The Tesla Primary usually consists of a spiral, or inverted conical coil of wire or tubing. Copper tubing is often used because at the frequency of operation of a Tesla Coil current tends to flow over the surface of conductors, so the inner core becomes redundant. Large diameter tubing or wire are used to keep the resistance of the coil low at the operating frequency. Inverted conical (or 'Saucer') shaped primaries have a slope of ten to thirty degrees. Flat spiral primaries usually have an internal diameter that is about 1.5 to 2 inches or so larger than the Secondary diameter . This Primary was wound in an inverted conical section of 10 degrees. The Primary consisted of 12 turns soft copper  tubing with a diameter of 1/4" and a spacing of 1/4" between the conductor surfaces and a spacing of 1 1/2" between the Primary and Secondary.  Tab on picture to the left to enlarge.