120 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



wire, the battery which supplies the electric current must be sup- 

 posed to have a greater or less electromotive force according as 

 the wire is moving with or against the side force due to the mag- 

 netic field if the strength of the current is to be kept constant. 



The action described above in connection with the motion of a 

 man on a swinging bridge span may be perceived in a very strik- 

 ing way by holding weights in one's hands, swinging round and 

 round on one's heel, and drawing the weights inwards or extend- 

 ing them outwards repeatedly. 



The facts outlined above in connection with the moving wire, 

 constitute what is called Lends law, a more elaborate statement 

 of which will be given later. 



63. Induced electromotive force. Faraday discovered in 1831 

 that a momentary electric current is produced in a coil of wire 

 when a magnet is thrust into a coil or withdrawn from the coil, 

 or when an iron rod upon which the coil is wound is magnetized 

 or demagnetized. The motion of the magnet in the first case or 

 the varying magnetism of the iron rod in the second case, pro- 

 duces a momentary electromotive force in * the coil and this elec- 

 tromotive force in its turn produces a momentary current if the 

 coil forms a portion of a closed circuit. The electromotive force 

 and electric current produced in this way are called induced elec- 

 tromotive force and induced current. 



Examples of Lenz's law. A current induced in a coil when a 

 magnet is thrust into the coil is in such a direction as to tend to 

 push the magnet out of the coil, and the work done in moving the 

 magnet against this opposing force is the work which goes to 

 produce the induced current. The current induced in a coil when 

 a magnet is withdrawn from the coil is in such direction as to 

 tend to draw the magnet into the coil and the work done in mov- 

 ing the magnet against this opposing force is the work which 

 goes to produce the induced current. When an iron rod with a 



*One should always speak of the electromotive force between two points, never of 

 the electromotive force in a circuit, except only when one is speaking of an induced 

 electromotive force. 



