I've been puzzled by this one for some time now...No one has
come up with a satisfactory explanation.
OK!...we all know that in an electric circuit the electric-current
moves through an electric conductor at TSOL.

QUESTION: What precisely is moving at TSOL in the electric conductor?

Is it the electrons?

No!...Electrons have rest mass and CANNOT move at the speed-of-light!

The best estimate I've read is that electrons in an "electric
conductor" move in an electric current [shifting from one electron-hole
to another] at 10 percent of TSOL, more-or-less.
If they moved at say 20 percent of TSOL in a conducting wire there would be obvious relativistic effects.

So back to the original question: (since it's not the electrons moving at
TSOL in an electric conductor, what is moving at TSOL in an electric
conductor?
Or put it another way: What is the substance/composition of an electric-current that's moving at the speed of light in an electric-conductor?
It cannot be electrons with their rest-mass.

Jacob