2 ELEMENTS OF ELECTRICAL ENGINEERING. 



other ; that is, unlike magnetic poles attract each other, and like 

 magnetic poles repel each other. 



Distributed and concentrated poles. The poles of a magnet, 

 that is, the seats of the attracting or repelling forces, are dis- 

 tributed over considerable portions of the bar, generally the end 

 portions. This is especially the case with short, thick bars. In 

 the case of long, slim magnets, however, the poles are usually 

 more nearly concentrated at the ends of the bar. In the former 

 case the poles are said to be distributed, in the latter case the 

 poles are said to be (approximately) concentrated. 



The elementary theory of magnetism in its simplest aspects 

 applies to long slim magnets ; that is, to magnets with concen- 

 trated poles and the following discussion applies primarily to 

 such magnets. The various definitions may be applied, however, 

 to magnets with distributed poles although rigorous statements 

 are in this case very complex, involving elaborate use of differ- 

 ential ajid integral calculus. 



2. Strength of pole. The poles of a magnet attract iron with 

 greater or less force, according to the size of the magnet and ac- 

 cording to the thoroughness with which the magnet has been 

 magnetized. The poles of a magnet are said to be strong when 

 they attract iron or steel with relatively great force. 



Unit pole. Measure of pole strength. A magnet pole is said 

 to have unit strength, or to be a unit pole, when it exerts an 

 attraction or repulsion of one dyne upon an equal * pole at a dis- 

 tance of one centimeter, and the force in dynes with which a unit 

 pole acts upon a given pole at a distance of one centimeter is 

 adopted as the numerical measure of the strength of the given 

 pole. 



A magnet pole of m' units strength at a distance of one centi- 

 meter from a pole of unit strength is, according to the above 

 statement, attracted or repelled with a force of m 1 dynes. If the 

 unit pole is replaced by a pole of m" units strength the force be- 



" For the purpose of this definition the poles of two entirely similar magnets which 

 have been similarly magnetized, may be considered to be equal. 



