1. What a Thermistor Is and Why Heat Lowers Its Resistance
The name "thermistor" is thought to come from shortening the English phrase "thermally sensitive resistor." It is a semiconductor device where electrical resistance changes greatly with temperature, used for measuring or controlling temperature.[5] When people say "thermistor," they usually mean an NTC thermistor, where resistance drops as temperature rises. These are made from ceramics created by sintering oxides of elements like manganese, nickel, and cobalt.[1]
The change in resistance is much larger than in metal. A rule of thumb for NTC thermistors is that resistance changes by about 3–5% per degree Celsius.[1] If the resistance is 100, it might drop to around 95–97 for each degree rise. This is just a rough guide; the actual change depends on the temperature range and the specific type. Because small temperature differences create clear electrical differences, these parts are easy to use as thermometers.
2. How Heat Increases the Carriers That Carry Electric Current
When electricity flows, it is carried by free electrons and holes (the spaces left after an electron leaves). These carriers determine how well current flows. In metal, the number of carriers hardly changes with temperature. Instead, heat makes metal atoms vibrate, which hinders the movement of carriers, so resistance increases.[1]
In the semiconductors used for NTC thermistors, heat causes the number of carriers to increase. While movement also becomes harder due to vibration, the increase in carriers wins out. As a result, resistance drops.[1] A common misunderstanding is that movement becomes easier in semiconductors when heated. In reality, movement still becomes harder, but the effect of more carriers is stronger.[1]
This tug-of-war between increased movement difficulty and increased carrier numbers creates the opposite behavior seen in metals compared to semiconductors.
3. How the Thermistor Went From Faraday's 1833 Discovery to a Useful Part
According to records, the phenomenon of resistance dropping with temperature was discovered by Faraday in 1833.[3] Some secondary sources say he used silver sulfide, but the original paper has not been confirmed.[3] For a long time, this remained just a scientific curiosity with no practical use. It stayed that way until 1930.[3]
In 1930, Samuel Ruben proposed using this phenomenon to measure high temperatures.[3] His US patent, filed in March 1930 and registered in November 1935, describes using a layer of cuprous oxide (copper oxidized at about 1000°C for about 30 minutes) to detect temperature via resistance drop.[4] This material is different from modern manganese-nickel-cobalt thermistors. However, NTC thermistors are described as having been commercialized by Ruben in the 1930s.[1]
4. How NTC Thermistors Measure Temperature and Control Current
NTC thermistors are used for temperature detection and control in smartphones, air conditioners, and cars. They also help control current in power supplies.[1] Because small temperature differences create large resistance changes, even small and cheap parts become easy to read for temperature.
In electronic thermometers, NTC thermistors detect body temperature by measuring resistance changes. In "predictive" models, the thermometer estimates the final temperature from the initial rate of rise to display a result quickly. One manufacturer’s instructions state that for about 2 minutes and 30 seconds after starting, it shows a predicted value, and after that, it shows the highest measured value.[6] Times vary by model, so this is just one example of how thermistors are used in medical devices.
5. How PTC Thermistors Differ From NTC Thermistors
PTC thermistors are ceramics made from barium titanate with a tiny amount of rare earth elements added. When the temperature exceeds a certain point (the Curie point), the crystal structure changes, making it hard for electrons to move. Consequently, resistance increases suddenly.[2]
This property is used in self-heating devices. Examples include resettable fuses that limit fault current using their own heat, and self-regulating heaters that maintain a constant temperature by balancing heat generation and heat loss.[2] Murata Manufacturing Company began mass-producing PTC thermistors in 1961 and holds the trademark "Posistor."[2]
So, within the family of "thermistors," there are two types: NTC, which conducts better when hot, and PTC, where resistance suddenly increases above a certain temperature.
6. How to Find Out Which Thermometer Type You Have at Home
You can look at the manual for an electronic body thermometer at home. Check if it uses terms like "predictive" or "measured." If so, see how long it takes to display the temperature.
You might also check manuals for air conditioners or smartphones to see if they mention parts that detect temperature or functions that change based on heat. Note that specific part names are often not listed in consumer manuals.[1][2]