TERRESTRIAL MAGNETIC POLES. 



409 



The magnetic equator separates those points on the earth for 

 which the potential is positive, from those where it is negative. On 

 either side of the equator the absolute value of the potential de- 

 creases continuously. 



436. TERRESTRIAL MAGNETIC POLES. The term terrestrial 

 magnetic poles, is ordinarily applied to those points of the surface 

 where the potential is a maximum or minimum. A pole is a point 

 where the level surface becomes a tangent to the surface of the 

 Earth; the force there is evidently vertical. 



The number of poles is at least two, for there are at least two 

 points at which the level surfaces are tangents to the surface of the 

 sphere; but there may be a far greater number. Suppose, for 

 instance, that there are two poles P and P' (Fig. 93) situate in the 



Fig. 93 



positive region that is to say, on the southern hemisphere. These 

 poles might belong to the same level surface which had two points 

 of contact with the surface of the sphere ; but more generally we 

 shall consider them as belonging to two different surfaces of poten- 

 tials V m and V' m , V^ being greater than V' m . 



Since the points P and P' are points of maximum, the potential 

 decreases in all directions around each of them, and we may always 

 choose a value Vj of potential lower than V' m , such that the inter- 

 section of the surface V 1 with the sphere, gives two closed curves 

 S and S', insulated from each other, and each of which surrounds 

 one of the points ; we may then take a value V 2 so small that the 

 same curve of intersection comprises the two points. 



