ELEMENTARY ELECTRICITY AND MAGNETISM. 3 



comes m" times as great, or m'm" dynes. That is, a pole of 

 m' units strength at a distance of one centimeter from another pole 

 of m" units strength is attracted or repelled with a force of m'm" 

 dynes. 



3. 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 established experimentally in 1 800 

 by Coulomb who measured the force of attraction of two magnet 

 poles at various distances apart. A long slim magnet was sus- 

 pended horizontally by a wire, thus forming a torsion pendulum. 

 One pole of another long slim magnet when brought near to one 

 of the poles of the suspended magnet caused a twist of the sus- 

 pending wire and from the observed angle of twist the value of 

 the attracting or repelling force was calculated. 



Complete expression for the force of attraction of two magnet 

 poles. According to the previous article two poles m' and m" 

 attract or repel each other with a force of m'm" dynes when they 

 are one centimeter apart, therefore, according to Coulomb's law, 

 the poles attract or repel with a force of m'm" jr 2 dynes when 

 they are r centimeters apart. That is 



m'm" 



*~-yr CO 



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

 poles, r is their distance apart in centimeters, and F is the force 

 in dynes with which they attract or repel each other. 



Algebraic sign of magnet pole. The poles m' and m" are 

 alike in sign when both are north poles or when both are south 

 poles. In either case the product m'm" is positive, and in this 

 case the force F in equation (i) is by experiment a repulsion ; 

 therefore it is convenient to consider the force F as a positive 

 force when it is a repulsion. On the other hand rn' and m" are 

 opposite in sign when one is a north pole and the other a south 

 pole. In this case the product m'm" is negative and the force F 



