The photograph shows a spark coil, a device used in early experiments on electricity.
The diagram shows the parts of the spark coil.
A coil of wire P is wound around an iron core. A second coil S is wound around P.
The ends of S are connected to two thin metal rods with a gap G between them, knownas the spark gap.
With the switch closed there is a current in P. When the switch is opened, a large
potential difference is produced across S and a spark is observed across G.