ground connection

The other end of every wire must be brought back into the cave and fastened independently, and without any ground-connection.

You don't want any ground-connection except the one through the negative brush.

To begin with, it was a "metallic circuit;" that is, two wires were to be used instead of one wire and a "ground connection."

The ground connection is made by soldering a wire to a cold water pipe.

Maybe there was a ground connection of some kind, if not by the wire.