carriers in the material suddenly increases by many orders of magnitude, so its resistance drops and it becomes a conductor. This is called electrical breakdown. The physical mechanism causing breakdown differs in different substances. In a solid, it usually occurs when the electric field becomes strong enough to pull outer valence electrons away from their atoms, so they become mobile, and the heat created by their collisions with other atoms releases additional electrons. In a gas, the electric field accelerates the small number of free electrons naturally present (due to processes like photoionization and radioactive decay) to a high enough speed that when they collide with gas molecules they knock additional electrons out of them, called ionization, which go on to ionize more molecules creating more free electrons and ions in a chain reaction called a Townsend discharge. As these examples indicate, in most materials breakdown occurs by a rapid chain reaction in which mobile charged particles release additional charged particles. Dielectric strength and breakdown voltage The electric field strength (in volts per metre) at which breakdown occurs is an intrinsic property of the insulating material called its dielectric strength. The electric field is usually caused by a voltage difference applied across the material. The applied voltage required to cause breakdown in a given insulating object is called the object's breakdown voltage. The electric field created in a given insulating object by an applied voltage varies depending on the size and shape of the object and the location on the object where the voltage is applied, so in addition to the material's dielectric strength, the breakdown voltage depends on these factors. In a flat sheet of insulator between two flat metal electrodes, the electric field is proportional to the voltage difference divided by the thickness of the insulator, so in general the breakdown voltage is proportional to the dielectric strength and the length of insulation between two conductors However the shape of the conductors can influence the breakdown voltage. Breakdown process Breakdown is a local process, and in an insulating medium subjected to a high voltage difference begins at whatever point in the insulator the electric field first exceeds the local dielectric strength of the material. Since the electric field at the surface of a conductor is highest at protruding parts, sharp points and edges, for a conductor immersed in a homogeneous insulator like air or oil, breakdown usually starts