below to eliminate any chance of confusion as to their usage and to 

 provide an additional background to this discussion. 



1. Magnetic field 



A magnetic field is a region within which a magnet can experience 

 a torque. The earth surrounds, and is surrounded by, a magnetic field 

 which is a vector field. The complete measurement of the earth's field 

 at any place consists of determining both the direction and magnitude 

 of that field. 



2. Magnetic intensity 



The scalar magnitude of a field vector is called intensity. Total 

 intensity denotes the scalar value of the magnetic field vector. 



3. Magnetic elements 



The magnetic field vector commonly is defined by various 

 coordinate systems. These are: 



a. Horizontal intensity (H), Vertical intensity (Z), Declination (D) 



b. North horizontal intensity (X), East horizontal intensity (Y), 

 Downward vertical intensity (Z) 



c. Total intensity (F), Inclination (I), Declination (D) 



d. Horizontal intensity (H), Inclination (I), Declination (D) 



Transformations from one system to another, or to any orthogonal 

 system, readily can be made. Sometimes other systems are used, 

 particularly when only field variations are of interest. 



4. Common units of measure 



The c.g.s. unit of magnetic field intensity is the oersted which 

 is equivalent to a force of one dyne per unit north magnetic pole. In 

 geomagnetism where the field is small, the gamma (ft) is used as a 

 unit of intensity. One hundred thousand gammas equal one oersted. 

 Directions are commonly expressed in degrees of arc. 



5. Gradient 



The c.g.s. unit for a spatial gradient is oersteds per centimeter, 



XIV- 2 



