260 A LIGHTNING SPIRAL OBSERVP^D NEAR BASEL. 



regular arc the s])irals, as is to be expected. In order to fulfill 

 the conditions above mentioned, a very high fre(iuency is of course 

 required, 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 arc 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 Avith 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 Avhose presence would be indicated by marked deflec- 

 tions of the nuignetic needle," I addressed comnninications to the 

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

 ologische Observatoriuni at Potsdam. The Swiss meteorological bu- 

 reau A\as, luifortunately. unable to furnish the desired information, 

 but I leceived froni the director of the observatory at Potsdam the 

 following negative reply : 



So fai- :is {'oiiconis our oitservatory. it may Ito said in answor to your inquiry 

 that on the day in (luestion neither the terrestrial nia!j;netic instruments nor the 

 apparatus for ohserving atmospheric electricity (the latter records, within mod- 

 erate limits, not only the potential difference but also the discharging capacity 

 ot)served) showed any extraordinary behavior. 



It is interest'iug to note that the photograph indicates that tlie par- 

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

 have pulsed successively in the same direction in order to produce the 

 direction of rotation here shown. 



" See Kr. Birkeland, Expedition Norwegienne de 1899-1900 pour I'etude des 

 aurores boreales, p. 1.3, 1901. Jacob Dybwad, Cristiania. 



