A strain gauge is an instrument used to determine the amount of strain in an object. It was invented in 1938 by Edward Simmons and Arthur Ruge. The instrument consists of a flexible insulating backing that supports a foil pattern.
The instrument is attached to a specific object with an adhesive such as superglue. The foil is deformed along with the object during the process of measuring the gauge. Any strain deforms the foil and changes its electrical resistance. The change in resistance is then related to the strain via the measurement known as the gauge factor. In addition, the change in resistance is normally determined through the use of a Wheatstone bridge, a device used to measure electrical resistance.
Strain gauges are usually made from constantan. It is an alloy containing approximately 55 percent copper and 45 percent nickel. This prevents the strain gauge from being affected by an external temperature.
Semiconductor strain gauges are used to measure smaller quantities of strain. They generally have larger gauge factors than foil gauges. Semiconductor strain gauges are more sensitive to changes in temperature and are more fragile than those made of foil patterns.
Another type of gauge is used for the purpose of determining biological strain. This is known as the mercury-in-rubber strain gauge. This type of strain gauge is used primarily in procedures involving blood flow and tissue swelling. It is made of an amount of mercury inside a rubber tube. This tube is then wrapped around a specific body part such as a leg or a toe to measure the strain it experiences.