ELEMENTS OF ELECTRICITY AND MAGNETISM. 



drawn across a magnetic field, the surface being at each point at 

 right angles to the field. Of course, this chosen surface will be 

 curved if the lines of force are not parallel straight lines, which, 

 in general, they are not. Imagine lines of force drawn through 

 the field so that the number of lines which pass through each 

 square centimeter of this surface is equal to the intensity of the 

 magnetic field at that part of the surface. Then the magnetic flux 

 passing through any area anywhere in the field will be equal to 

 the number of these lines that cross that area. The unit of flux 

 (that is, the flux across a square centimeter at right angles to a 

 field of which the intensity is one gauss) is therefore called the 

 line of force or simply the line, and a magnetic flux is usually 

 specified as so many lines. The name maxwell has, however, been 

 internationally adopted as the name for the unit of magnetic flux. 



39. Total magnetic flux emanating from a magnet pole of strength 

 M. Proposition. The number of lines of force (the number of 

 maxwells of flux) which emanate from a magnet pole of strength 

 Mis 



(19) 



Proof. Imagine a spherical surface of radius r drawn with 



the pole M at its center, as rep- 

 resented by the dotted line in Fig. 

 34. The area of this spherical 

 surface is 477T 2 (neglecting the 

 small portion of the sphere which 

 falls inside of the material of the 

 slim magnet at the point ) ; the 

 magnetic field at the spherical 

 surface due to the pole M is 

 everywhere at right angles to 

 the surface, and its intensity is ev- 

 erywhere equal to M/r 2 , according to equation (17). Therefore, 

 according to equation (18), the magnetic flux <E> across the spher- 

 ical surface is equal to 477V 2 times M/r 2 , which is equal to 



Fig. 34. 



