70 



EXPLORATION GEOPHYSICS 



case, 10 cm. apart. With the dimensions of the bar magnet as given, for a 

 pole strength of 5 c.g.s. units, the magnetic moment would be equal to 5 x 

 10 = 50 c.g.s. units. This follows from the definition of the magnetic 

 moment, viz., pole strength times the distance between the theoretical poles. 

 The formula for magnetic moment is 



M — 2 ml 



(2) 



The exact position of the magnetic poles in a bar magnet cannot be 

 located with precision, as such poles are small areas or regions. It is there- 

 fore customary to use formulae that involve measurements of distance 

 taken from the center or middle line of a bar magnet, when one is used 

 in calibrating instruments or in certain experiments which will be discussed 

 later. 



I 



m- 



m 



MAGNETIC 



m+ AXIS 



L/l2-f i- 



-r- 



I 1:5/I2L 



^^MIDDLE LINE 



Fig. 9. — Diagram of a bar magnet. 



Magnetic Axis. — The magnetic axis of a magnet, as is apparent from 

 Figure 9, is usually defined as a line joining the poles. More strictly con- 

 sidered, the magnetic axis is a line drawn from the negative to the positive 

 pole of a bar magnet, i.e., from south to north. 



If a bar magnet is carefully and properly magnetized and is of sym- 

 metrical shape, the magnetic axis usually will coincide with the geometrical 

 axis of the bar for its long dimension. In some bar magnets or magnetic 

 needles the geometric axis and the magnetic axis are not identical. Allow- 

 ance must be made for that fact, and suitable procedures adopted to over- 

 come this defect, when making magnetic measurements. 



Characteristics of Magnetic Fields 



A magnetic field is defined as the zone surrounding a magnet or mag- 

 netic body, within which its influence can be traced or detected. According 

 to this definition, the extent of a magnetic field, such as that surrounding 

 a magnetic ore body for example, would depend on the sensitivity of the 

 apparatus with which it was measured. 



