ELEMENTARY LAWS OF CONTINUOUS CURRENTS 15 



terms are not synonymous. Thus a machine may be gen- 

 erating 100 volts in its armature while the difference of 

 potential of its terminals may be only 90 volts. 



8. Magnetic Field; Law of the Magnetic Circuit. All 

 the space surrounding a magnet is said to constitute a 

 magnetic field. The imaginary lines from the north pole 

 of a magnet to its south pole really spread out and fill 

 all space. 



Strength of Magnetic Field. The strength of such a 

 magnetic field is reckoned in lines per sq.cm. and a field 

 is said to have unit intensity (one line per sq.cm.) when 

 the field acts upon a unit magnetic pole with a force of 

 one dyne at the point considered. The intensity of a mag- 

 netic field is usually represented by the symbol H. The 

 field near a magnetic pole is comparatively strong but 

 decreases rapidly in intensity with increasing distance 

 from the pole. 



The earth itself is a magnet and the field at the earth's 

 surface acts upon a unit magnetic pole with a force of about 

 0.2 dyne. Such a field would be represented by one line 

 for every 5 sq.cms. of area. The strength of field used in 

 electrical apparatus is generally of a density of several 

 thousand lines per sq.cm. A field of one line per sq.cm. 

 is said to have an intensity of one gauss. The strength 

 of field in the air gap of a generator might for example 

 be given as 10,000 gausses. 



Continuity of Magnetic Lines of Force. A magnetic line 

 of force is always continuous, i.e., it closes on itself. A 

 line of force may be traced from the north pole of a magnet, 

 through an air space, into a piece of iron, through another 

 air gap, then into the south pole of the magnet, and through 

 the magnet to the north pole again, thus closing on itself 

 and forming a complete loop. The path taken by this 

 line of force may be called the magnetic circuit. The 

 magnetic circuit of a generator for example would be through 

 a pole into an air gap and so to the armature core, then 



