The Leyden jar is, however, only one form of the piece of electrical apparatus known as an electrical condenser, and many other forms exist. For example, a flat sheet of glass with foil above and below, or several such piled one on top of another. An eminent electrician whom I know has recently made some of two tin patty pans put bottom to bottom, nearly but not quite touching, the whole being enclosed in a solid block of paraffin wax. And I might describe many other forms, but whatever they may be every one is essentially two conductors with an insulator between.

Now when a condenser has been charged its charges remain for a considerable time unless they be given a chance to escape. Suppose you have a charged condenser and that you take a wire and with it touch simultaneously both the conductors, the surplus on one "plate" will rush through the wire and make good the deficiency upon the other; it will thus in an instant become discharged.

Now several scientific men had suggested, before Hertz's time, that when that occurred something else happened too. They thought that the charge did not simply rush from one plate to the other instantly, but that it oscillated to and fro for a period; that the surplus rushing round overshot the mark, so to speak, and not only made up the deficiency but caused a surplus on the opposite plate, after which this new

surplus rushed back again through the wire, doing the same thing, though to a less and less degree, several times over before a condition of perfect rest was reached. To use a simple analogy, it was thought that the surplus swung to and fro like the swinging of a pendulum. We know that a pendulum swings because of its inertia, and electricity possesses a property very like inertia which, it was thought, would cause it to behave in the same way.

The Ether waves travel at the rate of 186,000 miles per second, so that if, as was thought, a sudden current of electricity gives rise to a wave, currents which succeed each other at the rate of one per second would produce waves 186,000 miles apart. A hundred currents per second would give a wave length of 1860 miles. A thousand per second would give 186 miles. But a thousand succeeding currents per second are difficult to produce, and 186 miles is so very much greater than the tiny fraction of an inch, which is the length of the light and heat waves, that Hertz had to find some way of making currents succeed each other faster even than a thousand times per second.

So he thought of these oscillating currents which were supposed to occur when a condenser was discharged, and he rigged up a condenser with an induction coil and a spark gap in a way which he thought would do what he wanted.

There is not room here to explain the Induction Coil, indeed it is so well known that it will be quite sufficient to state that it is an apparatus which takes steady current from a battery and gives back instead

a lot of little spurts or splashes of current at a rate of, say, fifty or one hundred splashes per second, according as we adjust the little vibrating spring which forms a part of the coil. We can so connect this to a condenser that each splash will charge it up; and we can combine with it a spark-gap, that is to say, a gap between two knobs, so that every time it is charged it immediately discharges again through this gap. Thus we may have, say, one hundred splashes per second, and each splash is followed by several oscillations across the air-gap, the oscillations taking place at the rate of perhaps a million per second. Each series of oscillations is called a "train."

Now a million per second gives a wave-length somewhere about what Hertz wanted, so he arranged his apparatus as just described.

For a condenser he used two metal plates a little distance apart, the air between forming the insulating material. He set up his apparatus in a large room, and having started the coil he moved about with a nearly complete hoop of wire, the ends of which nearly touched. Working in darkness he found after a while that sometimes he could see little sparks, very small but just visible across the gap between the ends of the bent wire. Those sparks only occurred when the coil was in action, and so he knew that the one was the result of the other's work. By careful painstaking experiment he found that the sparks were unquestionably caused by waves, and that the waves moved with the same speed as light, also that they could be reflected and refracted just on precisely the