What is change in resistance of an electric resistance strain gauge?
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Lord Kelvin presented to the Royal Philosophical Society the results of an experiment involving the Îelectrical resistance of copper and iron wire when subjected to strainâ in 1856. Kelvinâs observations are consistent with the relationship of electrical resistance to some of the physical properties of a conductor:
R = rL/A
where R is the electrical resistance, ris the conductivity constant, L is the length of the conductor and A is the cross sectional area. Resistance is directly proportional to the length and inversely proportional to the cross sectional area. The electrical resistance of a metal wire increases as it is stretched as a result of the decreased cross sectional area and an increase in the length of the wire. Conversely, as the wire is compressed and the length decreases, with a corresponding increase in cross sectional area, the electrical resistance of the material decreases.
The relationship between length and cross sectional dimension can be expressed by Poissonâs ratio:
n = -(dD/D)/dL/L = eL / ea
where n is Poissonâs ratio, D is the cross sectional dimension and L is the length, eL is the lateral strain and ea is the axial strain. It basically states that as the length decreases (compression) for a material, the cross sectional dimension increases and vice versa for an increase (tension) in length for a material.
Lord Kelvinâs discovery was not put to any practical use until the 1930âs. Carlson is credited with one of the first recorded instances of a wire resistance strain gauge being applied to measure stress in 1931. The use of a bonded wire gauge to measure strain was conceived at about the same time by Simmons and Ruge in 1938. A wire gauge was mounted and bonded between two thin pieces of paper. The general construction of a bonded wire type strain gauge is shown in Figure 16.

Figure 16 General arrangement of a bonded-wire type strain gauge [4]
The bonded wire gauge has largely been replaced by the foil gauge which has been in production since the 1950âs. This type of gauge consists of a metal foil grid which is bonded onto an epoxy support. Printed circuit techniques are used in the manufacture of bonded foil strain gauges. Foil configurations can be varied and complicated. (Figure 17)
Figure 17 Different Foil Gauge Configurations [4]
The selection of a strain gauge for a particular application is affected by the following gauge characteristics: grid material and construction, backing material, bonding material, gauge protection and gauge configuration. Gauge design incorporates as many of the following features as possible: high gage factor, high resistivity, temperature insensitivity, high electrical stability, high yield point, high endurance limit, ease of working, low hysteresis, low thermal emf with other materials and durability. A variety
R = rL/A
where R is the electrical resistance, ris the conductivity constant, L is the length of the conductor and A is the cross sectional area. Resistance is directly proportional to the length and inversely proportional to the cross sectional area. The electrical resistance of a metal wire increases as it is stretched as a result of the decreased cross sectional area and an increase in the length of the wire. Conversely, as the wire is compressed and the length decreases, with a corresponding increase in cross sectional area, the electrical resistance of the material decreases.
The relationship between length and cross sectional dimension can be expressed by Poissonâs ratio:
n = -(dD/D)/dL/L = eL / ea
where n is Poissonâs ratio, D is the cross sectional dimension and L is the length, eL is the lateral strain and ea is the axial strain. It basically states that as the length decreases (compression) for a material, the cross sectional dimension increases and vice versa for an increase (tension) in length for a material.
Lord Kelvinâs discovery was not put to any practical use until the 1930âs. Carlson is credited with one of the first recorded instances of a wire resistance strain gauge being applied to measure stress in 1931. The use of a bonded wire gauge to measure strain was conceived at about the same time by Simmons and Ruge in 1938. A wire gauge was mounted and bonded between two thin pieces of paper. The general construction of a bonded wire type strain gauge is shown in Figure 16.

Figure 16 General arrangement of a bonded-wire type strain gauge [4]
The bonded wire gauge has largely been replaced by the foil gauge which has been in production since the 1950âs. This type of gauge consists of a metal foil grid which is bonded onto an epoxy support. Printed circuit techniques are used in the manufacture of bonded foil strain gauges. Foil configurations can be varied and complicated. (Figure 17)
Figure 17 Different Foil Gauge Configurations [4]
The selection of a strain gauge for a particular application is affected by the following gauge characteristics: grid material and construction, backing material, bonding material, gauge protection and gauge configuration. Gauge design incorporates as many of the following features as possible: high gage factor, high resistivity, temperature insensitivity, high electrical stability, high yield point, high endurance limit, ease of working, low hysteresis, low thermal emf with other materials and durability. A variety
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