i6o PHYSICAL SCIENCE 



of the velocity with which they move under an 

 electric force ; and of their mean free path within 

 the metal, that is, of the average distance an 

 electron moves between its collisions with other 

 electrons. As we have seen, when the metal is 

 heated, the electrons begin to leave it, and stream 

 away into the surrounding space. At any constant 

 temperature equilibrium is set up between the 

 electrons leaving the metal owing to the effect 

 of temperature, and those drawn back again by 

 the residual positive charge on the metal. We 

 may look on the system as analogous to a liquid 

 in equilibrium with its own vapour. 



In the last chapter we saw that it was necessary 

 clearly to distinguish the electric current and the 

 heating effect of the current from the flow of the 

 energy by which the current was maintained. 

 The energy passes through the surrounding 

 medium, through the luminiferous sether. The 

 current is merely the line along which the energy 

 of the aether can be dissipated as heat. Faraday 

 and Maxwell showed that the medium invoked to 

 explain the phenomena of light was also competent 

 to explain electric and magnetic manifestations. 

 An electric force is a state of strain in the aether, 

 and the immediate function of an electric machine 

 or voltaic battery is to set up such a state of strain. 

 If the poles of the battery are insulated from each 

 other, the state of strain is maintained, the poles 

 are attracted towards each other with a small 

 force, but nothing else happens. Faraday, as 

 we have seen, represented this state of strain by 

 drawing lines of force, or tubes of force, which 

 map out the electric field, and everywhere follow 

 the direction of the electric force. The tubes of 



