View in elevation
Free terminal and circuit of large surface
with supporting structure
and generating apparatus
The intensity of the effect of the circuit with a free, or elevated, toroid is proportionate to the quantity of electricity displaced which is determined by the product of the capacity of the circuit, the pressure, and the frequency of the currents employed.
The primary and secondary coils make up an air-core transformer. Tesla coils are composed of two coupled LC circuits - The tank circuit is series LC circuit composed of the capacitor, spark gap, and primary coil; The second LC circuit, which is a series resonant circuit, is the secondary coil and the toroid. The toroid is one terminal of a capacitor, the other terminal being the ground.
The terminal consists of a metallic frame, sometimes a circular ring covered with smooth half spherical metal plates (constituting a very large conducting surface). The terminal is of relatively small capacity, charged to as high a pressure as practicable. The elevated conductor, its outer surface, is on which the electrical charge chiefly accumulates. It has itself a large radius of curvature, or is composed of separate elements which, irrespective of their own radius of curvature, are arranged in close proximity to each so other and so, that the outside ideal surface enveloping them is of a large radius.
The frame is carried by a strong platform and rests on insulating supports. The circuit comprises of a coil in close inductive relation with a primary, and one end of which is connected to a ground-plate, while its other end is led through a separate self-induction coil (whose connection should always be made at, or near, the center in order to secure a symmetrical distribution of the current) and a metallic cylinder to the terminal. The primary coil may be excited in any desired manner, such as an alternator, capacitor, or condenser. The important requirement is that a resonant condition be established.
It is practicable to produce in the tesla coil's resonating circuit immense electrical activities, measured by tens and even hundreds of thousands of horsepower. Adjustments should be made with particular care when the tesla coil is one of great power, not only on account of economy, but also in order to avoid danger. If the points of maximum pressure should be shifted below the terminal, along the coil, a ball of plasma might break out and destroy the supports or anything else in the way. The destructive action may take place with inconceivable violence. It is advisable to begin the adjustments with feeble and somewhat slower impressed oscillations, strengthening and quickening them gradually, until the apparatus has been brought under perfect control. To increase the safety, one or more elements or plates of somewhat smaller radius of curvature or protruding more or less beyond the others (in which case they maybe of larger radius of curvature) so that, should the pressure rise to a value, beyond which it is not desired to go, the powerful discharge may dart out there and lose itself harmlessly in air.
The conductor of the shaft to the terminal is in the form of a cylinder with smooth surface of a radius much larger than that of the spherical metal plates, and widens out at the bottom into a hood (which is slotted to avoid loss by eddy currents and the purpose of saftey). The secondary coil is wound on a drum of insulating material, with its turns close together. When the effect of the small radius of curvature of the wire itself is overcome, the lower secondary coil behaves as a conductor of large radius of curvature, corresponding to that of the drum (this effect is applicable eslewhere). The lower end of the upper secondary coil, if desired, may be extended up to the terminal should be somewhat below the uppermost turn of the primary coil. This lessens the tendency of the charge to break out from the wire connecting both and to pass along the support.
A Tesla coil can ultilize the phantom loop to form a circuit to induct energy off of the earth's magnetic field. In operation, high-voltage sparks may strike out in all directions from the toroid into the air. This coil is part of Tesla's wireless transmission of electric power distribution system (US1119732 - Apparatus for Transmitting Electrical Energy - 1902 January 18). This type of coil may have been used as the basis for the Wardenclyffe Tower project.
This tesla coil produces currents or discharges of very high tension for various valuable uses. In modified form, this type of transformer, called a flyback transformer, provides the voltage needed to power the cathode ray tube that makes (non-LCD) televisions and monitors possible.
The Colorado Springs laboratory of Tesla possessed the largest Tesla Coil ever built, known as the "Magnifying Transmitter". The Magnifying Transmitter is not identical to the classic Tesla Coil, but is based in principle on this coil. The world's largest currently existing Tesla coil was made by Greg Leyh. It is a 130,000 Watt Tesla coil, part of a 38 foot tall sculpture. It currently resides at a farm near Auckland, New Zealand. 
Nikola Tesla, Tesla patents, Wardenclyffe Tower