INTRODUCTION
Thermistors are thermally sensitive resistors and have either a negative (NTC) or positive (PTC) resistance/temperature coefficient. NTC thermistors, or NTCs for short, are semiconductors made from specific mixtures of pure oxides of nickel, manganese, copper, cobalt, tin, uranium, zinc, iron, magnesium,

titanium, and other metals sintered at temperatures above 1800 ° F (982 ° C). The type and proportion of oxides used, the sintered atmosphere, and the sintering temperature dictates the resistance and temperature coefficient of the NTC thermistors. Their distinguishing characteristics are a high temperature coefficient and the fact that their resistance is a nonlinear function of absolute temperature. This makes them very good for narrow span measurement, but more difficult to handle for widespan applications. PTCs are  manufactured from silicon (silistors) or barium, lead, and strontium titanates with the addition of yttrium,manganese, tantalum, and silica (switching PTC thermistors). Silistors have lower sensitivity than NTCs, but their resistancetemperature characteristic is more linear. Switching PTCs have extremely high sensitivity in a narrow range of temperature centered at the Curie temperature of the material. used outside the laboratory. The credit for the development of thermistors as we know them today must be given to Bell Laboratories. They started to manufacture them some 60 years ago, naming them from the term  thermally sensitive resistors. The Bell project resulted in the development of thermistors stable and reproducible enough to make their large-scale use worthwhile in telephone work around 1940. The industry in general did not accept these sensors until the 1950s. Bad experiences with commercially available thermistorshampered their acceptance. Variances in resistance at a given temperature and in rate of change of temperature  made individual calibration a requirement. Overcoming this difficulty, one manufacturer patented a process for interchangeable thermistors, resulting in production of probes interchangeable to 0.05 ° F (0.03 ° C). Boosted by aerospace industry requirements, the 1950s and 1960s witnessed the