# Du Noüy ring method

A du Noüy ring tensiometer. The arrow on the left points to the ring itself.
Close up of the ring drawn out of the liquid.

The du Noüy ring method is one technique by which the surface tension of a liquid can be measured. The method involves slowly lifting a ring, often made of platinum, from the surface of a liquid. The force, ${\displaystyle F}$, required to raise the ring from the liquid's surface is measured and related to the liquid's surface tension, ${\displaystyle \gamma }$:

${\displaystyle F=2\pi \cdot (r_{i}+r_{a})\cdot \gamma }$

where ${\displaystyle r_{i}}$ is the radius of the inner ring of the liquid film pulled and ${\displaystyle r_{a}}$ is the radius of the outer ring of the liquid film.[1]

This technique was proposed by the French physicist Pierre Lecomte du Noüy (1883–1947) in a paper published in 1925.[2]