Large Apps-Spottiswoode induction coil

A large induction coil made in 1877 by British instrument maker Alfred Apps for British scientist William Spottiswoode. One of the largest induction coils ever constructed, it could produce a spark 42 inches (106 cm) long, corresponding to a voltage of roughly 1,200,000 volts. It was 44 in. in length, 20 in. in diameter, mounted on 3 insulating wood posts. This drawing shows the coil only; the interrupter, capacitor and liquid batteries needed to generate the primary current are not shown. Its primary winding, seen extending from the ends of the secondary coil, consisted of 1344 turns of .096 in. copper wire wound on a 3.56 in. core made of parallel iron wires. There were actually two primary windings, which could be exchanged; the other was for higher current work. The secondary consisted of 280 miles of wire wound in 341,850 turns. Powered with 5 quart-sized liquid Grove cells, it gave a spark of 28 in., with 10 cells 35 in. and with 30 cells 42 in. Information from William Spottiswoode, "Description of a large induction coil". The London, Edinburgh, and Dublin Philosophical Magazine, Vol. 3, No. 15, Jan 1877, p. 30. Image alterations: cropped out caption.