Sparks and discharges

Spark is a fascinating physical phenomenon that occurs in nature e.g. as lightning. If the electric field intensity in air exceeds a certain value, the ions are accelerated to such a speed that they are able to ionize other molecules. This causes an avalanche creation of new ions and a conductive channel is formed that discharged the charge. The air is heated to a high temperature, expands rapidly and collapses again, which causes the typical sound and light. In the first row we can see sparks from the Van de Graaff generator; the third picture shows a discharge starting from a pin tip. The pictures in the subsequent rows show sparks made with Rhumkorff generator that uses electromagnetic induction for creating the high voltage. Note the orange parts of some sparks and the detail of the conductive channel in the last picture.