are defined such that two equal poles will exert a force upon one another of one 

 dyne when separated by a distance of one centimenter. If p x and p 2 have the 

 same sign, the force is one of repulsion. If they have opposite signs, the force 

 is one of attraction. 



If r be the vector drawn from p t to p 2 , then the force Fi acting on px 

 will be given by 



f. = - -*£*- ■ (29> 



and the force F 2 acting on p 2 by 



f. = -&?- • (30) 



We associate with each point in the space about a pole p a vector H 

 defined by 



H = -£- • (31) 



The vector H, called the magnetic intensity, represents the force in dynes 

 which would be exerted on a unit pole if placed a distance r from a pole p. 

 The unit of H in the electromagnetic system is called the oersted. The unit 

 used in geophysical prospecting is equal to 10 — ° oersteds and is called the 

 gamma. 



If the pole p be placed at a point whose coordinates are (u,v,w) , then the 

 magnetic intensity H at a point whose coordinates are (x,y,z) would be given by 



H 



(x—u) . (y — v) {z—w) 

 + j + k 



(32) 



r s r s 



where r ■= [(x — u) 2 + (y — v) 2 + (z — iv) 2 ] 1 / 2 . This may be written 



H = -i-|-<JL)-|-2-<-£-)-k -!-(.£) , (33) 



from which we have 



H. = - — ~ (-£) (34) 



d* r 



H y = -^- (-L ) (35) 



dy r 



H z = - -2- (-£) (36) 



7)z r 



The quantity p/r whose negative derivatives give the field components is called 

 the potential of p. 



599 



