Thermistor

A thermistor is a resistor whose resistance changes strongly with temperature. In the NTC example shown here, warming lowers resistance and cooling raises it. NTC means Negative Temperature Coefficient. PTC types have a Positive Temperature Coefficient: their resistance rises as they warm over their specified operating region. A measuring circuit turns the resistance change into a useful signal.

The stated resistance belongs to a reference temperature, often 25 °C. Check the exact type, temperature range and resistance curve. A thermistor has no polarity, but measuring current can warm it and change the reading.

In SparkVille’s greenhouse, the gardener wants seedlings to stay snug through a chilly night. An NTC thermistor changes resistance as the air warms or cools. A separate controller reads that change and switches the heater — the thermistor only senses the temperature.