Here is a very early insulator, used in Germany in the late 1840's and up to 1852, at the first German line. It was designed by Werner von Siemens. As can be seen it is threadless, and the insulators were fastened to the iron pin by molten sulphur. Because of the tension of the wire at the rather thin part of the body around the wire groove, many insulators failed when the top part popped right off. This lead to a improved design in 1852....but that is in the next picture!