Just thinking aloud about a superposition demo. Suppose you use carbon
paper with two silver-painted spots and you measure E (grad of V) at a
given location. Then you take another paper and the second spot is at
another location. You measure E at the same spot as before and it is
different. Finally you "superpose" by painting three spots on the third
paper and measure E again. This should, in principle, be an easy way
to show that E3=E1+E2 (vectors).
Ludwik Kowalski