the chain above. J. Pantaleone (Am. J. Phys. 85, 414, 2017) measured the fountain and supported a force from the container.

Honest caveat: The mechanism is still debated. Later work argues that the upward “kicks” from the pile are not the main cause and that the chain's own inertia, its tension and the way it leaves the pile can drive the fountain (for example arXiv:1810.13008 and arXiv:1912.08682). The proportionality of the two heights is the measured result; the reason for it is not settled.

On the sticker

Chainfount cradles a glowing jar as a white-gold bead chain arcs up out of it. Printed text: CHAINFOUNT with the line h1 ∝ h2. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.