Lightning can be defined as "a rapid exchange of electrical charges between a cloud and the ground." Depending on whether the base of the lightning bolt comes from the cloud or the ground, lightning can be classified in four categories: negative descending, negative ascending, positive descending and positive ascending. In fact, "negative ascending" lightning bolts are by far the most frequent: 90% if we take an average of the figures given by 16 specialists in different countries. We shall therefore describe the mechanism of this type of discharge below.

When the potential gradient at the base of the cloud reaches a sufficient value (about 10kV/cm), a jet of electrons is projected groundward and produces a luminous track in the sky. Depending on the author, this first discharge is called a bolt, tracer, arrow or pilot discharge. The "tracer" covers 30 to 50 meters at a speed ranging from 60 to 50,000 km/s. After a pause of 30 to 100us, a second tracer follows the path taken by the first, extending it by about 50 meters. A third tracer is released, then a fourth and so on, the end of the tracer always getting closer to the ground. The electrical field increases progressively between the tracer and the ground, and when it reaches a sufficient level, a positive ascending jet leaves a point on the ground and joins the tracer. This ascending jet can be as long as 150 meters. An intense current then passes through the conducting channel thus produced. This is the main discharge, which can be followed by a series of secondary discharges, all taking, not stepwise but in a single stroke, the path ionized by the main discharge.

Conductor Types

Franklin Cone

A FRANKLIN lightning rod protects the volume of a cone in which the radius of the base is equal to the height. This method is satisfactory for Church Spires, tall industry chimneys and tower buildings in which the zones to be defended are contained within the cone. Where this method can be usefully employed, the cost of installation is reasonable and nearly always justifiable.

A FARADAY cage installation comprises multiple air terminals, not less than one foot high, fixed at all salient points on the roof and bonded together with copper roof conductor and down conductor to form a cage having a mesh not greater than 100 feet and with air terminals at the intersections. This method is not satisfactory because it leaves the areas of roof between the conductors unprotected unless these areas are defended by air terminals or roof conductors at higher levels.

The building illustrated is 300 ft. x 300 ft. x 100 ft. high. The efficiency of this method is lower than that of the Franklin method because any point discharge current available is divided at random between the multiple air terminals, whereas if would be more effective if it would be concentrated at one salient air terminal, as in the Franklin method. The Faraday method is expensive, and particularly so when we consider the large expanse of roof which remains undefended.

It is only rarely that the cost can be considered to be justifiable.

The inspiration of these devices springs from J.B. Szillard, a colleague of Madame Curie A paper "Sur un paratonnerre au Radium" was read by him to the Academy of Sciences in Paris on 9th March, 1914. The first ionizing air terminal was patented by Gustav P. Capart, also a colleague of Madame Curie.

In 1953 Alphonse Capart, the son of Gustav, improved on his father's device, the final result is known as PREVENTOR.