I think that's a valid test. When I've measured my cars that all have coolant less than three years old they typically measure about 250 millivolts.

If you place two dissimilar metals in an electrolyte, even a weak electrolyte you have an electrolytic cell, which is essentially a battery - see any chemistry textbook.

If the "dry ends" of the metal are not connected there is no complete circuit and electrolytic corrosion should be minimal just like a battery doesn't loose charge fast unless the circuit is completed by connecting a load between the terminals. For this reason radiators should be electrically isolated from vehicle ground by rubber mounting. You should still see a few hundred ohms resistance between the radiator and ground due to the coolant path, but they should not be shorted.

In the case of aluminum heads and cast iron cylinder blocks, they are shorted together, so your best defense is frequent coolant changes with Zerex G-05 or MB antifreeze. Poor antifreeze maintenance can cause the head to erode at the block interface, which can cause head gasket failure.

Coolant is an electrolyte, but it has sophisticated additives to slow elecrolytic corrosion to a very low rate. These additives get depleted with time, and since electrolosis is a fact of life even when the engine isn't running (although the reaction rate is higher at operating temperature), Mercedes' recommended coolant change interval is strictly time based.

Duke