ELEMENTARY THEORY OF MAGNETISM. 3 



ble to select a pair of magnets such that the north pole of one 

 magnet would repel the north pole of the other with a force 

 of one dyne when they (the two north poles) are one centimeter 

 apart; each pole of such a pair is called a unit pole. That is, 

 a unit pole is a pole which will exert a force of one dyne upon 

 another unit pole at a distance of one centimeter. 



5. Strength of pole. Let us choose a slim magnet with unit 

 poles, and let us use one of these poles as a "test pole." Any 



given pole 



strength m-^ unit test pole 



one S&itimeter^ / 



an dynes / / \ tn dyne* 



Fig. 2. 



given magnet pole is said to have more or less strength according 

 as it exerts more or less force on our "test pole" at a given dis- 

 tance. And the force m (in dynes) with which the given pole 

 attracts or repels (or is attracted or repelled by) the unit test 

 pole at a distance of one centimeter is taken as the measure of 

 the strength of the given pole. That is, a given pole has m units 

 of strength when it will exert a force of m dynes on a unit pole at a 

 distance of one centimeter, as indicated in Fig. 2. 



6. Attraction and repulsion of magnet poles. Unlike poles 

 attract and like poles repel each other, as stated in Art. 2. When 

 the two attracting or repelling poles ^ rf neg 



are unit poles their attraction or Q^--^-- -~^~^*O 

 repulsion is equal to one dyne Qe^^-^-^zr" 1 

 when they are one centimeter apart, ni f =s2units ~~ ~~ "-^Q 

 and the attraction or repulsion of Fig 3 



two poles whose respective strengths 



are m f and m" is equal to m'm" dynes when the poles are one 

 centimeter apart. One may think of each unit of m' as exerting 



