10 



ADVANCED ELECTRICITY AND MAGNETISM. 



Fig. 11. 



Showing the unequal forces with 

 which a non-uniform field acts upon 

 the poles of a magnet. 



vibration. This equation is true only when the amplitude of 

 the vibrations is very small, that is, when the angle never 

 becomes large. 



(b) Behavior in a non-uniform field. The forces which are 

 exerted upon the poles of a magnet in a non-uniform magnetic 



field are, in general, not equal 

 in value and not opposite in 

 direction, and therefore such 

 forces tend not only to turn 



\. ^^\^ tne magnet but also to impart 



i to it a motion of translation. 



Thus Fig. ii shows the two 

 forces F and F' which are ex- 

 erted upon the poles of a small 

 magnet by a non-uniform mag- 

 netic field. The forces T^and F' 

 are not the same in value and 

 not opposite in direction. 



In order that a particle of iron may be attracted by a magnet it 

 is necessary for the particle of iron to be magnetized, and also 

 it is necessary for the magnetized particle to be in a non-uniform 

 magnetic field. If the particle of iron is in a uniform field, 

 equal and opposite forces are exerted upon its two poles, and it 

 tends only to turn and point in a certain direction. 



The magnetic field near a flat ended magnet pole is approxi- 

 mately uniform (lines of force parallel straight lines) as shown in 

 Fig. I2a. Near the sharp corners of the pole, however, the field 

 is distinctly non-uniform (lines of force diverge strongly). 

 Therefore particles of iron are not appreciably attracted by the 

 flat end of the pole, whereas the sharp corners of the pole 

 attract particles of iron very strongly. This is strikingly shown 

 by passing a flat ended magnet pole over a table on which a very 

 few iron filings have been placed. The filings are all caught 

 by the corners of the pole. 



The lines of force in the neighborhood of a sharp pointed 



