English: Drawing of one of the first induction coils, built by British physicist William Sturgeon in 1837. The primary of the coil (B) was 260 ft. of bell wire wound on a wooden bobbin with an iron core, and the secondary was 1300 ft of thinner wire, insulated with a wax coating, wound on top, and soldered to the primary wire to make an autotransformer. The primary was powered by a liquid battery cell (0). The zinc sawtooth interrupter wheel (D) dipping in mercury was turned by hand to break the primary current, to create the flux changes necessary to induce a voltage in the secondary. The magnitude of the voltage produced was judged by how strong a shock it gave when the copper handles (H) were held.