You can't directly measure V because for a superconducting system, R is exactly 0, and I is nonzero. You can't easily have components in line with the current either for the same reason unless they're superconducting -- and practically making switches, diodes etc for superconductivity applications is a gigantic pain.
Quench protection systems exist to make the whole of the magnet dump its current uniformly, not in one tiny area, and are very complex. This is a neat extension of the idea with B field monitoring as a proxy for I.
Ok, I was under the impression that V is slightly nonzero during quenching, but I suppose the nonzero R parts get shunted by parts that are still superconductive.
Quench protection systems exist to make the whole of the magnet dump its current uniformly, not in one tiny area, and are very complex. This is a neat extension of the idea with B field monitoring as a proxy for I.