It is with respect to ground, the positive pole of the battery is connected to ground.

The telegraph system figured this out very quickly. Most water in nature has at least a bit of salt in it, which is present as positive sodium ions and negative chloride ions. By making the outdoor wiring negative with respect to ground, the chloride ions are repelled, and such wires corrode much more slowly than those that're positive with respect to ground.

Since most of the telegraph network, later the telephone network, is outdoors, this is a pretty big deal.