192 JAMES CLEKK MAXWELL 



in terms of these currents the electric momentum of 

 each wire circuit; let this be done, and let half the 

 sum of the products of the corresponding velocities 

 and momenta l>e formed. 



In maintaining the currents in the wires energy is 

 needed to supply the heat which is produced in each 

 wire ; but in starting the currents it is found that 

 more energy is needed than is requisite for the supply 

 of this heat This excess of energy can be calculated, 

 and when the calculation is made it is found that tho 

 excess is equal to half the sum of the products of tho 

 currents and corresponding momenta. Moreover, if 

 this sum be expressed in terms of the magnetic force, 

 it is found to be equal to p. I!~/8 TT, which is the mag- 

 netic energy of the field. Now, when a dynamical 

 system is set in motion against known forces, more 

 energy is supplied than is needed to do tho work 

 against the forces; this excess of energy measures tho 

 kinetic energy acquired by the system. 



Hence, Maxwell was justified in taking the mag- 

 netic energy of the Held as the kinetic energy of tho 

 mechanical system, and if the strengths of the currents 

 in the wires be taken to represent the velocities of tho 

 driving-points, this energy is measured in terms of 

 the electrical velocities and momenta in exactly the 

 same way its the energy of a mechanical system is 

 measured in terms of the velocities and momenta of 

 its driving-points. 



The mechanical system in which, according to 

 Maxwell, the energy is stored is the ether. A state of 

 motion or of strain is set up in the ether of the field. 

 The electric forces which drive the currents, and also 



