English: Photo of an early induction coil, built by Irish clergyman Nicholas J. Callan in 1863 at Maynooth College. This was his largest coil, finished just before his death, one of the largest in the world at the time. The iron core was 43 inches (109 cm) long. The secondary windings were 21 inches (53 cm) in diameter and consisted of 150,000 feet of 34 gauge iron wire They were made in 3 separate pancake shaped rings separated by air gaps, so wires carrying large voltage differences would not lie adjacent to each other, reducing the risk of insulation breakdown and arcover. He probably intended to add more secondary windings. At the left end is a vibrating mercury 'contact breaker' in the primary circuit, actuated by the magnetic field in the primary, which interrupted the primary current. When powered by 4 'iron cell' battery cells, it could produce 15 inch (38 cm) sparks, corresponding roughly to a potential of 210,000 volts. Alterations to image: removed caption, which read: "Dr. Callan's large induction coil, completed in 1863, still preserved at Maynooth College".