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Q: What is Inductance?

A: An inductor is an electronic component consisting simply of a coil of wire. If a constant electric current is running through the wire then this produces a magnetic field. If the current changes, so does the magnetic field. The unit of measurement for inductance is the henry (H), named after Joseph Henry, an American physicist who discovered it independently around the same time as the English physicist Michael Faraday did.

One single henry is the amount of inductance that is required to induce one volt of electromotive force when the current is changing at one ampere per second. There are three laws that are linked to inductance. These are:

  • Oersted’s Law: which states, that a constant electric current generates a magnetic field around the conductor.
  • Faraday’s Law: which states, that a changing magnetic field induces a current in a conductor.
  • Lenz’s Law: which states, that this induced current is in the opposite direction of the change in current that produced the magnetic field. This phenomenon is called self-inductance.
So what does this all mean and what is the benefit? Well, because the inductor changes its magnetic field in opposition to a current spike or current drop, it tends to maintain the current at its previous level, thereby resisting the change. This tends to maintain the current at a constant level. In other words, an inductor creates a kind of inertia within the current flow that resists rapid fluctuations in much the same way that a large body resists changes in its velocity.

One important application for inductors is that they tend to block high-frequency signals while letting lower-frequency oscillations pass. This is the opposite function of capacitors, which allow AC current to flow freely flow while blocking DC once the capacitor is charged. This is why DC capacitors are often used as low-pass or high-pass filters. If the circuit is an AC circuit and it is susceptible to DC noise (as would be the case with microphone inputs or connections between audio components), then a DC capacitor in series before the rest of the circuit will only allow AC signals through. This is known as circuit blocking. However, if a capacitor is placed between a signal and a ground, then it will prevent AC signals from passing. This is known as a DC decoupling circuit and they are used frequently to remove ripple voltages from DC power supplies so that they deliver cleaner voltage.

Thus, by combining the two components (i.e. capacitors and inductors) in any circuit, it is possible to selectively filter or generate oscillations of almost any desired frequency. However, modern circuitry rarely uses inductors anymore because they can achieve virtually all the same results with micro-circuits and capacitors.

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