It would lead us into matters beyond the scope of elementary lectures if we were to attempt to summarize all the evidence which exists tending to show that the phenomena of electricity and magnetism must depend upon actions taking place in some medium called the _electro-magnetic medium_. All the great investigators at the beginning of the last century, when electrical and magnetic phenomena were beginning to be explored, came to this conclusion, and in the writings of Joseph Henry, of Ampère, and of Faraday we find references again and again to their conviction that the phenomena of electricity imply the existence of a medium exactly in the same way as do the phenomena of optics. It is only, however, in recent years that we have had evidence before us, some of which will be reviewed in the next lecture, which affords convincing proof that the luminiferous æther and the electro-magnetic medium must be the same. The consideration of the simplest electrical effects is sufficient to show that, if this medium exists, it possesses at least two properties, one of which is that it offers an elastic resistance to the production of electric strain in it by means of electromotive force. A question which is sure to arise in the minds of those who consider this subject carefully is, What is the nature of an electric strain? And the only answer which we can give at the present moment is that we must be content to leave the question unanswered. We do not know enough yet about the mechanical structure of the electro-magnetic medium, or æther, to be able to pronounce in detail on the nature of the change we call an electric strain. It may be a motion of some kind, it may be a compression or a twist, or it may be something totally different and at present unthinkable by us, but, whatever it is, it is some kind of change which is produced under the action of electromotive force, and which disappears when the electromotive force is removed. Clerk-Maxwell, to whom we owe some of our most suggestive conceptions of modern electricity, coined the phrase _electric displacement_ to describe the change which we are here calling an electric strain. One essential element in Maxwell’s theory of electricity is that an electric strain or displacement, whilst it is being made or whilst it is disappearing, is in effect an electric current, and it is for that reason sometimes spoken of as a displacement current. We have seen that every electric circuit possesses a quality analogous to inertia, that is to say, when a current is produced in it it tends to persist, and it cannot be created at its full value instantly by any electromotive force. Just as we cannot, at the present moment, pronounce in detail on the real nature of electric strain, so we cannot say whether that quality which we call inductance of a circuit is dependent upon a true inertia of the electro-magnetic medium or on some entirely different quality more fundamental.