The first bonded metal wire strain gauge was developed in 1938. The metal foil type strain gauge consists of a grid of wire (a resistor) approximately 0.001 inches (0.025 mm) thick, which is directly connected to the strained surface and is made of a thin layer of epoxy. When a load is applied to the surface, the resulting change in surface length is transmitted to the resistor, and the corresponding strain is measured based on the resistance of the foil wire, which changes linearly with the strain. The foil diaphragm and the adhesive must work together to transmit strain, and the adhesive must also function as an electrical insulator between the foil grid and the surface. When choosing a strain gauge, you must consider not only the strain characteristics of the sensor, but also its stability and temperature sensitivity. Unfortunately, the ideal strain gage material is also sensitive to temperature changes and tends to change resistance as it ages. For short-term applications, this may not be a serious problem, but for continuous industrial measurements, temperature and drift compensation must be included.