What mechanical principle did Leonardo da Vinci illustrate with an early ball-bearing design?

Leonardo da Vinci illustrated the principle of reduced friction with an early ball-bearing design.

In the Codex Atlanticus, Leonardo drew balls placed between rings so that one surface could rotate against another with less sliding friction. The rolling elements were intended to keep the rings from rubbing directly together.

Modern ball bearings use the same broad principle, although they have carefully manufactured races, cages, lubrication, and standardized materials. Leonardo’s sketch did not amount to a complete modern bearing system, and historians debate how practically usable the early arrangement would have been.

The invention is often associated with Leonardo because his drawing is one of the earliest known designs resembling a ball bearing. The key idea is rolling contact, not electricity, magnetism, or steam.