He saw that it was at least possible that the difference of conducting power between the earth and the wire might give one an advantage over the other, and that thus a residual or differential current might be obtained.

He combined wires of different materials, and caused them to act in opposition to each other, but found the combination ineffectual.

The more copious flow in the better conductor was exactly counterbalanced by the resistance of the worse.
Still, though experiment was thus emphatic, he would clear his mind of all discomfort by operating on the earth itself.

He went to the round lake near Kensington Palace, and stretched 480 feet of copper wire, north and south, over the lake, causing plates soldered to the wire at its ends to dip into the water.

The copper wire was severed at the middle, and the severed ends connected with a galvanometer.