So soon as the motion ceases the force is gone. But it is most important to notice that no power need be exerted by the engine to call forth a manifestation of electro-motive force, because there is not as yet any appreciable opposition to the rotation of the armature.
Third. The instant the free ends of the armature wires are joined, the attraction, or potential as it is called, diminishes, a current of electricity rushes through the wire, and the mysterious opposition to rotation at once springs into existence, the belt grows taut on the driving side, the engine takes more steam and labors harder and harder, while a constant stream of mechanical power must be supplied by it to the dynamo to maintain that motion which a minute before went on so easily and freely.
The electrical current passing out from the dynamo is constantly carrying energy away from it. This loss must be incessantly supplied by the steam engine, and this demand is brought about by the opposition to rotation set up within the machine through reaction of the electro-motive force on the material of the conductor and on the magnetic lines. Thus we have here the constant characteristic of a closed system where invariably the product of a reaction opposes the primitive cause of the change.
Thus far the phenomena just quoted exemplify the rule. It would not have been worth while to take so much time to describe the dynamo if nothing more was to be learned from it, but there is. This semi-living machine, whose elements are so simple compared with those of a really living structure, enables us, because of its mechanical simplicity, to go one step further in our analysis and to inquire how the result of the change reacts on the exciting cause. It is known beyond doubt that in a working dynamo the action of the current is twofold. It not only tends to stop the armature, but it actually diminishes the magnetism of the fields, and so lessens the electro-motive force by attacking it at the very place of its origin. Let me repeat: the magnetism and the rotation create the electro-motive force; this latter creates the current; then the current in turn reacts both to oppose the rotation and to cut down its own initial cause; and, further, this reaction on the cause is found always to require an appreciable time.
Here, I think, we have struck a new principle. In electrical matters it has been known only a few years, and has had no applications in other sciences, but I venture to think it is somewhat general, and that illustrations of it may be found elsewhere, one or two of which I will endeavor to submit.
A spiral spring supporting a weight does not manifest this principle, for the cause—that is, the weight—is not lessened by the pressure it produces. The same is true of all static states; but when motion occurs, then this new principle may often be observed.
