When an object is subjected to mechanical stress, it contracts or
expands. This is called the Poisson effect to honor the French
scientist Simeon Poisson. Spring elements that exhibit Poisson
effects are used with the addition of strain gauges that vary in
electrical resistance when under stress. To measure the stress,
strain gauges are attached to the spring element surface that
contracts or expands and the mechanical changes in the surface
are transmitted to the gauge. The intensity of the electrical
resistance variation in the strain gauges is proportional to the
intensity of the applied force. Figure 1 shows the basic ringtype
geometry of a load cell. The four wide-black rectangles
represent the placement of the strain gauges. Compared
with the older devices, the actual strain gauges have evolved
and can be considered vector strain sensors. They are able to
indicate force, intensity, direction, and the method of applying
the mechanical force.