366 Trot'. E. Edlund on Atmospheric Electricity 



earth is as great as is required. Outside of the equatorial 

 regions this resistance is less, as has been said, and violent 

 tempests are more rare. Finally, at a still higher latitude the 

 resistance is so slight that the discharges are transformed into 

 slow and continuous currents, giving rise to the phenomenon 

 called the aurora borealis. 



That portion of the electric fluid which does not descend 

 into the earth by disruptive discharges in the equatorial re- 

 gions is carried by the tangential component of the force of 

 induction towards higher latitudes, while its distance from 

 the terrestrial surface is augmented by the vertical component 

 of that force. These currents of electric fluid receive accessions 

 everywhere during their journey towards the poles through 

 the inductive force of the earth's magnetism incessantly im- 

 pelling into the atmosphere fresh quantities of that fluid from 

 the subjacent terrestrial surface. In proportion as the dis- 

 tance to the poles diminishes, the vertical component of the 

 induction-force also diminishes, and the dipping needle conti- 

 nually approaches nearer to the vertical ; and in consequence 

 the resistance opposed by the force of magnetic induction of 

 the earth to the flow of the electropositive fluid to the earth 

 grows less with the diminution of the distance to the poles. 

 When the difference of electric tension between the atmosphere 

 and the earth has become sufficiently great to overcome the 

 resistance opposed by the induction-force of the earth and 

 the subjacent strata of air, the electric fluid flows from the 

 atmosphere to the earth. The places where this phenomenon 

 takes place evidently form a circle around the pole. This 

 circle is characterized by the circumstance that in every point 

 of its circumference the vertical component of the force of 

 terrestrial induction must have nearly the same value. In 

 the vicinity of the poles the atmosphere receives only a feeble 

 charge of electricity, the vertical component of the induction- 

 force being there but very small (as indicated by formula A), 

 and the tangential component directing towards those regions 

 only an insignificant quantity of the electric fluid which enters 

 the atmosphere in lower latitudes. 



The electric fluid impelled into the atmosphere by the ter- 

 restrial force of magnetic induction descends again therefore 

 in two ways to the earth — either by powerful disruptive dis- 

 charges, or by more or less continuous feeble currents. The 

 former mode of discharge takes place chiefly in the equatorial 

 regions, the latter especially in high latitudes. The fluid 

 which is not discharged disruptively in the equatorial regions 

 is conducted by the induction-force towards higher latitudes, 

 where the discharge takes place by means of continuous cur- 



