Science Art: Phone Tester for Electric Circuits, Patent No. 1,187,500; issued to G. B. Raymond.
These are not headphones, exactly. This is a thing made of telephone parts designed to help electrical tinkerers do better tinkering.
It’s from a page called “Latest Patents”… Read the rest “Science Art: Phone Tester for Electric Circuits, Patent No. 1,187,500; issued to G. B. Raymond.”