ELEMENTARY THEORY OF ELECTROMAGNETISM. 39 



25. Definition of magnetic flux. Figure 32 represents a plane 

 area of a square centimeters at right angles to a uniform mag- 

 netic field of which the intensity is H gausses. The product 

 aH is called the magnetic flux across 

 the area. 



The unit of magnetic flux is the 

 flux across an area of one square 

 centimeter (a = i) when the area is 

 at right angles to a magnetic field 

 of which the intensity is one gauss 

 (H = i), or it is the flux across n 

 square centimeters of area when the 

 area is at right angles to a magnetic 

 field of which the intensity is i/wth of a gauss. The unit of 

 magnetic flux is properly called the maxwell; the common usage 

 however is to call the unit of flux a line of force for the follow- 

 ing reason: 



Fig. 32. 



Fig. 33. 



Consider any magnetic field whatever, and imagine a surface 

 or shell BB which is everywhere at right angles to the field 

 as shown in Fig. 33. Imagine the surface BB to be divided 

 up into small squares such that one unit of magnetic flux crosses 

 each square, and imagine lines of force to be drawn so that one 

 line of force passes through each square. Then the magnetic 

 flux across any other surface or shell A A placed anywhere in the 



