Les decharges frappant les surfaces isolantes et l'electrostatique

Abstract

When an unidirectional voltage is applied to a point-to-plane (point I, plane B) electrode system with an insulating plate inserted between them, a corona effect takes place at the point I when the voltage increases, and another one of the opposite polarity occurs when the voltage decreases. The ions released by these corona effects are trapped at the air—insulating plate interface, where they form a circular layer. In order to know the characteristics of this layer, two methods are used: (i) an induction probe, which must contact the insulating surface F, measures the charge density at different distances from the layer centre; (ii) the dust patterns method is used also in order to materialize the regions of each polarity. When the plane electrode B remains attached to the back side of the insulating plate during the detection, the two methods notably contradict each other. These contradictions are explained by the fact that the probe contacting F detects only the charge trapped on F; however, the powder detects the algebraic sum of the trapped charge on F and of the induced charge on B. Finally, it may be shown that the powder materializes the edge effect of the pseudo-condenser formed by the plane electrode B and the circular charged region on F. The study of the characteristics of the charged layer trapped on F shows that as long as the electric field near I tends to exceed the critical value ± g, the ions in the air distribute on F in such a manner that an electrostatic equilibrium is obtained as with conductors; because g is not zero, such an equilibrium cannot be realized completely. The contradictions observed between the two detection methods disappear if the charges are detected after B has been removed far from the insulating plate. The phenomena are disturbed, however, by ionisation of the air space that forms between the plate and B during their separation. This ionisation causes the rather imperfect transfer of the charge layer, previously induced on B, to the back side of the plate where it is trapped. The study performed here shows that gas discharge phenomena on striking insulating walls allow many experimental demonstrations of the classical Laws of Electrostatics. Before the availability of films and plates of highly polymeric materials, simple, practical applications of these Laws were very scarce.