An eudiometer is an instrument used to measure the volume change of gases. Early incarnations were meant for measuring the amount of oxygen in the atmosphere. Made of glass, the eudiometer typically has an elongated tube shape with a measuring scale, similar to a barometer or a thermometer.
Each eudiometer has one end closed, with the other end open for filling up with water. It is usually immersed in a container of water, with the closed end facing upward. When immersion occurs, a gas sample enters the instrument. This creates an electric spark between the two wires sealed into the eudiometer and enables a graduation within it to measure the change in gas volume. Some users of the instrument rely on mercury for immersion instead of water.
The eudiometer is most commonly manufactured as a graduated cylinder. This means that it resembles a glass container with measuring marks at its sides. The eudiometer is usually available in measurement scale ranges of 50 to 100 milliliters (mL), or in grams. The graduated cylinder form first came into use in the mid-20th century and has since been the most popular type of eudiometer.
Although the tall, narrow cylinder is the most common shape, the eudiometer comes in other forms. Some of them come in a U-shape, with one end slightly longer than the other. There are also T-shaped cylinders, characterized by small, truncated arms.
The term "eudiometer" is of Greco-Roman origin. "Eu" means "good," and "dio" means God. The latter part is a reference to God's residence as in the atmosphere, heavens or sky. The "meter" part indicates measurement.
The first mention of an eudiometer was in 1777, when Italian physicist Alessandro Volta wrote in his letters about a laboratory device that can measure the air's quality. The instrument's invention, however, is often attributed to another Italian physicist, Marsilio Landriani, who described the instrument in a 1885 publication entitled Ricerche fisiche intorno alla salubrità dell'aria, or Physical Researches On the Salubrity of Air. He theorized that air can be chemically analyzed, and gases in the atmosphere can be isolated using a laboratory device.
Englishman Joseph Priestley predated both men by using an instrument to discover gases such as ammonia and hydrogen chloride and oxygen. The instrument he used, however, was described as a pneumatic trough.