LIGnTNING CURRENTS //V BURIED CABLE 289 



insulated wire is installed along the outside of the copper shield for meas- 

 urement of voltage drop m the copper jacket, in connection with routine 

 corrosion surveys. This facilitated measurements of the charge transferred 

 along the shield by lightning currents, as described in the following. Mag- 

 netic link instruments intended to measure the steepness of the wave front 

 were also employed, but lacked the sensitivity required for accurate meas- 

 urements and are not discussed further here. 



In addition to these measurements of current, charge, and voltage, in- 

 volving the cable structure, observations were also made of the incidence 

 of strokes to ground as described later in this paper. 



2.1 Crest Ctirrent Measurements 



To measure crest currents in the cable, magnetic links' were mounted at 

 distances of 1.6, 5.7, 12.7, and 36.4 inches from the center of the cable, to 

 cover a current range from 1 to 220 ka. The readmgs on these magnetic 

 links indicated the total current in the cable, that is, the sum of the currents 

 in the copper shield, the lead sheath, and inside cable conductors. 



2.2 Measurements of Charge 



These measurements were made by means of surge integrators* In 

 principle this instrument consists of a shunt Ro across which is connected 

 a coil of inductance L. When a surge current 7o(/) passes through the 

 shunt, the current 7(0 in the coil is given by; 



l'^-^ = Ro hit) 



lit) = J j' hit) 



dt 



= ^ QoiO 



where Qoit) is the charge which has passed through the shunt up to the 

 tune t. By measuring the crest value / of the current /(/), the total charge 

 may be obtained from the relation: 



This relation is always valid if the surge current rises to a peak value and 

 then decays uniformly. The relation should provide a good approxima- 

 tion to the total charge conveyed by natural lightning strokes, even if 

 there are small oscillations. 



The shunt i^o consisted of about 26 feet of the copper shield over the cable, 



