82 ADVANCED ELECTRICITY AND MAGNETISM. 



The magnetomotive force along a path which links with one 

 turn of wire carrying one ampere of current is called one ampere- 

 turn. The magnetomotive force of any coil, in ampere-turns, 

 is equal to the product of the current flowing in the coil in am- 

 peres and the number of turns of wire in the coil. In magnetic 

 calculations it is usually convenient to reduce ampere-turns of 

 magnetomotive force to c.g.s. units (gilberts), which is done by 

 multiplying ampere-turns by 4?r/io, according to equation (2) 

 of Art. 49. 



The product, field intensity multiplied by length of path, 

 gives magnetomotive force, so that the quotient obtained by 

 dividing magnetomotive force by length of path is field intensity. 

 When a magnetomotive force which is expressed in ampere-turns 

 is divided by length of path, we have a magnetic field intensity 

 expressed in ampere-turns-per-centimeter, or in ampere-turns-per- 

 inch, as the case may be. 



51. Magnetizing force in iron. When an iron rod is placed 

 in a magnetic field and is magnetized thereby, the actual magnetic 

 field along the rod depends not only upon the original cause of 

 the field but also upon the newly created magnetic poles of the 

 rod itself. Thus when an iron rod is placed in a coil of wire 

 through which an electric current is flowing, the field along the 

 rod is due to the combined action of the coil and the poles of the 

 rod. 



The magnetizing force at a given point in an iron rod is defined 

 as the net resultant field intensity 3f at the point due to the com- 

 bined action of the external original cause and the newly created 

 magnetic poles of the rod. The influence of the poles of a 

 magnetized rod is always such as to reduce the intensity of the 

 original field along the rod. 



When the iron rod is slim, its poles are weak, and if the rod is 

 long, its poles are at considerable distances from the middle 

 portions of the rod. In this case, therefore, the poles of the rod 

 do not produce any appreciable weakening of the original field 

 along the middle portions of the rod. When the iron rod is in 



