2 RELATION BETWEEN QUANTITY AND TENSION. 



quantity of the electricity flowing in the primary wire, we are struck with the fact, that 

 whenever the one has increased, the other has diminished. No matter what the other 

 conditions may be, whether the communication is made by a long wire or a short one, 

 wliether the plates are near or far apart, whenever the quantity is diminished, the tension 

 increases; and whenever the quantity increases, the tension is diminished. 



290. The remarkable analogy of the ponderable elastic fluids, which, when their vol- 

 ume is diminished, or, in other words, condensation takes place, experience an increase 

 of tension or elastic force, is here too broadly indicated to be mistaken. 



291. When I first saw that removing the plates to a greater distance apart deter- 

 mined a given rise in the elastic force of the current, for a time it appeared to me that 

 Dr. FARADAY'S theory of the tension being due to the affinity of the zinc for oxygen 

 must certainly be incorrect. A more extensive acquaintance with the facts has reversed 

 that opinion. If the tension be determined by the affinity of the metal for oxygen, which 

 must be a constant force, how comes it to pass that moving the plates to a greater dis- 

 tance apart can cause it to increase ? This apparent paradox, when properly under- 

 stood, forms a fine illustration of the truth of the doctrine advanced in the 5th, 7th, and 

 8th series of that philosopher's researches. In what follows I shall, therefore, regard 

 those doctrines as established. 



292. Let us take a given pair of plates, and connect them together by a slender wire. 

 We find that the quantity that the plates generate is diminished, and its tension is in- 

 creased ; but that this has not happened either by gain of momentum or inductive 

 influence in the channel of communication, and we are compelled to refer the effect 

 to the resistance of the wire, placing the plates and the electrolyte between them in a 

 state of force. If this be the action of a resisting medium, we might suppose that by 

 continually increasing it we should continually increase the tension, and when it be- 

 came infinitely great, the tension would be so too. But what is the true action of a 

 slender wire, connecting in this way a pair of plates 1 A certain amount of electricity 

 passes along it, but not the whole quantity that the plates could generate in a given time ; 

 yet we cannot suppose that all that does pass comes from the whole surface exposed, and 

 not from a fractional part thereof. The water and zinc are ready to generate, and, as 

 it were, attempting to drive a fresh quantity of electricity through the wire; and, accord- 

 ingly, as the quantity that actually passes becomes a greater and greater portion of 

 what the system actually tends to put in motion, the tension becomes less and less. 

 The tension would therefore become zero if the whole circle wires, plates, and elec- 

 trolyte could carry all that the zinc and water could generate. The limit prescribed 

 to its diminution is the conducting power of the electrolyte, which is the worst con- 

 ductor of the system. 



293. This hypothetical condition, of a tension ranging near zero, is most nearly ap- 

 proximated to in a thermal pair. 



294. Suppose, now, that everything remains the same as respects wires, electrolyte, 

 distance of plates, &c., except that the dimensions of both plates are doubled. Shall 

 we increase the tension 1 No ; for although the surface in action is doubled, and the 

 absolute quantity which the system could generate is doubled, yet the quantity that 



