Induction demonstration, Riess spiral

Two coils of copper wire are wound around a central iron core. As soon as an electric current runs through the first coil, a magnetic field is generated. This field also encloses the second coil. When the current is interrupted via the manual interrupter, the magnetic field in both coils will disappear. This generates a voltage pulse (induction pulse) in the second coil. This phenomenon is called electromagnetic induction and it is the basis for the working of dynamos, electromotors and transformers. The efficiency of this air transformer is high because much wire from the first (primary) coil can come close to the wire of the other (secondary) coil. The magnetic field generated by a current-carrying wire is largest close to the wire. Riess' spiral inspired the German physicist Heinrich Hertz to further investigate electromagnetism and induction. Eventually, his research would lead to the discovery of electromagnetic radiation.