This chapter introduces our third passive device, the inductor. Inductors are fundamentally different from both resistors and capacitors in terms of their construction and their operation. Inductors do, however, share certain broad traits with capacitors. First, they are energy storage devices. In the case of the inductor, energy is stored in a magnetic field, similar to the case of the capacitor which utilizes an electric field. Further, in the ideal case, inductors do not dissipate power. Also like capacitors, when inductors are placed in DC circuits they are not ohmic, meaning that their current-voltage characteristic does not respond to Ohm's law. Instead, their currentvoltage characteristic is dynamic in nature. In some respects, though, their current-voltage behavior is opposite to the way in which capacitors behave, and thus they offer their own unique performance characteristics.
Inductors have a long history of use in electronic systems. In fact, one of the most common uses in the home is as an integral part of a typical loudspeaker system. They are used in modern switch-mode DC power supplies in products such as personal computers and televisions. In audio and communications systems they are used in filters and tuning circuits alongside capacitors. Like a capacitor, for any application that needs to smooth out a varying voltage, store energy or filter a signal; an inductor is a likely candidate.
Unfortunately, real-world inductors generally do not behave as close to their desired ideal operation as do realworld capacitors. The secondary effects of inductor construction limit their performance; perhaps the most notable factor being their potentially large equivalent series resistance. They are also susceptible to external magnetic fields which can introduce noise and interference, degrading signal quality. For these reasons, there are areas where, given a choice, capacitors would be preferred over inductors. But this is by no means a broad condemnation and there are areas where the use of inductors is essential. Beyond this, the very concept of inductance is important in that informs designers of the practical limits of performance of their circuits.
The concept of electromagnetic induction was first discovered by English scientist Michael Faraday in the early 19th century. He noticed that if he wrapped two wires around an iron ring and introduced a current in one of them, then a transient (shortlived) current would appear in the second coil of wire. He noticed a similar effect when he slid a bar magnet through a wire coil. Around the same time, American Joseph Henry discovered much the same independently of Faraday and performed considerable research in this area. In his honor, the unit of inductance, the henry (abbreviated H), is named after him. The symbol for inductance is L, named after physicist Heinrich Emil Lenz. Inductors are commonly referred to as coils or chokes, for reasons that will be apparent shortly