Page "Debye sheath" Paragraph 7

:" Figure 1 shows graphically the condition that exists in such a tube containing mercury vapor.

The space between filament and plate is filled with a mixture of electrons and positive ions, in nearly equal numbers, to which has been given the name " plasma ".

A wire immersed in the plasma, at zero potential with respect to it, will absorb every ion and electron that strikes it.

Since the electrons move about 600 times as fast as the ions, 600 times as many electrons will strike the wire as ions.

If the wire is insulated it must assume such a negative potential that it receives equal numbers of electrons and ions, that is, such a potential that it repels all but 1 in 600 of the electrons headed for it.