I've posted this somewhere else, but I think this is the best place to. Why can I not get a tuning fork to resonate when I play a concert A in the guitar, or vice versa? The former, I understand, may be connected to mass, but what about the latter?

Dunno, I used to be able to get a piece of cigarette paper to jump off an acoustic guitar A string when I held an A tuning fork on the bridge. - That is , good resonance. If you are talking about electric, it is probably because not enough energy is transmitted to the string. - The fork doesn't lose much energy moving the bridge as it would on an acoustic.
Works for my guitars

The effect is deadened if the note is even slightly out of tune, and it's certainly less pronounced on an electric, but it should still work. Perhaps your intonation is off a bit, or your fretting is pulling the note too sharp?
Thank you, very much for your answers. I am in fact talking about an electric guitar and what I'm actually trying to do is to get resonance peaks where the guitar doesn't have any of its own. Not an easy job I guess.
That doesn't make any sense to me. What do you mean by "get resonant peaks" ?