English: A homemade Tesla coil transformer from 1908. It consists of a primary winding of about 12 turns of thick copper wire on a large cylindrical form around a secondary winding of thousands of turns of fine wire. Unlike most modern coils this has a bipolar output: both ends of the secondary are attached to high voltage electrodes at top, and during operation a spark jumps between them. The primary is located midway between the high tension ends, to discourage arcs striking it.