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.
The Seebeck Effect
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, 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.
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Thermostat: A Simple Device
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 — often copper and aluminum, tungsten and nickel, or some combination of these 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 thermometer tells you the temperature of the room or the outdoors. A thermostat acts to control the temperature of the room by heating or cooling that room to a preferred temperature based on settings you have chosen.
Conductivity of a Thermistor
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.
A thermistor doesn't actually read the temperature the way a thermostat does. Instead, the resistance of a thermistor changes as the temperature changes. So how much the resistance changes depends on the material used in the thermistor.
Thermostat and Thermistor Contrasts
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.