364 Prof. E. Edlund on Atmospheric Electricity 



ciently great to induce a discharge towards the earth. This, 

 as is the case in the ordinary experiments of the laboratory, 

 may take place in two ways — that is, either by an instanta- 

 neous discharge, or by a more or less continuous current. 

 This difference of discharge depends on the following circum- 

 stances : — 



The action of a magnetic pole p upon another magnetic 

 pole q takes effect along the right line which joins p and q ; 

 but the action of the magnetic pole p upon a current-element 

 situated at the same point as q, on the contrary, operates in a 

 plane normal to the said right line. The component along 

 this right line of the action of the pole p upon a current-ele- 

 ment is consequently equal to nil. What has just been said 

 may be applied also to the case of two magnets acting upon 

 each other. If, for instance, a magnet sn (fig. 10) acts upon 

 another magnet s'n', and the latter is movable so as to be able 

 to take any position whatever in relation to sn, the magnet 

 s'n 1 will place itself in the direction of the resultant of the 

 action exerted upon it by the magnet sn. The action of sn 

 upon a current-element situated at the point m, or in the same 

 place as s'n', on the contrary, will take place in a plane nor- 

 mal to the same resultant ; the component, then, along this 

 resultant of the action of sn upon the current-element is equal 

 to nil. If the circle abed represents a section of the earth, the 

 magnet s'n' shows the direction of the dipping needle at the 

 point m, since we suppose s'n' to be in the direction of the re- 

 sultant of the action exerted upon it by the earth's magnetism. 

 Therefore the action of terrestrial magnetism upon a current- 

 element situated in the atmosphere has zero for its component in 

 the direction of the dipping needle. 



At the equator the dipping needle takes a horizontal posi- 

 tion. The action of the earth's magnetism takes effect here, 

 as the above formulae show, in a vertical direction upward. 

 If then, the electric fluid of the atmosphere can precipitate 

 itself vertically into the earth, the force which produces this 

 effect mnst be «ufficiently great to overcome not only the 

 electric resistance of the subjacent strata of air, but also the 

 action of tl e earth's magnetism upon the electric fluid of the 

 atmosphere ; or, in other words, the force must exceed the sum 

 of the two obstacles to the motion of the electric fluid. We 

 must further observe that the action of the earth's magnetism 

 in the vertical direction upon a current-element situated in 

 the atmosphere is, according to formula (A), greater at the 

 equator than to the south or north of that circle. 



At higher latitudes the dipping needle has not a horizontal 

 position, but makes a greater or less angle with the plane of 



