by Chris Woodford. Last updated: April 22, 2016.
If you had to write a soundtrack for the 20th century, electric guitars would almost certainly be playing the tune. No other instrument defines the angry rebelliousness of the modern age quite like it. Who could forget Chuck Berry, Jimi Hendrix, the Rolling Stones, or Nirvana—some of the greatest exponents of guitar-driven rock? But if you think playing an electric guitar is all about attitude and dexterity, think again: it's actually about the science of electromagnetism. Let's take a closer look a how these amazing instruments turn electricity into sound.
Photo: A Schecter Synyster Gates electric guitar in its distinctive black mahogany with silver stripes. The fingerboard is made of ebony. Note the Seymour Duncan pickups underneath the strings. Photo by courtesy of Gabriel Pollard, published on Flickr under a Creative Commons Licence.
What's the difference between acoustic and electric guitars?
An ordinary (acoustic) guitar makes sound entirely by vibration. When you pluck a string, it vibrates back and forth, transmitting sound energy into the hollow wooden body of the guitar, making it (and the air inside) resonate (vibrate in sympathy) and amplifying the sound (making it considerably louder).
If you've ever seen an electric guitar, you'll have noticed that most of them have solid bodies that are thinner (and sometimes much smaller) than those of acoustic guitars. Although most electric guitars are wooden, the material from which they're made is not critical. As George Beauchamp (pioneer of the modern electric guitar) pointed out in his patent back in the 1930s: "The body may be varied considerably in size, shape and construction, and may be constructed of various materials without departing from the spirit of the invention"; his original design suggested the body could be made from "a simple integral casting of metal such as aluminum." Early electric guitars were made from all kinds of materials, including molded Bakelite (one of the first plastics) and sheets of soldered brass.
Why does the material matter less than with an acoustic guitar? Because the body of an electric guitar is much less important in producing and amplifying the sound; all it really has to do is hold the strings so they're long and tight enough to make the kind of sound frequencies we want to hear. Although resonance still plays an important part in giving an electric guitar its tone, solid-body electric guitars generate most of their sound through an entirely different process from acoustic guitars. In fact, even though acoustic and electric guitars look similar, and you play them in a broadly similar way, they are quite different instruments.
Photo: Pick up an acoustic guitar and what you have in your hands is mostly empty space! Inside, the large wooden body (which is called the sound box) is filled with nothing more than air. As you pluck the strings, the wooden body of the guitar vibrates, the air inside the body vibrates too, and it's the vibrations of the wooden body and the air that amplify the string sound so you can hear it. Acoustic guitars have those big holes in the middle of the case, under the strings, so the amplified sound can get out and travel to your ears. In other words, the hole works the same way as the hole in the end of a trumpet, flute, or any other instrument. Photo by Greg Pierot courtesy of US Navy.
Electromagnetism in electric guitars
Electric guitars are powered by electromagnetism—and electromagnetic induction to be precise. That might not sound familiar, but you've probably used it if you've ever ridden a bicycle at night with a dynamo-powered light. A dynamo is a simple electricity generator with two basic parts: a rotating coil of wire that spins around inside a hollow, curved magnet. As the coil spins, it cuts through the magnet's field. This makes electricity flow through the coil. Two electrical connections from the coil are wired up to a lamp and the electricity generated makes the lamp light up.
To cut to the chase, we can say that a changing magnetic field generates or "induces" electricity. It's also true that a changing electric field generates magnetism. If you feed electricity through a coil of wire, you generate a magnetic field around it. That's how you can make a magnet controlled by electricity—better known as an electromagnet. Electricity and magnetism are really two different aspects of a single phenomenon: electromagnetism.
What does all this have to do with guitars? Crudely speaking, the metal strings of an electric guitar are a bit like dynamos: they make electricity when you move them. Under the strings, there are electricity-generating devices called pickups. Each one consists of one or more magnets with hundreds or thousands of coils of very thin wire wrapped around them. The magnets generate a magnetic field all around them that passes up through the strings. As a result, the metal strings become partially magnetized and, when they vibrate, make a very small electric current flow through the wire pickup coils. The pickups are hooked up to an electrical circuit and amplifier, which boosts the small electric current and sends it on to a loudspeaker, making the familiar electric guitar sound. Usually, the amplifier and loudspeaker are built into a single unit called an "amp."
Photo: Electromagnetism at work: a closeup view of the pickups under the strings of an electric guitar. Photo by Arif Patani courtesy of US Navy.