sanduhr

A Sanduhr, or hourglass, is a device for measuring time through the controlled flow of granular material between two glass bulbs connected by a narrow neck. The elapsed time depends on the amount of material, the size and shape of the neck, the grain size, and the properties of the material.

Typically consists of two glass bulbs connected by a narrow neck, mounted in a frame. The container is filled with fine sand or similarly grained material; dry, clean sand is preferred to avoid clumping. When inverted, the upper bulb empties into the lower bulb at a steady rate until all sand has moved.

Origin and development: The exact origin is uncertain; earliest unambiguous mentions date from the 14th century in Europe. Hourglasses spread as portable timekeepers in maritime and ecclesiastical contexts and remained common until the advancement of mechanical clocks and later quartz and electronic timers.

Operation and variation: The hourglass is turned to start timing, with sand flowing under gravity through a narrow passage. The duration can be calibrated by adjusting the amount of sand, the size of the neck, and the grain type. Hourglasses have been produced for minutes (3–5 minutes), hours (30, 60, 90 minutes), and longer intervals for ships.

Contemporary use: Today, sandglasses are widely used as decorative objects, teaching aids, novelty timers, or ceremonial devices; they remain a recognizable symbol of the passage of time.