What Is a Stalagmometer?

A stalagmometer is a device consisting of a capillary tube with a wide, bulb-shaped middle section and a narrow bottom that together allow a liquid to form a drop as it falls from the tube. It is used in a process known as the drop-weight method to compare the number and weight of drops of a particular liquid to a reference liquid, such as water. The instrument is used by chemists and physicists to measure a liquid's surface tension. Stalagmometers also are known as stactometers and stalogometers.

During experiments, a stalagmometer is suspended vertically and a liquid is added to the tube and allowed to flow out. The narrow bottom of the tube ensures that the liquid forms a drop at the end. Liquids with low surface tensions will create smaller drops than liquids with a high surface tension.

When the amount of liquid in the drop, or its volume, reaches its maximum value, the liquid will fall from the tube. The maximum value of a liquid is the point when the weight of the liquid in the drop reaches its equilibrium state in surface tension. In other words, the liquid begins to drop from the bottom of the capillary tube when the weight of the droplet is equal to the circumference multiplied by the surface tension.

Surface tension is a physical characteristic of fluids. Molecules of liquids are attracted to one another because of cohesive forces, or positively and negatively charged electrons. Liquids often form droplets because the molecules readily form a sphere shape to minimize the surface area of a fixed volume.

Capillary tubes come in sizes ranging from 2.5 milliliters to 5.0 milliliters. The 2.5- and 5.0-milliliter stalagmometers are the most commonly used sizes. The 3.5 milliliter stalagmometer is used to measure viscous liquids — or liquids that are thick and don't flow quickly.

Colloids are one type of fluid that a stalagmometer may be used to measure. This fluid type consists of a substance that has been broken into microscopic particles and mixed evenly throughout another liquid. An example of a colloid is homogeneous milk. Liquid medicines that are colloids are used in intravenous (IV) therapy and to replace fluids in a person's body. A stalagmometer is used to measure a medicine's surface tension and drop rate, which is used to determine how to administer the proper amount of the fluid to the patient.

Another use for a stalagmometer is to measure the surface tension of wetting agents — such as chromic acid, acid drips and nickel — to determine the amount needed to create an electroplating bath in chemistry. Electroplating is used to create a thin layer of a substance on the surface of a material, such as electroplated gold. This process is used to give a substance a physical property that it otherwise may not have, such as corrosion resistance.