An electrical circuit must be completed to function. Some telegraph designs used only one wire, and used the earth as the return to complete the signal.

A simple model for a telegraph line is seen in the diagram below. A basic telegraph setup may use one wire on poles with glass insulators attached to the pole. All electrical circuits must have an outgoing path and an incoming path to the power source (e.g. a battery). The circuit is considered complete because the current can flow from the negative (-) terminal of the battery to the positive (+) terminal. The current loop is completed to the battery through the Earth (Yes, the Earth is a good conductor because the cross-section of such a conductor is enormous).

The comment was asking if that’s what is happening, since there appears to only be a single wire. I am not an expert on WW1 field telephones, but I do believe a phone can work on the same wire setup as the telegraph. It is a plausible question.

The spool seems to contain cable with just one wire. To form a circuit two wires are needed.

I assumed that maybe the ground is the second wire.

Edit: exactly what setsneedtofeed said. I just read it afterwards.

Can you hear me now?