ELECTROMOTIVE FORCE AND CURRENT. 



chosen that the number of lines which pass through a square 

 centimeter of a surface which is at right angles to their 

 direction is also the number representing the strength of 

 the field. Thus the number of lines of force per square 

 centimeter gives the force which would act on a unit magnetic 

 pole at the point considered. 



Whenever an electric conductor moves in such a magnetic 

 field so as to cut the lines of force, an electromotive force, 

 or E.M.F., is produced J in the conductor. The numerical 

 value (in C.G.S., or " absolute " units) of this electromotive 

 force is the number of lines cut through by the conductor 

 in 1 sec. 



Thus 



E.M.F. = Rate of cutting magnetic lines. 

 Expressed in volts (the " practical " unit of electromotive 

 force), the electromotive force will be the above number of 

 units divided by 10 8 (since one volt is equal to 10 8 absolute 

 units of electromotive force), so that 



E.M.F. (in volts) = Rate of cutting magnetic lines x 10~ 8 . 



If the conductor moves in a direction parallel to the lines 

 of force it cannot be said to cut them. In such a case no 

 electromotive force results. 



If there are a number of conductors, instead of one 

 only, in each of which is induced an electromotive force, 

 these electromotive forces will produce a resultant 

 electromotive force if the conductors are connected together. 

 This will be equal to their sum, if the conductors 

 are connected together in such a manner that the electro- 

 motive forces are all directed towards the same end of the 

 composite conductor, which they then form. 



The direction in which this electromotive force acts 

 depends upon the relative direction of the lines of force and 

 the movement of the conductor. If the conductor moves at 

 right angles to the lines of force there will be three directions 

 mutually perpendicular to each other, viz. : (1) The direction 

 of the lines of force (from the north to the south pole), (2) 

 the direction of motion of the conductor, and (3) the direction 

 in which the electromotive force is induced, i.e., the direction 

 along the conductor in which it tends to produce a current 



{ This electromotive force must be considered to be an experimental fact. 

 The relation existing between the value of the electromotive force and the 

 rate at which the conductor cuts the lines is a result of the definition given 

 to the unit of electromotive force. 



