Engineers or experimenters working with ordinary gases have no difficulty in determining the physical properties and state of the gas. Any thermometer or pyrometer can measure the temperature of the gas; a manometer the pressure; a flowmeter, the stream velocity; finally, highly developed chemical and physicochemical methods of gas analysis yield a determination of the chemical composition. Plasmas represent a different situation. Every measurement is a problem. There are many cases in which temperature measurements by different methods on the same plasma yield results which disagree by a factor of ten. Also we find that many experimenters working with plasmas do not even know such basic quantities as the charged particle density with precision.
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