pipe that an exact equality of pressure in the two vessels is attained. The electrical actions which take place in connection with an electric discharge between two conductors, one of which is charged positively and the other negatively, are exactly analogous to the above-described experiment with two air-vessels, one of which has air in it under compression, and the other has had the air removed from it. You will notice, however, that the oscillations of the air in the pipe in the air-vessel experiment depend essentially upon the fact that air is a substance which has _inertia_, or mass, and you will naturally ask what is it which has inertia, or its equivalent, in the electrical experiment? The answer to this question is as follows: Every electric circuit has a quality which is called _inductance_, in virtue of which an electric current cannot be started in it instantly, even under any electromotive force, and conversely when the current is started it cannot be immediately brought to rest. From the similarity of this quality of the circuit to the inertia of ordinary material substances, it has been sometimes called the _electric inertia_ of the circuit. The word “inertia” really means inactivity, or laziness, but the term as used in mechanics implies something more than mere inactivity. It involves the notion of a persistence in motion when once the body is set moving. When a material substance is in motion it possesses energy, and has the power of overcoming up to a certain point resistance to its motion. This energy-holding power, or capacity for storing up energy of motion, which is characteristic of all material substances, is a consequence of their inertia. The fact is otherwise expressed by stating that the _mass_ of a material substance is one element in the production of energy of motion.