266 ELEMENTARY LESSONS ON [CHAP. v. 



(a) The magnetic potential at any point is the work 

 that must be spent upon a unit magnetic (N. -seek- 

 ing) pole in bringing it tip to that point from an 

 infinite distance. 



(b) The magnetic potential at any point dite to a 

 system of magnetic poles is the sum of the separate 

 magnetic potentials due to the separate poles. 



The student must here remember that the potentials due 

 to S. -seeking poles will be of opposite sign to those due 

 to N. -seeking poles, and must be reckoned as negative. 



(c) The (magnetic) potential at any point due to a 

 system of magnetic poles may be calculated (com- 

 pare with Art. 238) by summing ^lp the strengths 

 of the separate poles divided each by its own 

 distance from that point. Thus, if poles of 

 strengths m\ ;;/", m'" 9 etc., be respectively at 



distances of /, r", r'" , (centimetres) 



from a point P, then the following equation gives 

 the potential at P : 



, r m' m" m'" 



VP = -7 f ?' + ?"- + 



or V, = ^ 



(d) The difference of (magnetic) potential between 

 two points is the work to be done on or by a 

 unit (N.- seeking) pole in moving it from one 

 point to the other. 



(e) Magnetic force is the rate of change of (magnetic) 

 potential per unit of length. 



(f) Equipotential surfaces are those (imaginary) stir- 

 faces surrounding a magnetic pole or system of 

 poles, over which the (magnetic) potential has 

 equal values. Thus, around a single magnetic 

 pole, supposing all the magnetism to be collected 

 at a point far removed from all other poles, the 

 potential would be equal all round at equal 



