The relative change (expansion or contraction), expressed as a ratio, in a linear dimension of a body, corresponding to a change of 1^o in the body's temperature. The coefficient of expansion, as it is generally called, may be in terms of the Celsius (SI), Fahrenheit, or other thermometric scale. To a very great extent, its magnitude is peculiar to the material' a steel tape, for example, has a coefficient of expansion about 25 times as great as has an invar tape. The coefficient is usually expressed as a decimal fraction. In a measure of length such as the length of a base line, it enters as a correction which is the product of the coefficient of expansion of the apparatus used, the length measured, and the difference between the temperature at which the measurement was made and the temperature (standard or calibration) at which the length of the apparatus is known.