MAGNETIC METHODS 



91 



earth's magnetic field shows quite definitely that the earth is not sym- 

 metrically or uniformly magnetized. 



The Earth's Magnetic Field 



Declination. — It is apparent that terrestrial magnetic measurements 

 are, for practical purposes, confined to points on or near the earth's surface. 

 A magnetic needle suspended so that it can turn freely about its center of 

 balance (a compass, for example) will eventually come to rest with its 

 magnetic axis directed along the lines of force of the earth's field. On this 

 basis the magnetic field at a point of observation can be specified in part as 

 follows. 



The magnetic meridian at any such point is the vertical plane fixed by 

 the direction of the magnetic lines of force. It is the direction assumed 

 by a free magnetic needle when at rest. A compass is usually employed for 

 determining the magnetic meridian or magnetic north line. 



Magnetic declination is the angle between the astronomic meridian, or 

 true north direction, and the magnetic meridian at the station of observa- 

 tion. It is measured in a horizontal plane and is designated as east or 

 as west declination according as the magnetic meridian is east or west of 

 true North. Declination is sometimes called variation or variation of the 

 compass. 



Inclination or Dip. — Magnetic mclination is the angle, measured in a 

 vertical plane, which the lines of force make with the plane of the horizon 

 at the point of measurement. It is called + if below the horizon and — if 

 above it. A dip needle is an instrument closely similar to a compass, 

 designed so that its needle can rotate in a vertical plane, and used to 

 measure the inclination or dip angle. 



Vectorial Re presentation. - 



magnetic field can be con- 

 veniently represented by a 

 vector (T). This vector starts 

 at the point of observation 

 and extends in the direction 

 of the line of force of the 

 field. Its vector length is pro- 

 portional to the strength of 

 the total intensity of the field. 

 See Figure 24. 



The total intensity can be 

 resolved into two components 

 in the plane of the magnetic 

 meridian, namely, H or hori- 

 zontal intensity, and the ver- 



-At any point on the earth's surface, the 



ASTRONOMIC 

 NORTH 



POINT OF 

 OBSERVATION 



D' DECLINATION 



I -- DIP OR INCLINATION 



Fig. 24. — Vectorial representation of the earth's 

 magnetic field at a point of observation. 



