Now just about half a century ago a celebrated
professor of Cambridge University brought all these facts about light, heat, magnetism and electricity together and by skilful reasoning showed that but one Ether sufficed to explain all these things. He showed how magnetic and electric forces acting together could produce waves like those of light and heat. And finally he demonstrated by figures that waves so formed would necessarily travel at the very speed at which light and heat are known to move.
This is known as the electro-magnetic theory of light. And not content with showing the nature of things already known, Professor Clerk-Maxwell added a prophecy that there were other waves in existence of longer wave length, which no one then knew how to make or to detect if made.
Following up this prophecy many investigators sought these waves, and the first to find them was Professor Hertz of Carlsruhe in Germany. Fortunately for his position in the minds of English people he died before the War, so that his name is not sullied by the stupidities of which German professors in more recent days have been guilty. On the contrary, his writings show him to have been a kindly, modest, genial soul, and particularly gratifying is his generous assertion in one of his books that had he not himself discovered these waves he is certain Sir Oliver Lodge would have done so. He seemed quite anxious to share the credit of his discovery with his "English colleague" as he called him.
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.