12 PRINCIPLES OF ELECTRICAL DESIGN 



2. Definitions. Magnetomotive Force. The difference of mag- 

 netic potential which tends to set up a flux of magnetic induction 

 between two points is called the magnetomotive force (m.m.f.) 

 between those points. The unit m.m.f. known as the gilbert 

 will set up unit flux of induction between the opposite faces of 

 a centimeter cube of air. If we consider any closed tube of 

 induction linked with a coil of S turns carrying a current of 

 / amperes, the total ampere turns producing this induction are 

 SI, and the total m.m.f. is, 



m.m.f. = YQ SI gilberts. 1 



Magnetizing Force. The magnetomotive force per centimeter 

 is called the magnetizing force, or magnetic force. The symbol H 

 is generally used to denote this quantity which is also referred to 

 as the intensity of the magnetic field, or simply field intensity, 

 at the point considered. The magnetomotive force is, therefore, 

 the line integral of the magnetizing force, or, 



m.m.f. = ZHdl 



where dl is a short portion of the magnetic circuit expressed in 

 centimeters over which the magnetizing force H is considered of 

 constant value. Thus H = 0.4*- X ampere-turns per centimeter 



SI 

 = 0.47r-y or, if it is preferred to use ampere-turns per inch 



(not uncommon in engineering work), we may write H = 0.495 

 (SI per inch). 



1 What the practical designer wants to know is the number of ampere- 



4?r 

 turns required to produce a given magnetic flux. The factor TQ constantly 



enters into magnetic calculations as it is required to convert the engineer's 

 unit (ampere-turn) into the C.G.S. unit (gilbert). It should not be neces- 

 sary to explain its presence here, because this is done more or less lucidly in 

 most textbooks of physics. It should be sufficient to remind the reader 

 that the introduction of this factor is due to the physicist's conception of 

 the unit magnetic pole which he has endued with the ability to repel a 

 similar imaginary pole with a force of 1 dyne when the distance between 

 the two unit poles is 1 cm. Now, since, at every point on the surface of a 

 sphere of 1 cm. radius surrounding a unit magnetic pole placed at the center, 

 a similar pole will be repelled with a force of 1 dyne, there must be unit flux 

 density over this surface; that is to say, a flux of 1 maxwell per square centi- 

 meter. The surface of the sphere being 4r sq. cm., it follows that 4ir lines 

 of flux must be thought of as proceeding from every pole of unit strength. 

 The factor 10 in the denominator converts amperes into absolute C.G.S. 

 units of current. 



