64 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



alike. // is possible to select, from such a set, two similar magnets 

 of which the two north poles, for example, repel each other with a 

 force of one dyne when they are one centimeter apart. Each of 

 the poles is then said to be of one unit strength, and the strength 

 m of any other pole is equal to the force in dynes with ivJucJi tJiis 

 otJter pole is acted upon by a unit pole at a distance of one centi- 

 meter. The force with which two poles, of strengths m' and m" , 

 respectively, attract each other when they are at a distance of one 

 centimeter apart is m'm" dynes. This is evident when we con- 

 sider that each of the m' unit poles, which may be thought of as 

 being combined to give the pole ;#', attracts each of the m" unit 

 poles which may be thought of as being combined to give the 

 pole m", with a force of one dyne. 



33. Coulomb's law. The force of attraction or repulsion of 

 two magnet poles is inversely proportional to the square of the 

 distance between them. This fact was discovered in 1800 by 

 Coulomb, who measured the force of attraction of two magnet 

 poles at different distances apart and found the force to vary 

 inversely with the square of the distance. A long slim magnet 

 was suspended horizontally by a wire, thus forming a torsion 

 pendulum. One of the poles of another long slim magnet was 

 brought near to one of the poles of the suspended magnet, the 

 force action between the two poles produced a twist in the 

 suspending wire, and the value of the force was determined from 

 the observed amount of twist. 



Complete expression for the force of attraction of two magnet 

 poles. According to the previous article, two poles attract or 

 repel each other with a force of m'w" dynes when they are one 

 centimeter apart, therefore, according to Coulomb's Law, the 

 poles attract or repel each other with a force of m'm" Jr 2 dynes 

 when they are r centimeters apart ; that is, 



F-*- 05) 



in which m' and m" are the respective strengths of the two 



