Half a croissant, on a plate, with a sign in front of it saying '50c'
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Floating-Disk Clock

A clock with zero sliding friction.
  [vote for,

This consists of a bouyant disc floating in the center of a slightly larger container of water (from one to several mm more). The disc is set into motion at a speed of one rotation per 12 hours. The container has three small water inlets evenly spaced around the circumference. The inlets leak the water very slowly --- just enough to keep the disc rotating at the correct speed and keep it from drifting into contact with the sides and incurring friction. The water drains out through a tube in the center of the container which extends upward about two thirds of the disc's thickness. The bottom of the disc has a relatively roomy recess to accommodate this tube.

To make it responsive to changes in the water's flow rate, the disc has either gear teeth around its circumference or turbine-style spiral ridges on the bottom. If gear teeth are used, then the water from the inlets is aimed at a slight angle to direct the flow clockwise.

A one-gallon reservoir should be able to power the clock for a month. The same water used to power the clock is pumped back into the reservoir. A closed system helps ensure highly consistent operation.

The disc could also be hollow, with sector-shaped, water- filled chambers radiating from the center of the bottom on the inside. This would serve as a speed governor, since the water in the sectors would be flung toward the outer edge in proportion to the speed of rotation.

The clock could be made all or mostly of glass or clear plastic, and the water colored for better appearance.

Alvin, Aug 26 2016


       [+] nice.
FlyingToaster, Aug 26 2016

       "Viscous" is the word used to describe friction in liquids, and water does not have zero viscosity. However, liquid helium can have zero viscosity (and thus earns the label "superfluid"), at about half the temperature of its boiling point (becomes called "Helium II").
Vernon, Aug 27 2016

       Might be good scaled up as a park feature, where you can actually walk on the rotating face.
wjt, Aug 27 2016

I've accepted that there is no way to completely eliminate all types of friction. Even in outer space, there is the occassional wandering molecule which can strike a body in motion and influence it to a minuscule degree. The purpose was to avoid _sliding_ friction which exists in every type of mechanical clock except water clocks. Sliding friction may be very slight in mechanical clocks, but it's still there and it is sure to vary over time and affect accuracy. Flowing water by contrast, should be fairly stable, if kept at a constant temperature in a closed system.
I imagined the clock as around one foot in diameter, but there's no reason it couldn't be much larger. If people were allowed on such a large clock, they would need to see that the energy imparted by their walking averaged out to zero.
Alvin, Aug 29 2016

       // Flowing water by contrast, should be fairly stable, if kept at a constant temperature in a closed system. //   

       Brownian motion?
notexactly, Aug 29 2016


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