JAMES CLERK MAXWELL 323 



to the reservoir. The water will rise in the tube, but the rise will 

 stop when hydrostatic equilibrium is attained — that is, when the 

 downward pressure of the water in the tube above the point of appli- 

 cation 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 bend 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 when 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 com- 

 pared 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 fluids. 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 in 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 

 considered as a current round a closed circuit. It went, they thought, 

 from one armature through the wire, the battery, the other wire, to the 

 other armature, and there it stopped. Maxwell, on the contrary, sup- 

 posed that in the form of a current of displacement it passes through 



