temperature coefficient of resistance. Depending on type of the thermistor in question the may be either positive or negative. If is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, or posistor, there are two types of PTCr, switching thermistor and silistor. If 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 as close to 0 as possible, so that their resistance remains nearly constant over a wide temperature range. Instead of the temperature coefficient k, sometimes the temperature coefficient of resistance ("alpha sub T") is used. It is defined as This coefficient should not be confused with the parameter below. Construction and materials Thermistors are typically built by using metal oxides. NTC thermistors are manufactured from oxides of the iron group of metals: e.g. chromium (CrO, Cr2O3), manganese (e.g. MnO), cobalt (CoO), iron (iron oxides), and nickel (NiO, Ni2O3). PTCs are usually prepared from barium (Ba), strontium, or lead titanates (e.g. PbTiO3). Steinhart–Hart equation In practical devices, the linear approximation model (above) is accurate only over a limited temperature range. Over wider temperature ranges, a more complex resistance–temperature transfer function provides a more faithful characterization of the performance. The Steinhart–Hart equation is a widely used third-order approximation: where a, b and c are called the Steinhart–Hart parameters and must be specified for each device. T is the absolute temperature, and R is the resistance. The equation is not dimensionally correct, since a change in the units of R results in an equation with a different form, containing a term. In practice, the equation gives good numerical results for resistances expressed in ohms or kΩ, but the coefficients a, b, and c must be stated with reference to the unit. To give resistance as a function of temperature, the above cubic equation in can be solved, the real root of which is given by where The error in the Steinhart–Hart equation is generally less than 0.02 °C in the measurement of temperature over a 200 °C range. As an example, typical values for a thermistor with a resistance of 3 kΩ at room temperature (25 °C = 298.15 K, R in Ω) are: B or β parameter equation NTC thermistors can also be characterised with the B (or β) parameter equation,