Martin Thomas L. Electronic Circuits

Second edition. — Englewood Cliffs: Prentice-Hall, 1955. — 707 p.The ordinary electric circuit elements of resistance, inductance, and capacitance are generally assumed to be linear and bilateral. Over some specified range of operation, the current through such components is a linear function of the applied voltage, and current flows with equal ease in either direction. As a result, it is possible to characterize such devices by single constants. Hence, it is said that a given component is a resistor of so many ohms, a capacitor of a certain number of farads, or an inductor of a specified number of henries. Actually, such a description is true only over a specified range of operation, and is therefore simply an equivalent representation of the device within certain operating limits.
In some cases, thermistors for example, the device might be very nonlinear over its customary operating range. It is not possible then to represent the component by a single equivalent. Instead, the characteristics are usually described graphically by showing a plot of the current through the device as a function of the applied voltage. This is called a current-voltage characteristic. For ferromagnetic or ferroelectric components, a plot of flux density as a function of field intensity is usually used.