Figure 1 Acrystal of potassium sodium tartrate (KNaC 4H44OO6 ),),known as Rochelle salt also
If normal atmospheric air is subjected to an electric field greater than a few MVm−1(10 6Vm −1) the air becomes conducting and sparks are produced (Figure 2). A strong enough field can be produced by applying a p.d. of a few kV across a gap of a few mm.
In the absence of a strong field, air is a good insulator. But even normal air contains a small proportion of free electrons, which have been dislodged in collisions between molecules with much higher than average kinetic energy. In an electric field, these electrons are accelerated.
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