To understand negative resistance ( or conductance) we must define what positive resistance ( or conductance ) is. These primary elements act like absorbers of electromagnetic energy.Negative resistance ( or conductance) does almost the reverse. Negative resistance ( or conductance) does not actually generate energy. Rather they act to transform and release energy from a related source ( such as a battery ) into electromagnetic form. Negative resistance is defined as ” the property of a two terminal device with an internal source of energy which is controlled either by current through, or voltage across the terminals, but not by both”. Mathematically the difference between positive or negative resistance is illustrated by the slope at a point on a plot of the current – voltage characteristic of a device or system. The current is the ordinate and the voltage is the abscissa. If the current through the device or system increases with increased applied voltage ( positive slope) , then the sign of the slope is positive and the resistance is considered positive. If the current decreases with increasing applied voltage, then the slope will be negative. In this case the sign of the resistance ( or conductance ) will be negative i.e the device is a negative resistance ( or conductance).