The electric and acoustic guitar are superficially similar, but they produce sound in totally different ways. Acoustic guitars make sound from vibrations of the body, driven by the vibrating bridge, which is in turn driven by the vibrating strings. The player controls the body’s vibrations by plucking and strumming the strings. All the power of the vibrations has to come from the player’s hands.

Electric guitars generate a little sound from their bodies directly, but it’s almost inaudible. The sound you’re hearing mostly comes from the speaker cone in the amplifier, driven by current from the wall. This current is controlled by a much weaker current originating in the guitar’s magnetic pickups. As the metal strings vibrate, they agitate the pickups’ electromagnetic field, sending a fluctuating current down the cable and into the amp circuitry. Good amps respond dramatically to very subtle touches on the electric guitar’s strings that would be inaudible on an acoustic instrument.

Jimi Hendrix was one of the first guitarists to think of his instrument as a way to modulate an electrical signal first and foremost. He didn’t just pluck and strum the strings; he scraped them and swatted them and played with their tension. And he produced his most distinctive sounds by letting the amp itself vibrate his guitar’s pickups. All of these techniques are at work in his iconic performance of “The Star-Spangled Banner” at Woodstock:

Jimi’s guitar is feeding back and heavily distorted. He also throws in a little wah-wah pedal. The time is free, or as the classical musicians say, rubato – drums don’t always need to keep a steady beat. Jimi interlaces the melody notes with inharmonic screams and yowls, produced by scraping the pick against the string’s winding. He throws in a few unresolved tritones at 1:35 and some terrifying divebombing sounds at 2:00. At around 2:30 he quotes part of “Taps.” This performance has been criticized as anti-American, but Jimi said in interviews that he considered himself to be patriotic.

The first generations of electric guitarists considered feedback to be bad, a technical mishap to be avoided. Jimi discovered ways to use it as a musical expression in its own right. If the amp is loud enough, its sound can physically shake a guitar’s pickups enough to produce a current. That current gets sent to the amp, which then vibrates the pickups harder, which sends even more current to the amp, which produces even more sound. This feedback loop builds rapidly, getting louder and louder. Every beginner electric guitarist discovers feedback accidentally by leaning their guitar against their amp without turning the volume down. Feedback can also be seeded spontaneously by the slight hum produced by any electrical system that uses alternating current, or by radio waves. Cheap, poorly shielded pickups and cables make great radio antennas. I used to live on Roosevelt Island, right across the East River from a Con Ed power plant with a whole bunch of big transformers. If I didn’t face due north while playing electric guitar, I picked up all kinds of radio signals and other electromagnetic noise. It was nice for experimental music, but not so great for producing a clean sound.

Feedback is more likely, and a lot louder, when the guitar is overdriven, its signal boosted and compressed to bring out and sustain overtones that are normally inaudible. Feedback has a mystical quality, an evolving life of its own. It’s unpredictable and hard to control exactly. It would be pointless to try to score a feedback composition because there are too many variables at work; it’s an intrinsically improvisational medium. The results can be annoying or boring, or they can be transcendant. You can experience the visual equivalent by pointing a video camera at the monitor showing its own output.

Jimi was also a pioneer in his exploration of the electric guitar’s microtonal possibilities. The conventional way to control a guitar string’s pitch is to press it against the frets, changing its length. You can also bend the strings, changing both their length and tension, for more nuanced pitch intervals. Jimi’s guitars have an additional pitch control, the whammy bar, which lifts the bridge, allowing very precise control of all six strings’ tension simultaneously. The whammy bar lets you play arresting microtonal chords effortlessly. It also quickly pulls the strings out of tune, which is why in the video Jimi is continually adjusting the tuning pegs whenever his left hand is free.

The video cuts out before this point, but at Woodstock Jimi segued from “The Star Spangled Banner” into “Purple Haze.” The song is based around a distinctive chord that has come to be nicknamed the Hendrix chord.

This chord is easy to play – any beginner could learn it – but intellectually it’s extremely intense. It contains every possible interval in the western tuning system (or implies them, I count the inversions too.)

Jimi didn’t invent the Hendrix chord. It had been a distinctive device in blues and jazz since before he was born. But where Duke Ellington and Thelonious Monk used the Hendrix chord for accents and embellishments, Hendrix pushed it front and center, using it as a cornerstone for songs like Purple Haze and Foxy Lady (in different keys than the one written here.)

The electric guitar doesn’t just offer a lot of the tonal and harmonic freedom. It also leaves the player’s mouth and feet free for more expression. You can use your feet to dance, or to control stomp boxes and expression pedals. Your voice is free for singing and talking. The electric guitar is some seriously advanced interface design.

Here’s a remix/cover of “Purple Haze” by my band Revival Revival, combining Jimi with Missy Elliot, M.I.A. and Miles Davis. Enjoy:

mp3 download, ipod format download

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