Only objects that conduct electricity will work, so this simple circuit doubles as a very basic "conductivity tester". Conductors and insulators Ordinary play dough conducts electricity quite well ...

Electricity will only travel around a circuit that is complete. That means it has no gaps. You can use a switch in a circuit to create a gap in a circuit. This can be used to switch it on and off.

This article was originally published with the title “ Powerful Electromagnet for the Use of Oculists ” in Scientific American Magazine Vol. 105 No. 26 (December 1911), p. 573 doi:10.1038 ...