ELEMENTS OF ELECTRICAL ENGINEERING. 



The force of attraction of the two unlike magnet poles of Fig. 

 i may be attributed to a tendency of the lines of force to shorten, 

 very much as if the lines of force were stretched elastic filaments. 

 This idea of tension along the lines of force of a magnetic field is 

 also useful in giving an insight into the cause of the side push 

 which a magnetic field exerts upon a wire carrying an electric 

 current as explained later. 



Not only do the lines of force in a magnetic field have a ten- 

 dency to shorten, but they push each other apart sidewise. That 

 is, there is a tension along the lines of force and a pressure at 

 right angles to the lines of force. The repulsion of the two like 

 poles in Fig. 2 may be attributed to this side pressure of the lines 

 of force. 



5, Direction and intensity of field near an isolated magnet 

 pole. Inasmuch as a test pole of north polarity is attracted 

 directly towards a given south pole and repelled straight away 

 from a given north pole, it is evident that the lines offeree of the 

 magnetic field surrounding an isolated magnet pole are radiating 

 straight lines, that the magnetic field near a north pole is directed 

 away from it, and that the magnetic field near a south pole is 

 directed towards it. 



Imagine a test pole of strength m' placed at a point distant r 

 from a given pole of strength m. The force with which the pole 

 m acts upon the test pole m' is: F= mm' jr*, according to 

 equation (i) ; but the force acting upon the test pole m' is equal 

 to the product of the strength of the test pole into the field in- 

 tensity at the test pole due to the given pole ;;/, according to 

 equation (2). Therefore 



or 



*-? (3) 



6, Magnetic flux. Consider a plane surface, s square centi- 



