Thermistors can be classified into two types, depending on the sign of formula_4. If formula_4 is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, or posistor. If formula_4 is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor. Resistors that are not thermistors are designed to have a formula_4 as close to 0 as possible, so that their resistance remains nearly constant over a wide temperature range.