CHEMISTRY - Thermistors

Thermistors :- A thermistor is a heat sensitive device whose resistivity changes very rapidly with the change of temperature. The temperature co-efficient of resistivity of a thermistor is very high, which may be positive or negative.
Thermistor are usually prepared from oxides of various metals such as nickel, iron, copper, cobalt etc. These compounds are also semi-conductor. The size of thermistor is very small. The thermistors are generally in the form of beads, discs or rods. A pair of platinum leads are attached at the two ends for electric connections. This arrangement is sealed in a small glass bulb.
A thermistor can have a resistance in the range of 0.1 ohm to 10 power 7 ohm, depending upon its composition. A thermistor can be used over a wide range of temperature.
A thermistor differs from an ordinary resistance in the following ways :-
1. The temperature coefficient of resistivity of a thermistor is a very high.
2. The temperature coefficient of resistivity of a thermistor can be both positive and negative.
3. The resistance of thermistor changes very rapidly with change in temperature.

Important application of thermistor :-

1. Thermistors are used to detect small temperature changes and to measure very low temperature. A typical thermistor can be easily measure a change in temperature of 10 power -3 degree celcious

2. Thermistor are used to safe-guard the filaments of the picture tube of a television set against the variation of current.

3. Thermistor are used to temperature control units of industries.

4. Thermistors are used in the protection of windings of generators, transformers and motors.

5. Thermistors are used for voltage stabilisation and remote sensing.

Super-Conductivity :- Professor K. Onnes in 1911 discovered that certain metals and alloys at very low temperature lose their resistance considerably. This photochemical is known as super-conductivity. As the temperature decreases , the resistance of the material also decreases , but when the temperature reaches a certain critical value ( called critical temperature or transition temperature ) , the resistance of the material completely disappeares I.e it becomes zero. Then the material behaves as if it is a super-conductor and there will be flow of electrons without any resistance whatsoever. The critical temperature is different for different materials.
Super-conductivity is a very interesting field of research all over the world these days. The scientists have been working actively to prepare a room temperature super conductor and have met with some success only.

Applications of super-conductors :-

1. Super conductors are used for making very strong electromagnets

2. Super conductivity is playing an important role in material science research and high energy particles physics.

3. Super conductivity is used to produce very high speed computers.

4. Super conductors are used for the transmission of electric power.