There may be some of my readers who are unacquainted with this interesting piece of electrical apparatus the Leyden jar. It is a convenient form of what is called an electrostatic condenser. This is two conductors, generally in the form of two plates with an insulator between them. In the Leyden jar the insulator is a glass jar, while the "plates" are coatings of tinfoil, one inside and the other outside. On connecting one coating to one pole of a battery, and the other to the other pole, they become charged, one positively and the other negatively. One, that is, acquires an excess of electricity, while the other becomes deficient to an exactly similar extent. When the two are afterwards connected by a wire the surplus on one flashes through it to make good the deficiency on the other. Rushing first of all from positive coating to negative, electrical inertia causes it to overshoot the mark and to recharge the jar with the charges reversed. Then current begins to flow back again, doing the same several times over, until at last equilibrium is established. The power to absorb and hold a charge of electricity, which is the characteristic of a condenser, is called "capacity." What, then, is "electrical inertia"? I have already referred to the effect which the creation of a magnetic field around a current has upon neighbouring conductors. It also has an effect upon itself. As soon as the current begins to flow it builds up the magnetic field, and in the process some of its energy is exhausted. On the original current ceasing, however, the magnetic field collapses back on to the conductor once more and in so doing restores that energy. This occurs whenever current flows, but it is specially noticeable in long conductors, like submarine cables. In them the battery has to act for a considerable time before any current reaches the farther end. It is in the meantime employed in building up the magnetic field around the wire. Then when the battery has ceased to act the current still comes flowing out at the farther end--the magnetic field is giving back the energy expended upon it. Thus a current is reluctant to start flowing through a conductor, and, having started, is disinclined to stop. This is called "inductance," and it has exactly the same effect upon the current that inertia has upon a body. What inertia is to a material body inductance is to an electric current. And lastly, the resistance which the conductor offers to the passage of the current is precisely analagous to the friction of the water in a pipe. So, we see, the "capacity" of the two coatings of the jar and the inductance which occurs in the connecting wire cause the current to oscillate to and fro for a while when the jar is discharged, which surging or oscillation is ultimately stopped by the resistance of the wire. The two coatings and the wire form what is called an oscillatory circuit. We can now resume our story.