LIGHT AND ELECTRICITY. 131 



viscous resistance, to the movements of electricity. Hence, there are two 

 kinds of currents; currents of displacement Avhicli traverse dielectrics 

 and ordinary currents of conduction which circulate in conductors. 



Currents of the first kind, having to overcome an elastic resistance 

 which continually increases, naturally can last but a very short time, 

 since a state of equilibrium will (juickly be reached. 



Currents of conduction, on the other hand, having only a viscous 

 resistance to overcome, must continue so long so there is any electro- 

 motive force. 



Let us return to the simile used by M. Coruu in his notice in the 

 Annuaire du Bureau des Longitudes for 1893. Suppose we have iu a 

 reservoir water under pressure. Lead a tube plumb downward into the 

 reservoir. The water will rise in the tube, but the rise will stop when 

 hydrostatic equilibrium is attained — that is, when the downward pres- 

 sure of the water in tlie tube above the i^oint of application of the first 

 pressure on the reservoir, and due to the weight of the water, balances 

 that first pressure. If the pipe is large, there will be no friction or 

 loss of head, and the water so raised can be used to do work. That 

 represents a current of displacement. 



If, on the other hand, the water flows out of the reservoir by a hori- 

 zontal pipe, the motion will go on till the reservoir is emptied; but if 

 the tube is small and long there will be a great loss of energy and con- 

 siderable production of heat by friction. That represents a current of 

 conduction. 



Though it would be vain, not to say idle, to attempt to represent all 

 details, it may be said that everything happens just as if the currents 

 of displacement were acting to beml a multitude of little springs. 

 When the currents cease, electrostatic equilibrium is established, and 

 the springs are bent the more, the more intense is the electric field. 

 The accumulated work of the springs — that is, the electrostatic energy — 

 can be entirely restored as soon as they can unbend, and so it is that 

 we obtain mechanical work wlien we leave the conductors to obey the 

 electrostatic attractions. Those attractions must be due to the pressure 

 exercised on the conductors by the bent springs. Finally, to pursue 

 the image to the death, the disruptive discharge may be compared to 

 the breaking of the springs when they are bent too much. 



On the other hand, the energy employed to produce conduction cur- 

 rents is lost, being wholly converted into heat, like that spent in over- 

 coming the viscosity of fiuids. Hence it is that the conducting wires 

 become heated. 



From Maxwell's point of view it seems that all currents are in closed 

 circuits. The older electricians did not so opine. They regarded the 

 current circulating in a wire joining the two poles of a pile as closed; 

 but if ill place of directly uniting the two poles we place them in com- 

 munication with the two armatures of a condenser, the momentary 

 current which lasts while the condenser is getting charged was not 



