Principles of Strain Gages
If external tensile force or compressive force increases or decreases, the resistance proportionally increases or decreases. Suppose that original resistance R changes by ΔR because of strain ε: the following equation is set up.

ΔR/R =Ks ·ε

Where, Ks is a gage factor, expressing the sensitivity coefficient of strain gages. General-purpose strain gages use copper-nickel or nickel-chrome alloy for the resistive elements, and the gage factor provided by these alloys is approximately 2.

Types of Strain Gages
Types of strain gages are classified into foil strain gages, wire strain gages, and semiconductor strain gages, etc.

Structure of a Strain Gage
The foil strain gage has metal foil on the electric insulator of the thin resin, and gage leads attached, as shown in Fig. 1 below.