El secreto de Nikola Tesla (1980)

Corona mechanism

Near sharp points the electric field may become very high, as indicated in the figure below. Cosmic rays and sunlight from above and radioactivity from the earth below produce free electrons in the air, also near the sharp point. There are two situations to be distinguished:

a sharp electrode at negative potential, and
a sharp electrode at positive potential.
(1) Point electrode at negative potential
In the strong electric field near the sharp electrode a force, F = eE, acts on the free electrons, which are thus accelerated away from the sharp electrode. When these electrons collide with nitrogen or oxygen molecules in air, they may ionize these molecules, resulting in a new free electron and a positive ion. These new free electrons can also contribute to the ionization process. The result is an electron avalanche directed away from the point electrode. Because the electric field decreases fast as a ******** of the distance to the sharp electrode (see the dotted curve in the figure below), the foremost electrons soon arrive in a region with an electric field that is too low to gain enough energy for ionization. The electrons drift with a relatively low speed and attach easily to neutral oxygen molecules, resulting in slow moving negative ions. A negative space charge is formed that decreases the electric field just in front of the sharp electrode. The positive ions increase the field very close to the point, but the ionization region is drastically reduced and the ionization process stops. The negative space charge now drifts away to the positive electrode, the electric field in front of the point electrode recovers, and the ionization process restarts. Because this recovery is only dependent on the time the negative ions need to reach the positive electrode, corona is observed as bursts of ionisation that are equidistant in time.

(2) Point electrode at positive potential
If the point electrode is at positive potential, free electrons are accelerated towards the point and cause ionization. Now it is positive space charge in front of the point electrode that decreases the electric field directly in front of the point, see figure below.

Further away from the sharp edge, however, the electric field is increased, extending the region of ionisation towards the other electrode. Thus, in contrast to the sharp edge at negative potential, the ionisation is enhanced by the space charge, and not decreased. This results in large discharges towards the other electrode. It is further observed that the breakdown voltage for a positive sharp electrode is lower than for a negative point electrode. Apart from ionisation of the gas, oxygen and nitrogen molecules will be raised to higher energy states through excitation by collisions with free electrons. On returning to lower energy levels, visible light is emitted, producing the coronal glow. Ultraviolet light is also emitted. The ionisation is also the source for shock waves in the air that produce the characteristic hissing sound.

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