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0 amp. 0 amp. With this nearly plane electrode the flat portions of the curve corresponding to the positive ion and electron saturation currents are particularly marked. 44 ma. In this range the current is a positive ion current which is limited by the rate at which ions reach the edge of the sheath. 7 For convenience we shall adopt the convention that the sign of a current is positive when positive current flows from the electrode into the space. 3* V 36 Studies of Electric Discharges in Gases at Low Pressures Between —20 and —12 volts the current increases rapidly owing to the electrons taken up because of their proper velocities.

It is believed that Class IV electrons are not a direct result of collisions between primary electrons and atoms but are more probably due to photo-electric emission from gas atoms due to ultraviolet radiation from atoms excited by primary electrons. In general, the velocities and number of these electrons decrease as the pressure is raised and increase as the anode voltage is raised. Class V. Secondary Electrons of Low Velocity. 7 volts at high pressures (1000 bars) to 10 volts at lower pressures (30 bars) and higher voltages (225 volts).

The resulting tungsten atoms become positively charged within the positive sheath, and migrate under the influence of the electric field in the opposite direction to that of the convection current. T h e tungsten thus accumulates at the boundary between the positive and negative sheaths in the form of minute solid particles or aggregates. If any transverse motion causes more rapid concentration at some places than others the increased recombi­ nation at these places makes the sheaths more sharply defined and still further increases the rate of accumulation of tungsten.