Strain is defined as the change in length DI of a material based on its original length I0.
The strain is generated e.g. by the influence of temperature or force or by physical effects such as e.g. magnetostriction or piezoelectricity.
The relationship between strain and its cause is described by material coefficients, for example, thermal expansion coefficient, elastic modulus, magnetostriction or piezo module.
The strain may have different amounts or different signs in different directions, because the material constants have different values for different directions or because the cause effects in a certain direction.
The strain unit is meter per meter.
In technics the strain is most specified in microns per meter (µm/m).
Many strains are in this order of magnitude, for example, the thermal strain of metal (10 ... 20 µm/m per Kelvin temperature change) or the elastic strain of machine parts (0.1 ... 1000 µm/m).
1 µm/m can be also written (by "way of shortening" of the unit) as 10-6 or 1 part per million (ppm) or 0.001 per thousand, or 0.0001 percent.
In the Anglo-American area the names "Microepsilon" and "Microstrain" are common. A Microepsilon (µe) is so much like 1 micrometer per metre (µm/m).
Many physical quantities are indirectly determined in which the strain is measured. There are countless scales in the technics, in trade and in the households, near pressure sensors, force sensors, acceleration sensors, torque sensors...
The strain gauge is used for converting the strain into an electric signal.
Thereby a bridge circuit is always applied.