Hertha Marks Ayrton 1854–1923

Making sparks

One way to make a spark, for example in a lighter for a gas barbecue, is to apply a stress to a crystal of potassium sodium tartrate (Figure 1). These crystals are piezoelectric, which means that flexing them produces a separation of charge and hence a potential difference. Applying this p.d. across a gap produces a spark that can ignite a flammable gas.

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.