Return stroke core diameter

This letter concerns the measurement of return stroke core diameter by examining discharge craters formed on an aluminum ‘needle’ mounted on a tower. It is found that mere equality between the Lorentz force per unit volume and the divergence of the kinetic pressure dyadic yields core diameters of the observed crater size, if the return stroke currents have been accurately measured and if the metallic boundary phenomena have been totally neglected. The part of the University of Arizona Lightning Facility pertinent to this letter may be described briefly as follows: An aluminum tube (the needle) 10 meters long is placed on top of a tower that is 49 meters high, the base altitude being approximately 2800 meters (Mt. Lemmon, Arizona). The portion of the needle involved in discharge contact is 5 cm in diameter, the end cap being of hemispherical shape. The bottom of the needle is connected to a 0.09-ohm, noninductive resistor (the ‘shunt’) approximately 2.74 meters long; the shunt is constructed of a resistance material known as Tophet ‘C’. The bottom of the shunt is connected to a triple-walled copper tube, through which runs the coaxial cable connecting the shunt to the termination, some 137 meters distant. The termination connects through a series resistor to a compressor network and from there to the tape recorder. Currents of either polarity, ranging from about 30 to 300,000 amp, can be measured from dc to slightly over 300,000 cps; the measurement accuracy is better than ±15% over the current range of interest.