Like a National guitar...
—Paul Simon, "Graceland"
Before the days of amplified guitars, there was a problem hearing acoustical guitars properly in the crowded 1920s Prohibition-Era Juke Joints and dance halls. Singers like Bessie Smith had the vocal cords to cut through the noise of the crowd in the smoke-filled rooms, but the guitar players couldn't compete with that or with the brass instruments of the bands.
To address this issue, the resonator-style guitar was invented in 1928 by John Dopyera. Essentially, he replaced the traditional open sound box of a normal guitar with a spun-metal cone or "resonator" which produced 4 to 5 times more sound volume than the regular acoustical guitar put out.
Obviously, the mellow wood tones of the guitar were gone, replaced by the twang of the steel. Some of these guitars were made to play flat on the player's lap, facing up, and some had round necks that allowed them to be held in the normal fashion. They were, and still are, ultra-suited to the Blues, which is what they were playing in those smokey Juke Joints back then.
Probably the epitome of those fine guitars was the National tricone which, as the name suggests, had three cones instead of one. The famous National tricone doesn't twang - it GROWLS. The National tricones were nickel over brass or, in some expensive models, silver plated. The shine they put out is what Paul Simon was referring to in his song.
Those old guitars are quite valuable as collectors items today, with a limited edition hand painted tricone going for maybe $4000 if you can find one.
John Dopyera left National in 1928 and, with his brothers, founded his own company called Dobro. That was a contraction for Dopyera Brothers, and was also a word which meant "good" in their native Slovak language. The Dopyera brothers put out a single-cone resonator that was inverted and bowl shaped that could put out more volume than even a National tricone could, so they say.
Today, the name "Dobro" has become synonymous with resonator guitars, much like Kleenex or Xerox have almost entered the public domain as generic names. However, Gibson, who currently owns the Dobro name, has announced that they intend to begin defending the trademark again. Good luck to them.
The names of the old Blues legends who used the resonator "steel" guitars - especially the National tricone - are too numerous to list. Some played them like regular guitars and some used a slide (they were often called "slide guitars" too) or a "steel." Often this was really a broken glass bottleneck. This they used to change the pitch of the strings in a continuous sound rather than using the frets.
Eric Clapton, the British rocker, is more famous for his modern amplified guitars, but over the years built up quite a guitar collection. One of these was an old Dobro he found in Nashville that he had Randy Wood replace the neck on. Randy also decorated the now famous guitar. Clapton is shown playing that resonator guitar in this 1976 studio session. The guitar probably didn't see the actual light of day in an actual public performance until a few years later during the "Eric Clapton Unplugged" live concert. Several of the songs from that concert were American Depression-Era Blues songs that were just perfect for a resonator.
If you are interested in hearing what a resonator guitar sounds like, here is one of those songs from that concert. I recommend headphones.