INTERACTIONS OF MOVING ELECTRONS 209 



in the magnetic field of a coil which is fixed in space 

 with reference to the conductor. In every case, as 

 was first stated succinctly by Lenz, the direction of 

 the induced current is such as to oppose the cause 

 inducing it. This is, of course, an electrical statement 

 of the principle of the equality and opposition of ac- 

 tion and reaction which had been noted by Newton. 



If in one of two parallel loops of wire a stream of 

 electrons is started in a clockwise direction there is 

 induced in the other loop a stream in a counterclock- 

 wise direction. The e.m.f. in the second circuit exists 

 only while the current in the first coil is building up, 

 that is, only while there are more electrons starting than 

 stopping. The direction of the current in the second 

 coil is obviously such that the coils tend to move apart. 

 An induced current of the same direction would occur 

 in the second coil it it were moved toward the first. 

 Conversely if the second coil were moved away from 

 the first the induced current would be in the opposite 

 direction, that is, in the direction to oppose the motion 

 inducing it. These phenomena are usually stated in 

 terms of magnetic lines of force and the motion of 

 positive charges. The idea of energy is frequently 

 obscured thereby and the motion of electrons is en- 

 tirely neglected. 



In discussing energy we found that work results from 

 a relative motion of the parts of a system to a configu- 

 ration of smaller potential energy. Two parallel con- 

 ductors carrying parallel streams of electrons tractate 

 and may thus do work in the same manner as a gravi- 

 tational system. We therefore say that the conductors 

 move to a position of smaller potential energy, The 



