Measuring instruments are tools used to measure physical quantities such as length, distance, angle, and force. Calipers, which have 8 letters, are used to measure the distance between two opposite sides of an object. Interferometers, with 15 letters, are used to measure the wavelength of light and other electromagnetic radiation. Gauges, with 6 letters, are used to measure pressure, vacuum, and other fluid properties. METERS, also with 6 letters, are used to measure the length of a particular object.