INTERACTIONS OF MOVING ELECTRONS 199 



If then the two wires are to tractate, energy must be 

 given to the system from some outside source. If 

 this condition is brought about by causing parallel 

 currents we may justly consider it to be due to a de- 

 crease in the availability of the energy of system (1), 

 for the molecules of the wires do not move more dur- 

 ing conduction than before but the free electrons do. 

 Individual electrons, of course, may start or stop; 

 some may move with velocities higher than the average 

 and some with less. The net effect, however, is just the 

 same as if there was a stream of electrons formed by 

 a definite number which move continuously with con- 

 stant velocity. The available potential energy of a 

 system of two electrons, moving hi parallel paths, is 

 less than if they were at rest by an amount equal to 

 the work done hi setting them into motion, that is, 

 their own kinetic energy. Of course, because of the 

 impacts which they make in then- travel, energy must 

 constantly be supplied equal hi amount to the energy 

 dissipated in heat hi the conductors, if the condition 

 is to be maintained. 



We recognize, then, two expenditures of energy from 

 the batteries which cause the streams of electrons. 

 The first of these is equal to the kinetic energy of the 

 electrons. It is made in the first few instants after 

 the battery is connected. The second is the constant 

 energy expenditure, the rate of which is expressed by 

 Joule's Law. The current, which flows in the first 

 moment or so before a steady condition is established, 

 depends not only upon conditions hi its own circuit 

 but also upon those in the adjacent circuit. When 

 the battery is removed from the circuit the moving 



