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The dynamic behaviour of the pulsed arc and its plasma in an experimental single-anode mercury-pool valve with anchored cathode spot was systematically investigated with the valve in periodic operation at mains frequency with steady resistance loads, and with superimposed short-duration anode-current pulses. The principal tentative conclusions were found to be as follows: (a) A change of instantaneous value of anode current alters quantitatively the structure of the plasma column of the valve from what it would be if the valve were carrying a direct current of the same magnitude. The magnitude of the plasma changing effect is related to the rate of change of anode current in such a manner that both electron as well as ion density exhibit individually a hysteresis effect, causing the anode voltage to be higher than its steady-state value when the current increases, and lower when it decreases. (b) Shortly after the start of commutation and as the anode current falls there is first a rise of electron density before it starts to decay. This may perhaps be caused by ionization resulting from collisions of pairs of metastable excited atoms. (c) A disturbance of the plasma column by a transient increase of anode current is followed by a period of restoration during which anode voltage, plasma space voltage and electron temperature are temporarily reduced, whereas electron and ion densities are temporarily increased. The intensity of optical radiation from the plasma during some period following the disturbance bears no direct relationship to the electron and ion densities during that time.