72 



EXPLORATION GEOPHYSICS 



Therefore, the strength of a magnetic field and of a magnetic force 

 is defined by its effect on a magnetic pole, usually a unit magnetic pole. 



In a uniform magnetic field, the lines of force are parallel and the 

 strength everywhere the same, and there are an equal number of such lines 

 per unit perpendicular section. In a divergent field, as shown in Figure 5, 

 the intensity varies from place to place; likewise the number of force lines 

 passing through a square centimeter of the cross-section is different in dif- 

 ferent parts of the field. 



Magnetic force is a vector quantity. Like other vector quantities, it can 

 be completely defined in a specific case when (1) its magnitude is known, 

 (2) its direction is shown by force lines and (3) its sense is stated as 

 attractive or repulsive. 



Magnetic field strength or intensity may be expressed in the following 

 units, all of which are equal : 



1. 

 2. 

 3. 



4. 



Dynes per unit pole 

 Gauss* 



Lines of force per cm- 

 Maxwells per cm^ 



Lines of force from a unit magnetic pole. — In keeping with the con- 

 vention that one line of force per square centimeter establishes a unit 

 magnetic field, a unit magnetic pole is, by agreement, equivalent to A-ir lines 

 of force. This not too obvious statement can be demonstrated as follows : 



1. The surface area of a sphere is Att r^ where r is the radius of the 

 sphere. 



2. A sphere of unit radius or with r = 1 cm. has a surface area of Air. 



3. Inscribe such a unit radius around a unit magnetic pole m. (See 

 Figure 10.) 



4. By definition, a unit magnetic pole is 

 one which acts on a like pole with a force of 

 one dyne when it is at a distance of 1 centi- 

 meter. 



5. Another magnetic pole (of like sign to 

 pole m of Figure 10) at any point on the 

 surface of the unit sphere would be acted upon 

 with a force of one dyne. 



6. Likewise by definition, a magnetic field 

 of unit strength (or with 1 line of force per 

 square centimeter of section) acts on a unit 

 magnetic pole in it with one dyne of force. 



7. A unit magnetic field is therefore set up 

 at every point on the surface of the unit sphere by pole m. 



8. There are Air square centimeters of surface area on the unit sphere. 



* The term oersted is preferred by some in place of gauss, based on a 1930 ruling 

 of the International Electro-technical Commission. The word gauss is, however, firmly 

 established in geophysical literature by long usage, and will be used in this text. 



AREA • 4TTr' • AlT-l • 4TT 

 Fig. 10. — Section through a 

 sphere of unit radius inscribed 

 about a unit magnetic pole m. 



