260 A LIGHTNING SPIRAL OBSERVED NEAR BASEL. 



regulai- arc the spirals, as is to be expected. Tn order to fulfill 

 the coiiditioiis abovx' mentioned, a very high l're((uency is of course 

 recpiired, such as may be obtained with the electrolytic interrupter. 

 Such spirals depend on the direction of the discharge in reference 

 to the magnetic field of the induction coil or of the earth. Stereo- 

 scopic instantaneous exposures of less than 0.001 second show the 

 image of an irregularly wound line turning sometimes left-handed 

 and sometimes right-handed. With ai"c discharges no spirals are 

 produced, probably because at the high temperatures accompany- 

 ing the arc the magnetic lines of force are no longer present. 



These conditions for the production of electric screw discharges 

 seem to have been present in marked degree with the lightning dis- 

 charge photographed. The discharge shows an extraordinary num- 

 ber of lines, partly in the spirals and partly running horizontally 

 in the general path. Each line corresponds to a partial discharge, 

 and the regularity of spacing of the lines in the spirals indicates a 

 very large quantity of electricity discharged and a very high fre- 

 quency. Undoubtedly the quantity discharged far exceeds any- 

 thing of the kind we can obtain from an induction coil. This 

 explains the great regularity of the lightning spirals as compared 

 with those obtained from the induction coil. In the observed case, 

 too, the earth's magnetic field was favorably situated, since the 

 magnetic field produced by the spiral discharge coincided with it. 



In order to determine if the lightning spiral produced any unusual 

 magnetic field whose presence would be indicated by marked deflec- 

 tions of the magnetic needle," I addressed connnunications to the 

 Swiss meteorological Zentralanstalt at Zurich, and to the kgl. meteor- 

 ologische Observatoriuni at Potsdam. The Swiss meteorological bu- 

 reau Avas, unfortunately, unable to furnish the desired information, 

 but T received from the director of the observatory at Potsdam the 

 following negative reply: 



So fiU" as fonoerns onr oijsorvatory, it may be said in answer to your inquiry 

 that on The day in ciuestion neitluT the terrestrial iiiafjuotic instruments nor the 

 apparatus for ohservin;^ atuiosplieric electricity (the latter records, within mod- 

 erate linuts, not only the potential difference hut also the discharsinj.': ca])acity 

 ol)served) showed any extraoi-dinary behavior. 



It is interesting to note that the photograph indicates that the par- 

 tial discharges of this flash could not have been oscillating, but must 

 have pulsed successively in the same direction in oi-der to produce the 

 direction of rotation here shown. 



« See Kr. Birkeland, Expedition NorwciiifMuie de 1899-1900 pour letude des 

 jTUroi-es boreales, p. 1-3, 1901. Jacob Dybwad, Cristiania. 



