Induction Coil - Construction Details

Construction Details

To prevent the high voltages generated in the coil from breaking down the thin insulation and arcing between the secondary wires, the secondary coil uses special construction so as to avoid having wires carrying large voltage differences lying next to each other. The secondary coil is wound in many thin flat pancake-shaped sections (called "pies"), connected in series. The primary coil is first wound on the iron core, and insulated from the secondary with a thick paper or rubber coating. Then each secondary subcoil is connected to the coil next to it, and slid onto the iron core, insulated from adjoining coils with varnished paper disks. The voltage developed in each subcoil isn't large enough to jump between the wires in the subcoil. Large voltages are only developed across many subcoils in series, which are too widely separated to arc over. To give the entire coil a final insulating coating, it was immersed in melted paraffin wax or rosin, and the air evacuated to ensure there are no air bubbles left inside, and the paraffin allowed to solidify, so the entire coil is encased in wax.

To prevent eddy currents, which cause energy losses, the iron core is made of a bundle of parallel iron wires, individually coated with shellac to insulate them electrically. The eddy currents, which flow in loops in the core perpendicular to the magnetic axis, are blocked by the layers of insulation. The ends of the primary coil often protruded several inches from either end of the secondary coil, to prevent arcs from the secondary to the primary or the core.