68 EXPLORATION GEOPHYSICS 



Equation 1 may be either positive or negative, depending on the sign 

 of the product of the two poles involved. If the product is plus (two poles 

 of like sign), the force is one of repulsion. If the product is negative 

 (poles of unlike sign), the force is attractive. 



The polarity in a bar magnet can be easily determined by bringing one 

 end of it near the end of a compass needle, the polarity of which is known 

 by its action in the earth's field, and noting whether that end of the compass 

 is attracted or repelled. 



Unit Magnetic Poles. — A system of units is established by defining a 

 unit pole to be such as to make F rr 1 in the first equation given. Hence a 

 unit pole is one which when placed at a distance of 1 cm. from a sirhilar 

 pole will be acted upon with a force of one dyne.* 



Fundamental Concept of Magnetized Matter. — Every positive pole 

 has associated with it a negative pole of equal strength, and these two poles 

 are always in the same piece of matter. It is impossible to have separate 

 positive or negative magnets ; hence, it is impossible to have a body charged 

 with positive or negative magnetism as we may have one charged with 

 electricity. A magnetized body of complex configuration may have any 

 number of poles and at each pole there is, relatively, a charge of magnetism, 

 but since the signs of the poles are opposite, the total charge is always zero. 



Thus, the most fundamental piece of matter in considering magnetism 

 from this point of view is a small body, (not carrying a single charge of 

 magnetism), but carrying two equal and opposite charges at a certain dis- 

 tance apart. This leads to the concept of a line-magnet, which is an ideal 

 bar magnet of infinitesimal width, finite length and with poles at the end- 

 points of the line. 



In the above case and in further mathematical developments, the 

 magnetic moment of a magnet is taken as equal to the product of the pole 

 strength and the distance between the poles. The symbol of magnetic 

 moment is usually "M." 



Magnetic Particle. — If we imagine the distance between poles to shrink 

 until it is infinitesimal in length, then the magnet becomes what is known 

 as a magnetic particle. 



If -f-w and — m are the pole strengths involved and ds is the distance 

 between them, the magnetic moment of one particle is m ds or M z= mds. 



Two magnetic particles have the same effect if their moments are equal ; 

 length and strength of pole have no significance if considered separately. 



Given two magnetic particles placed either side by side or end to end. 

 In either case, the moment of the resulting magnet will be the same. (See 

 Figure 7). 



* The dyne, or unit of force, in the c.g.s. system, is a force that will impart to a 

 gram mass an acceleration of 1 cm. per sec. per sec. 



