xlvi 



INTRODUCTION. 



as the force on a unit magnetic pole in a narrow cavity in the material, the cavity 

 being in one case perpendicular, in the other parallel, to the direction of the 

 magnetization: this definition however applies only in the ordinary electro- 

 magnetic units. There are a number of reasons for considering induction and 

 magnetizing force as two physically distinct quantities, just as electromotive 

 force and current are physically different." 



In the United States " gauss" has been used much more for the c.g.s. unit of 

 induction than for the unit of magnetizing force. The longer name of " max- 

 well per cm 2 " is also sometimes used for this unit when it is desired to distin- 

 guish clearly between the two quantities. The c.g.s. unit of magnetizing force 

 is usually called the " gilbert per cm." 



A unit frequently used is the ampere-turn. It is a convenient unit since it 

 eliminates 47T in certain calculations. It is derived from the "ampere turn per 

 cm." The following table shows the relations between a system built on the 

 ampere- turn and the ordinary magnetic units. 1 



TABLE II. 

 THE ORDINARY AND THE AMPERE-TURN MAGNETIC UNITS- 



1 Bellinger, International System of Electric and Magnetic Units, Bull. Bureau of Standards, 

 13, p. 599, 



