Linear and Non Linear Resistors
Linear resistors are those in which current produced is directly proportional to the applied voltage. Their current versus applied voltage is straight and linear. In other words, their resistance remains constant.
Non-Linear resistors are those whose current does not change linearly with changes in applied voltage. It is so because the current flow always results in production of heat, which either increases (as in metals) or decreases their resistance (as in insulators). Because of this change in resistance the current through such a resistor is not directly proportional to the impressed voltage. Circuits of this types are called non-linear circuits and the current carrying elements are known as non linear resistors. In the figure 'A' represents the case of a linear resistor, whereas 'B' and 'C' represents non linear resistors. In case of 'B', current increase more than proportionally with applied voltage, which means that for this material resistance decreases with rise in temperature.It is examplified by Thyrite, which is a non metallic non linear resistor.
Curve 'C' also represents a non linear resistor but one whose resistance increases more than proportionally with applied voltage. Example of such a resistor is a semi conductor material chemically similar to ceramic oxide.