Mariotte jar (constant-head reservoir)

Design and basic operation

The typical Mariotte jar is an airtight bottle or jar fitted with two small tubes. One tube serves as the outlet through which liquid leaves the vessel. The second tube opens to the atmosphere at its upper end but terminates inside the jar with its lower end submerged near the same level as the outlet opening. When the outlet is opened, liquid begins to flow and the internal pressure adjusts so that the vertical distance (the hydrostatic head) between the outlet and the free surface maintained at the submerged end of the air tube remains essentially constant.

Mechanically, the device works by trapping a pocket of air above the liquid. As liquid flows out through the outlet, the air pocket expands or contracts so that the pressure at the internal opening of the outlet corresponds to atmospheric pressure plus a fixed hydrostatic contribution determined by the tube placement. This keeps the discharge rate nearly independent of the remaining volume in the jar until the liquid level falls below the submerged end of the air tube.

Key characteristics

• Provides nearly constant flow (constant head) without active control.
• Requires an airtight reservoir and correctly positioned tubes.
• Stops providing constant flow once the liquid drops below the submerged air-tube opening.
• Performance depends on liquid viscosity, outlet size and ambient pressure.

Uses and examples

Mariotte jars are convenient where a steady, gravity-driven discharge is needed. Typical uses include: maintaining a constant head for permeability and infiltration tests in soils, supplying constant reagent flows in long-duration laboratory experiments, calibrated trickle feeds in small-scale irrigation studies, and educational demonstrations of hydrostatic principles and siphon behavior. Because it relies on atmospheric pressure and hydrostatic balance, it is best for incompressible liquids under modest flow rates.

History, name and related concepts

The device is named after Edme Mariotte, a 17th-century French physicist who studied the properties of gases and fluids; in some regions the device and related gas laws are historically associated with his name. The Mariotte jar is conceptually related to other methods for producing constant discharge—such as float valves, pressure-regulated tanks, or mechanical pumps—but distinguishes itself by its passive, geometry-based control. For conceptual background on the pressure principles that make it work, see pressure.

Practical considerations and limitations

• Leakage or imperfect sealing ruins the constant-head effect.
• Very low outlet diameters or viscous liquids reduce accuracy; the device assumes hydrostatic flow conditions.
• Not suitable for delivering precisely metered volumes over very long durations without periodic refilling and adjustment.

In summary, the Mariotte jar is a low-cost, robust way to obtain a steady gravity-fed discharge in many experimental and educational settings. Its simplicity and reliance on basic fluid-static balance make it a useful teaching tool and a practical component in field and laboratory setups where a constant head is desirable.