Thermistor are composed of sintered mixture of metallic oxides such as manganese, nickel cobalt, copper, iron and uranium. They are available in variety of sizes and shapes.
Thermistors may be in the form of beads, probes, rods and discs.
Some of the commercial form are shown in figures.
Smallest Thermistors are in the form of heads with a diameter of 0.15mm to 1.25mm. This is the most familiar type of Thermistor usually glass coated.
Beads may be sealed in the tips of soild glass rods to form probes. Glass probe have a diameter of about 2.5mm. the probes are used for measuring temperature of liquids.
Discs are made by pressing material under high pressure into cylindrical flat shapes with a diameters ranging from 2.5mm to 25mm. they are mainly used for temperature control.
Washer type Thermistor:
Washer type is usually long cylindrical units. Leads are attached to the ends of the rods. The advantage of this type is, it produce high resistance under moderate power.