THERMISTOR

Thermionic is a heat sensitive resistor usually made from semiconductor. It is a non-ohmic conductor and possesses higher –ve temperature coefficient for resistible (ρ). As the resistible increases (decreases) then corresponding resistance will increase (decrease). Here, due to –ve resistance resulting increase of current in the circuit. This determines the temperature of the hot body.

Thermistors with high negative temperature coefficient are used as resistance thermometer in very low temperature measurements of the order of10 K. Higher resistance at low temperature leads to accurate measurement of temperature in this range.

In general we can say that the use of thermistor for temperature measurements gives more accurate result in the low temperature region than measurement temperature using a platinum resistance thermometer.

THERMAL EQUILIBRIUM AND ZEROETH LAW OF THERMODYNAMICS

It is a common knowledge that a hot body looses heat and becomes cooler when put in contact with a cold body. Heat transfer always takes place from a body at higher temperature to another at lower temperature. This process continues until the temperatures of the two bodies become equal. The two bodies are said to be in thermal equilibrium. For example, take two glasses of water at different temperature (say) 40ºC and 20ºC.When they are mixed, the temperature of the first falls down and the temperature of the second rises. This process is continued until their temperatures become equal and exchange of heat stops. Its shows that thermal equilibrium is the stage where flow of heat between two bodies or systems stops; and this happens only when the bodies concerned are at same temperature. Two bodies in thermal contact are said to be in thermal equilibrium if there is no net transfer of heat them.

  

Sometimes whether two systems A and B are in the thermal equilibrium can be inferred without pulling them in contact.

It can be done by making use of third system C. Suppose that C and A are in the thermal equilibrium and that C and B are in thermal equilibrium. Then A and B should be in the thermal equilibrium. A great deal of indicates that if two systems, then they are in thermal equilibrium with each other. This postulate is called the Zeroeth law of thermo dynamics. The importance of the Zeroeth law is that it allows a useful definition of temperature. All bodies in thermal equilibrium have same temperature. A hot body is assigned higher value of temperature than a cold body. Heat flows from the body at higher temperature to the body at lower temperature. On the basis, the Zeroeth law defines temperatures of a system as the property which determines whether it is in equilibrium with other bodies or it is not.