Out of fascination about why a steel sounds like it does, and because the inner engineer is still in me, I got hold of some audio software which acts like a spectrum analyser. It does this by performing maths on the signal, known as a fast Fourier transform.
I wanted to know why a steel sounds different from, say, a conventional guitar. After a bit of mucking around to get the signal levels right, so I wasn't over-driving the circuits, I started to get some answers. The signal has to be clean: any trace of distortion produces higher harmonics which obscures the "steeliness".
A conventional guitar produces a fairly even set of harmonics in addition to the basic note you are picking. Those harmonics are lower in strength, the higher you go. It seems that a steel produces a "parcel" of harmonics even stronger than the fundamental, especially the 2nd, 4th, 5th and 6th.
I then looked to see if I could spot what makes "tone". I think I am seeing another parcel of harmonics higher up, around the 10th - 14th harmonic, when the "tone" is good. This would be what our ears tell us is that extra "edge" that we interpret as "tone", clarity or "bite". One thing is for sure, if there was ever any doubt about whether you can get the same tone without picks, this shows you that it's very hard. Picks definitely produce that extra "bite", according to the spectrum scope.
Fascinating stuff...