A thermostat and thermistor are two ways to use metal and its reactions to temperature to read and report temperature changes. Metals of different types, especially copper, tungsten and aluminum, change certain properties as they become hotter or colder. The changes can be used to measure how temperature changes. As the metals change properties, such as their conductivity to electricity, the result is a reliable guide to how temperature changes. Both methods of measuring use metals and their properties relative to temperature, though in different ways.
Most modes of measuring temperature are based on the Seebeck Effect. The effect refers to the simple property of certain metals to change in significant ways according to changes in temperature. Specifically, the certain metals change conductivity according to changes in temperature. The principle is that changes in temperature produce an electric effect, a potential, that alters how electricity flows through a metal. The principle can be used to measure temperature changes.
The common thermostat is a fairly simple device. It is based primarily around the comparative movements of two metals as they press down on an electric connector or contact. Two metals are in a thermostat. They are often copper and aluminum, tungsten and nickel, or some combination of those metals. When they experience a temperature change, their responsive movements produce pressure that either presses against the electric contact or pulls away from the electric contact. The device is calibrated to perform the requisite movement at just the correct temperature to press down on the contact.
A thermistor is based on the same principle as a thermostat, but it is used in a different manner. A thermistor uses a metallic oxide compound such as cobalt or manganese. The principle is that the conductivity of the metallic oxide changes according to temperature. Depending on the metallic oxide compound used, usually the conductivity increases as temperature increases; the amount of electricity moving through the compound changes according to temperature. Therefore, the device is calibrated to read changes in conductivity as reflecting changes in temperature.
In general, a thermostat is a cruder device than a thermistor. Because conductivity is based on the mobility of the electrons in the metal, even tiny changes can be reported in a thermistor. A thermostat only permits the metals to rise above or press upon a contact as temperature changes. A thermistor is more complex because it can read changes in conductivity and, thus, can express minute changes in temperature as conductivity changes.