INTERACTIONS OF MOVING ELECTRONS 207 



That is, a conductor of length I, placed at right angles 

 to a magnetic field H , and carrying a current i, is pushed 

 sidewise with a force of F. Hence 



F = ilH (3) 



is the most convenient form for the electromagnetic 

 relation defining current. 



While this equation is peculiarly useful, it success- 

 fully obscures the energy relations. All ideas of the 

 interactions of electrons in motion, of the relative 

 motions of the two bodies containing these electron 

 streams, of the decreasing potential energy which is 

 the fundamental cause of the motion, are left without 

 either expression or implication. It is impraticable, 

 however, at this late date to rewrite this defining 

 equation in terms of energy and electrons. 



The laws for the motions of conductors carrying 

 currents are all expressed in terms of the positive 

 carriers, although this is the exact reverse of the actual 

 mechanism of conduction in wires. In terms of elec- 

 trons the laws might be expressed as follows: The 

 direction of a magnetic field is that in which a so-called 

 north pole would move. In any coil or magnet in 

 which the direction of the rotating electrons is clock- 

 wise the field is directed toward the observer. Elec- 

 tron streams which are parallel, tractate. Streams 

 which are in opposite sense pellate. The motion of 

 a conductor at right angles to a magnetic field is at 

 right angles to both the direction of the electrons and 

 that of the field. These motions are related as the 

 thumb, center finger, and forefinger respectively of 

 the right hand. 



