Let us see then how these "Hertzian waves" are produced. In the year 1748 a Dutch experimenter named Cuneus thought he would try to electrify water. He got a glass flask and filled it with water into which he let drop one end of a chain connected to an old-fashioned frictional electrical machine. Thus he stood with the flask in his hand while a friend worked the machine. After a short time the friend stopped and Cuneus took hold of the chain to lift it out, when to his astonishment he received a shock which knocked him over, broke his flask and sent him to bed to recover. Unwittingly Cuneus had invented what became known thereafter as a Leyden jar, Leyden being the town in which he lived. It consisted, you will notice, of two conductors, the water and his hand, with an insulator, the glass, in between. To understand or rather to give ourselves a useful working explanation of how such an apparatus comes to be charged we must first imagine that everything contains a certain normal amount of electricity which we can by certain means add to or take away from at will. When we add some to anything we say we have given it a positive charge: when we subtract some we say that we have imparted a negative charge. Clearly, if we add some to one thing we must first obtain it from something else, and if we take some away from one thing we must do something with what we have taken, and so we add it to something else. Therefore whenever we charge anything positively we must charge something else negatively and vice versa. Now the ease with which we can thus charge two bodies seems to depend upon their nearness to each other, so that the easiest things to charge are two plates of metal separated by the thinnest possible insulator. Modern Leyden jars are usually formed of a thin glass jar with a lining inside and out of tinfoil. 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.