Adolph Rickenbacker invented the electric guitar.
From Frying Pan to Flying V: The Rise of the Electric Guitar
By the end of the 1930s, electronic amplification proved to be one of the most successful innovations for building a louder electric guitar, despite the misgivings of some traditionalists about the new technology.
The Guitar Site's History Page
Complete and detailed information on the classical and electric guitar can be found here.
Classical Guitar Illustrated History
Recommended reading - a resume of the history of classical guitar, main entrance.
A Brief History of the Guitar
A universally popular string instrument played by plucking or strumming.
Electric Guitar/Bass Timeline
EMP's Guitars feature tells the history of the electric guitar through images and RealPlayer sound clips.
The History of the Electric Solid Body Guitar
History, companies, different brands of electric guitars etc.
Related Innovations
Musical Instruments
