You may have seen small glass tubes containing three tiny ball bearings and assumed they were odd little curiosities from the past. In reality, these pieces were precision components used in bullseye levels, also called machinist’s levels or spirit levels. Unlike standard straight levels that rely on a single bubble to measure one direction, bullseye levels were designed to show levelness in two directions at the same time through a circular or dome-shaped capsule filled with liquid.
In some older European and industrial models, the usual air bubble was replaced with small steel ball bearings. Instead of floating like a bubble, the balls rolled toward the lowest point, making it easier to see whether a surface was level from different angles. In work environments with machinery or vibration, this design could be easier to read than a traditional bubble. Certain models even used three balls so that levelness could be judged with greater precision through their position inside the liquid.
