In physics and chemistry, plasma is a partially ionized gas, in which a certain proportion of electrons are free rather than being bound to an atom or molecule. The ability of the positive and negative charges to move somewhat independently makes the plasma electrically conductive so that it responds strongly to electromagnetic fields. Plasma therefore has properties quite unlike those of solids, liquids or gases and is considered to be a distinct state of matter. Plasma typically takes the form of neutral gas-like clouds, as seen, for example, in the case of stars. Like gas, plasma does not have a definite shape or a definite volume unless enclosed in a container, but unlike gas, in the influence of a magnetic field, it may form structures such as filaments, beams and double layers (see section 3, below).
Plasma was first identified in a Crookes tube, and so described by Sir William Crookes in 1879 (he called it "radiant matter").[1] The nature of the Crookes tube "cathode ray" matter was subsequently identified by British physicist Sir J.J. Thomson in 1897,[2] and dubbed "plasma" by Irving Langmuir in 1928,[3] perhaps because it reminded him of a blood plasma.[4] Langmuir wrote:
Except near the electrodes, where there are sheaths containing very few electrons, the ionized gas contains ions and electrons in about equal numbers so that the resultant space charge is very small. We shall use the name plasma to describe this region containing balanced charges of ions and electrons.[3]
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