12 ELECTROMOTIVE FORCE AND CURRENT. 



a coil is equivalent to the movement of that number 

 of lines from the outside to the inside of the coil. That is to 

 say, the formation of 1,000 lines of force passing through the 

 coil will produce the same effect as if the movement of a 

 magnet caused 1,000 lines of force to be cut by the conductor. 



As explained on page 2, an electromotive force is produced 

 in a conductor when it is made to cut lines of force. Hence 

 the change of field in a coil produced by a change of the 

 current in it will also produce an electromotive force. This 

 electromotive force is called the electromotive force of self- 

 induction. The magnitude of the electromotive force is 

 governed by the same rule as already given on page 2 for the 

 electromotive force induced by motion in a magnetic field. 

 The voltage induced in each conductor of the coil is numeri- 

 cally equal to the number of lines cut per second divided by 

 10 8 . The direction of the electromotive force thus produced 

 is always such as to oppose the change of the current to 

 which the electromotive force is due. 



Thus a current started in a conductor will produce a 

 magnetic field, which will in turn produce an electromotive 

 force opposite to the direction of the current. Similarly, 

 the stoppage of a current will cause an electromotive force, 

 owing to the disappearance of magnetic lines, in the direction 

 which tends to maintain the flow of current. Since an 

 alternating current is constantly changing both its magnitude 

 and direction, there will be electromotive forces constantly 

 induced, and always tending to oppose the change of current 

 occurring at the time. Because the induced electromotive 

 force always opposes the changes occurring in the current, 

 it is often called the " back electromotive force " or " counter 

 electromotive force " due to self-induction. These electro- 

 motive forces will be proportional to the rate at which the 

 magnetic lines change, and hence proportional also to the 

 rate at which the current changes, and not directly to the 

 magnitude of the current or to the number of lines of force in 

 the circuit. 



In order to make clear this relation between the electro- 

 motive force of self-induction and the current in a circuit, 

 it will be best to draw two curves representing respectively 

 the successive values of the current and the corresponding 

 values of the rate at which the current changes. This is 

 shown in Fig. 6 where curve I. shows the values of a current 

 whose maximum value is 12-5 amperes, and the dotted 



