MAGNETIC METHODS 71 



Field Strength. — -The strength or intensity uf a magnetic field is the 

 force which it exerts on a magnetic body or a magnetic pole at some point 

 in it. Quantitatively, the magnetic field intensity is the force in dynes with 

 which it acts on a unit magnetic pole located at a given point. The con- 

 ventional symbol for field strength is H. 



If for a given field of intensity H , the force is represented by F and 

 the pole strength by m, then : 



p = -^Hm (3) 



and 



^- 1 -^ m (4) 



— m 



. 1 . " 



The quantity — is a constant relating to the media involved as was 



discussed in Equation 1. 



In this connection it is of interest to note that the earth's magnetic field 

 has a strength (on the average) of about 0.6 dyne per unit magnetic pole, 

 while the earth's gravitational field is considerably greater, being on the 

 order of 1000 dynes. (The terminology here used is not necessarily pre- 

 ferred, as special names, viz., gauss and gal, respectively, are applied to 

 strength units of magnetic and gravity fields.) 



Lines of Force. — If an idealized pole is free in a magnetic field, it will 

 move; the path described by it is called a magnetic line of force. The direc- 

 tion in which a free north (or-j-) pole moves, in a field, is the direction of 

 the given line of force. A tangent to a line of force at any point gives the 

 direction of the resultant magnetic field at that point. 



For practical purposes, the lines of magnetic force can be traced by 

 moving a small compass needle from place to place in the field and plotting 

 its position and direction when at rest. The compass needle will orient 

 itself so as to be tangent to the line of force at the test point. Also, the 

 shape of a magnetic field can be clearly shown by the alignment of the 

 lines formed when iron filings (which are minute magnets or compasses) 

 are sprinkled into a magnetic field. Figure 6 illustrates the alignment where 

 iron filings were sprinkled over a sheet of white cardboard (non-magnetic) 

 underneath which was located a bar magnet. 



Lines of force do not, in fact, exist. They are a convenient mental 

 mechanism for visualizing and analyzing magnetic fields and will be so used. 



Unit Magnetic Field. — By agreement in the c.g.s. system, a unit mag- 

 netic field has been defined as 1 line of force per square centimeter (the 

 area taken at right angles to the force lines). It acts on a unit magnetic 

 pole with a force of 1 dyne. A magnetic field of unit intensity is also said 

 to have a strength of 1 gauss.* 



*NoTE : In some old European literature a field of 1 gauss was one having 10 lines 

 of force per square centimeter of section. 



