An electric guitar is a guitar that uses the principle of electromagnetic induction to convert vibrations of its metal strings into electric signals. Since the generated signal is too weak to drive a loudspeaker, it is amplified before sending it to a loudspeaker. Since the output of an electric guitar is an electric signal, the signal may easily be altered using electronic circuits to add colour to the sound. Often the signal is modified using effects such as reverb and distortion. Conceived in 1931, the electric guitar became a necessity as jazz musicians sought to amplify their sound. Since then, it has evolved into a stringed musical instrument capable of a multitude of sounds and styles. It served as a major component in the development of rock and roll and countless other genres of music.
Some electric guitars have a tremolo arm (sometimes called a "whammy bar" or "vibrato arm" and occasionally abbreviated as trem), a lever attached to the bridge which can slacken or tighten the strings temporarily, changing the pitch, thereby creating a vibrato or a portamento effect. The name "tremolo bar" is somewhat misleading. It would be more accurate and appropriate to call it a vibrato bar. Tremolo is a fluctuation of volume. Vibrato is a fluctuation of pitch, which is what the whammy bar produces. Early vibrato systems, such as the Bigsby vibrato tailpiece, tended to be unreliable and cause the guitar to go out of tune quite easily, and also had a limited range. Later Fender designs were better, but Fender held the patent on these, so other companies used Bigsby-style vibrato for many years.
Electric guitars usually have up to three magnetic pickups. Identical pickups will have different tones depending on how near they are to the neck or bridge, with bridge pickups having a bright or trebly timbre, and neck pickups being more warm or bassy. The type of pickup also affects tone, with dual-coil pickups sounding warmer, thicker, perhaps even muddy, and single coil pickups sounding clear, bright, perhaps even biting. Guitars do not have to be fitted with a uniform type of pickup: a common mixture is the "fat strat" arrangement of one dual-coil at the bridge position, with single coils in the middle and neck positions.
Where there is more than one pickup, selector switching is fitted. These often allow the outputs of two or more pickups to be combined, so that two-pickup guitars have three-way switches, and three-pickup guitars have five-way switches. Further circuitry is sometimes provided to combine the pickups in different ways. For instance, phase switching places one pickup out of phase with the other(s), leading to a "honky", "nasal", or "funky" sound. Individual pickups can also have their timbre altered by switches, typically coil tap switch, which effectively short-circuits some of a dual-coil pickup's windings, giving a tone like a single coil pickup.