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A Tesla coil is a high-voltage, air-core, self-regenerative resonant transformer that generates very high voltages at high frequency, named after its inventor Nikola Tesla. The coil achieves a great gain in voltage by means of a resonant circuit, unlike a conventional transformer whose gain is limited to the ratio of the numbers of turns in the windings. The Tesla coil's outer conducting surfaces, which are charged to a high potential, have large radii of curvature to minimise leakage of the oscillating charge.

Description

The intensity of the voltage gain of the circuit with a free, or elevated, toroid is proportional to the quantity of charge displaced, which is determined by the product of the capacitance of the circuit, the voltage (which Tesla called "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, which is a series LC circuit composed of the capacitor, spark gap, and primary coil; and the second LC circuit, a series resonant circuit, which 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 has relatively small capacitance, charged to as high a voltage as practicable. The elevated conductor, its outer surface, is where the electrical charge chiefly accumulates. It has a large radius of curvature, or is composed of separate elements which, irrespective of their own radii of curvature, are arranged close to each other so that the outside ideal surface enveloping them has a large radius.

The frame is carried by a strong platform and rests on insulating supports. The circuit consists 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 by any desired source, such as an alternator, capacitor, or condenser. The important requirement is that a resonant condition be established.

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 for safety). 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 elsewhere). The lower end of the upper secondary coil, if desired, may be extended up to the terminal and 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.

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