1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 Journal of Electrostatics 51}52 (2001) 319}325 The Franklin lightning conductor: conditions necessary for the initiation of a connecting leader Mose Akyuz, Vernon Cooray* Division on Electricity and Lightning Research, The A s ngstro ( m Laboratory, University of Uppsala, Box 539, 751 21 Uppsala, Sweden Abstract The attachment of a lightning #ash to a Franklin conductor takes place through a connecting leader that rises from the Franklin rod towards the down coming stepped leader of the lightning #ash. The successful launching of a connecting leader requires the inception of a streamer discharge from the rod and its transformation into a leader (i.e. streamer to leader transition). Here, the occurrence of these physical processes at the tip of the Franklin rod is investigated. The results shows that for Franklin conductors of radii less than about 0.35m arti"cial initiation of streamers at the conductor tip would not result in any increase in the attractive distance. For rod of radii larger than 0.35m it may be possible to launch a connecting leader by arti"cially triggeringofstreamerdischargesattheconductortipbutthegaininattractivedistancewouldnot be larger than about 30%.  2001 Elsevier Science B.V. All rights reserved. Keywords: Lightning; Franklin rod; Lightning protection; Step leader; Streamer 1. Introduction In negative cloud to ground #ashes, the "rst return stroke is initiated by a discharge that travels from cloud to ground, called a stepped leader. As the stepped leader approaches the ground, the electric "eld at the extremities of the grounded objects increasestosuchalevelthattheylaunchconnectingleaderstowardsthedowncoming stepped leader. The "rst return stroke is initiated at the instant of connection between the down coming stepped leader and one of the connecting leaders. The strike point of the lightning #ash is the point from which the connecting leader that made the successful connection to the stepped leader was initiated. With the knowledge, deri- ved mainly from laboratory experiments, available at present one can specify the * Corresponding author. Tel.: #46-18-471-58-09; fax: #46-18-471-58-10. E-mail address: vernon.cooray@hvi.uu.se (V. Cooray). 0304-3886/01/$-see front matter  2001 Elsevier Science B.V. All rights reserved. PII:S0304-3886(01)00113-9