How Inductors Work

Inductors form the cornerstone of our technological world.  From the generation and transmission of electrical power in the United States to the design and manufacture of virtually every electronic device in wide use today, inductors are a key component

What is an inductor?

A simple inductor is a coil of wire with a single terminal at either end which, when electric current passes through it creates a magnetic field, thereby allowing it to store energy.

How inductors work

When an alternating current passes through an inductor the magnetic field which is created expands and collapses at the frequency of the alternating current. If another inductor is placed near this inductor, the expanding and collapsing magnetic field intersecting the windings of the second inductor causes current to flow through the second conductor. This phenomenon is known as “induction” and such a device is known as a “transformer”. The inductance of the inductor is expressed in “henries”.

Inductive reactance

When an alternating current flows through an inductor, as the magnetic field expands the inductor resists the flow of the ac current. This phenomenon is called “reactance”. The inductive reactance of an inductor is expressed in ohms.

History/practical applications of inductors

Many scientists worldwide have made significant contributions to the field of electricity and magnetism and the development of the inductor. Most prominent were England’s Michael Faraday and America’s Joseph Henry, who worked in concert in the 1830’s investigating the relationship between electricity and magnetism. Although working independently, they both discovered many of the same things. However, Michael Faraday will probably be remembered more for his theoretical contributions while Joseph Henry will be remembered for developing immediate practical applications as a result of the discoveries made by the two scientists.

In 1600 on the eve of the Scientific Revolution, William Gilbert, British scientist and physician, known as “The father of electricity and magnetism”, published “De magnate”.  In this important book which he wrote entirely in Latin after 17 years of experimentation with electricity and magnetism,  he detailed the peculiarities of the magnetic compass and described the earth as a great magnet. Gilbert coined the word “electricity” from the Greek word “amber” and he was honored to have the unit of magnetomotive force, the “gilbert” named after him.

In 1819 Danish scientist Hans Oersted accidently observed that current flowing through a conductor caused the needle of a nearby compass to move. He concluded from this observation that the current actually created a magnetic field around the conductor.

In 1820 French physicist and mathematician Andre Marie Ampere (1775 – 1836) published a paper following Oersted’s discovery that explained electromagnetism.  He also helped develop the galvanometer, a device for measuring electric current flow. Ampere was honored by having the unit of electric current named after him.

In 1824 British scientist William Sturgeon invented the electromagnet by winding a horseshoe shaped piece of metal with 18 turns of uninsulated bare copper wire and connecting the ends of the wire to a battery. He coated the horseshoe shaped metal with varnish to insulate it from the wire. In 1825 he exhibited the electromagnet for the first time. During the exhibition Sturgeon’s electromagnet, which only weighed 7 ounces, was able to lift a weight of nine pounds.

American scientist Joseph Henry later made improvements to the electromagnet by using wire insulated with silk, and was able to create a powerful electromagnet by winding multiple windings of the insulated wire around a metal core. His electromagnet was able to lift 2,063 pounds! 

In 1830 Joseph Henry demonstrated the potential of Sturgeon’s electromagnet by sending a dc current over a mile of wire which activated an electromagnet, causing a bell to ring. Thus the electric relay was born. Samuel F.B. Morse, who invented Morse Code, successfully used Henry’s invention to develop the telegraph and exploit it commercially.  Although many people disputed who actually invented the telegraph, Henry or Morse, Joseph Henry nobly defended Samuel Morse’s patent.

Also in 1831 Michael Faraday discovered the phenomenon of induction by constructing the following experiment;  a paper cylinder wrapped with wire and the ends of the two wires connected to a galvanometer. He then moved a magnet in and out of the inside of the cylinder as he observed the galvanometer, which indicated that a small amount of current was being generated. This great discovery was to lead to the development of transformers, motors and generators.

The solenoid

The solenoid is an electromagnet with windings of wire wrapped around an air core.  If you’re familiar with sprinkler systems, you’ve probably noticed a small black cylindrical device on your sprinkler system zone control valve. The black material is a coating of rubber to protect the winding of the solenoid from water. When 24 volts ac is applied across the windings of the solenoid, a small metal cylinder is pulled into the core of the solenoid and opens a valve which allows water to flow to the sprinkler heads.

Another application of the solenoid would be as an ignition relay to apply 12 volt battery power to a dc motor called a “starter” which turns over your car’s engine to start your car.

The oscillator circuit

A basic course in alternating current theory tells you that in an ac circuit when you pass current through a pure resistance the current and voltage are “in phase”.  While doing a vector analysis of a simple ac circuit containing a resistor, a capacitor, and an inductor, however, you’ll discover that the current will lag the voltage through the inductor and lead the voltage through the capacitor by a varying number of degrees .  This phenomenon allowed circuit designers to develop the “tank circuit” consisting of a capacitor connected in parallel with an inductor. As the magnetic field created by the inductor expands and collapses, the capacitor charges and discharges. The result is an “oscillator” circuit.

All “tank circuits” have a “resonant frequency”, depending upon the value of the capacitance in farads and the value of the inductance in henries. As the frequency increases, the reactance of the capacitor decreases and the reactance of the inductor increases. When the inductive reactance becomes equal to the capacitive reactance, the circuit has reached what is known as its “resonant frequency” and its total impedance will be at a maximum at this point. Tank circuits are used in the design of radio equipment.

Electric power generation and transmission

Motors, generators, and power transformers which principally incorporate inductors in their design all form the basis for the production and transmission of electrical power in the United States today.

The DC power supply

Large inductors and capacitors are utilized as “ripple filters” to smooth out the ac component of the output of a dc power supply circuit following rectification.


Thanks to the magnificent minds of the world’s leading scientists and inventors since 1600, the inductor has become an integral part of our modern world, contributing in a large way to the life style we enjoy today.