Other people have answered what the resistance does (create a voltage divider that feeds a voltage to the A/D converter), so I'll describe ways to select the re

Other people have answered what the resistance does (create a voltage divider that feeds a voltage to the A/D converter), so I'll describe ways to select the resistance value. There are several specifications that might have to be met: * Power delivery limit of the energy source * Current specification for optimal operation of the thermistor * Voltage level of the A/D input The resistance value might be completely constrained by one of these requirements or, in tougher cases, might be constrained by more than one requirement (often in different directions). For example, if the thermistor has an effective resistance of about 1kOhm and requires a current of about 1mA, then you need to set the resistor to 4kOhm (assuming the A/D input draws negligible current). If this is the case, you're stuck on the other requirements. Your power consumption will be 5mW (5Vx1mA) and the nominal level of the A/D converter input will be about 4V (5Vx4k/(1k+4k)). Experienced circuit designers seem to magically come up with correct values because they are familiar with typical requirements and can quickly identify the limiting specification. You'll have to struggle a bit at the beginning to get the hang of it.