The Decay of a Spark Channel which has ceased to carry Current

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Abstract
The method used by Somerville and Williams to study the develop- ment of a spark column by passing through it a delayed current pulse has been extended so that the whole evolution and decay of the channel may be examined. The column produced by a 32 A spark in air with duration less than 100 nsec is found to expand initially in close association with a cylindrical shock wave; a period of slower expansion follows in which, presumably, conditions within the column approach pressure equilibrium. Finally the column cools and contracts by a process of radial thermal conduction. This third stage has been treated theoretically and an expression for the radius of the column as a function of time has been derived which gives satisfactory agreement with experiment. OMERVILLE and Williams (1959) have described a method of studying the initial S expansion of the conducting region of a spark channel. The channel is initiated by the passage across a gap of a rectangular current pulse, called the 'initiating' pulse. After the passage of this pulse the channel expands freely until, at a controlled time later, a second 'probing' pulse is passed through the gap. If a specially treated anode is used, the probing pulse leaves on it a number of small spots distributed over a circular area the radius of which is taken to be the radius of the conducting channel at the time of the passage of the probing pulse. Using initiating and probing pulses with currents of about 30 A and duration of about 30 nsec, and with delay times between the pulses of up to 1 pec, the initial period of expansion of a spark channel in air was investigated at pressures betweer, atmospheric and about 200 mHg. The same method may be used to study the decay of the conducting channel, provided that the delay between the initiating and probing pulses can be increased to some tens of microseconds. This paper describes an investigation of channel decay by this method. 5 2. APPARATUS

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