CHAP. I] FLUX AND MAGNETOMOTIVE FORCE 5 



The number of ampere-turns of the exciting winding is called 

 the magnetomotive force of the magnetic circuit, because these 

 ampere-turns are the cause of the magnetic field. One ampere- 

 turn is the logical unit of magnetomotive force. In the example 

 above, the magnetomotive force is equal to 1000 ampere-turns. 

 In electric machines the field excitation often reaches several 

 thousand ampere-turns, and the magnetomotive force is for con- 

 venience sometimes measured in kiloampere-turns, one kilo- 

 ampere-turn being equal to 1000 ampere-turns. 



3. Magnetic Flux. The magnetic disturbance at each point 

 within the ring has not only a direction, but also a magnitude. The 

 disturbance is said to be in the form of a flux, for the following 

 reason : One may think of the magnetic state as being due to the 

 actual displacement of some hypothetical incompressible sub- 

 stance along the lines of force; in this case the flux represents the 

 amount of this substance displaced through each cross-section of 

 the ring, and is analogous to total electrostatic displacement. Or, 

 as some modern writers think, there is an actual flow of an incom- 

 pressible ether along the lines of force. In that case the flux may 

 l.r t bought of as the rate of flow of the ether through a cross-sec- 

 tion. The viewpoint common to these two explanations gave 

 rise to the name flux which means flow. 



Some physicists consider the magnetic circuit as consisting 



of infinitely subdivided (though closed) whirls or vortices in the 



ether, the rotation being in planes perpendicular to the lines of 



. Each line of force is considered, then, as the geometrir 



f an infinitely thin fiber or tube of force, and the ether within 



each tube in a state of transverse vortex motion. The line of 



force represents the direction of the axis of rotation, and the flux 



may be thought of as the momentum of the rotating substance 



per unit length of the tubes of force. According to any of tin > 



views, the energy of a current is actually contained in the 



magnetic circuit linked with the current. 



Whichever view is adopted, the magnet ie flux can be defined as 

 um total of magnetic disturbance through a cross-section per- 

 pendicular to the lines of force. K\p< riment shows that the total 

 flu\ is the same through all complete cross-sections of a ma: 

 circuit. This could have been expected from the point of view 

 t or flow along the lines of force; each tube of 

 force being like a channel within which the displacement or the 



