Gas discharges

The passing of current through a gas is called a gas discharge. If the current exists only under an external ionising agent and stops when the agent stops, it is a non-self-sustained discharge. If the voltage is raised enough, the electrons are accelerated so much in the field that they ionise molecules themselves — now the discharge continues without an ionising agent: this is a self-sustained discharge. In dry air such breakdown needs a field strength of about 3·10⁶ V/m (N/C). Types of self-sustained discharge: spark (short and noisy; lightning is such a discharge), arc (very bright and hot; used in welding and searchlights), glow discharge (in a rarefied gas; used in advertising lamps and fluorescent lamps). Only the teacher performs such experiments. During lightning stay in a building or a car, and do not stay under a lone tree, in an open field or near water.