Practical examples of the memristor have been
developed only in the last decade.
History of Inductors
The unit of inductance is the henry, named after the
American scientist, Joseph Henry (1797–1878). Henry
was the first to wrap multiple coils of wire around a piece
of iron to create an electromagnet that was more powerful
than a solenoid coil with no iron core.
English scientist, Michael Faraday (1791–1867) — for
whom the unit of capacitance, the farad, is named — did
similar experiments at about the same time. Faraday
discovered that an alternating current in one transformer
winding will induce a current in another. Note the
similarity between the words, “induce,” and “inductance.”
The Solenoid
The simplest inductor is a solenoid coil, which is just a
helix of wire with no core (Figure 2). Passing a direct
current through a solenoid creates a magnetic force; a fact
discovered in the early 19th century.
A solenoid essentially stores the electrical energy of
flowing current in a magnetic field. When the core of the
solenoid is empty, this energy is stored in the physical
vacuum, also known as free space.
Since air has a low magnetic permeability, much of
the magnetic field potential afforded by applied currents is
lost, so the inductance is smaller than when a magnetic
core is introduced (see Figure 3).
If our solenoid coil is very long, it’s inductance L is
easily calculated:
L = N2µ0A/ℓ
By Dev Gualtieri, Tikalon LLC
( gualtieri@ieee.org)
July/August 2018 45
FIGURE 1. The four members of the passive
components family: the resistor R; capacitor C;
inductor L; and the memristor M. The component
values are determined by their response to
current i, voltage v, charge q, and magnetic flux f.
FIGURE 2. A simple air-core solenoid with its
magnetic field lines shown.
To post comments on this article and find any associated files and/or
downloads, go to www.nutsvolts.com/magazine/issue/2018/07.
FIGURE 3. A solenoid
coil wrapped on a
ferrite core.
