The Boy's Book of New Inventions

by Harry E. Maule

This is just an instance of what the scientists do not know about wireless, but it shows the many boy amateurs that there are still worlds for them to conquer in scientific research.

The central principle upon which the wireless telegraph works now is the same as it was when Marconi, through his marvellous invention, first received a signal from the other side of the Atlantic Ocean, but the inventors have learned much more about the details of the theory and it is in the improvement of devices for applying these laws of electricity that the development has been, rather than in the discovery of new theories. Nikola Tesla's invention for the wireless transmission of power by earth waves is a revolutionary departure from the usual wireless practice, but as we saw in the earlier chapter on this subject the Tesla invention has not yet been put in practical operation.

Though Guglielmo Marconi did not discover the laws of electricity upon which his invention is based, to him belongs all the credit for making use of the discoveries of the scientists of his day, and working out from them a practical system of wireless communication.

As many boys know, the wireless telegraph is possible through the radiation of electric waves. For instance, if a stone is thrown into a pool waves are started out in every direction from the point where the water is disturbed. The water does not move except up and down, and yet the waves pass on until they reach the side of the pool, or their force is expended.

The scientists before Marconi found out that when an electric spark was made to jump between two magnetic poles it started electric waves in every direction, much like the stone thrown into the pool, except at a speed that is reckoned at 186,000 miles per second.

Prof. Amos Dolbear, of Tufts College, Massachusetts, first made use of these waves in 1880, and a few years later Doctor Hertz, conducting experiments along the same lines, discovered them. Since that time these waves have been called Hertzian waves.

For many years scientists had understood that electrical waves or vibrations travelled through the ether in a copper wire, and that gave us telegraphy by wires, but it was a new thing to think of the waves travelling in every direction through space without wires. These early investigators found out that they could detect these waves by a device called a Hertzian loop, which was simply a copper wire bent into a hoop with the two ends close together but not touching. A spark would appear between the ends of the wire when the electric waves were sent out.

Marconi began his work where these scientists left off, as a very young man on his father's farm in Italy, but soon went to England, of which country his mother was a native, and placed the results of his experiments before the government authorities. Continuing his labors he soon had his wireless apparatus worked out in the form in which it first became known to the world.

It consisted of a transmitter, receiving machine or detector, and a set of antennæ or aerial wires from which the electrical waves were sent. For his transmitter, he created a spark between the two brass knobs on the ends of two thick brass wires by closing and opening an electrical circuit with a key, very much like, but somewhat larger than the regulation telegraph key. The space between the knobs was called the spark gap. For a dash he would hold down his key and make a large spark, and for a dot he would release his key quickly and make only a short one. Thus, he could send the regular Morse or Continental telegraphic codes of dots and dashes. These impulses were transmitted by wires to the aerial wires, or antennæ. The impulses left the antennæ as electro-magnetic waves, and went forth in all directions, only to be caught on the antennæ of another station aboard a ship or on land.

Here is where the receiver did its work, and the problem was a far more difficult one than the working out of the transmitter, for the waves as received were too weak in themselves to register a dot or a dash. In Marconi's first instruments he used a device called the "coherer." This was a glass tube about as big around as a lead pencil, and perhaps two inches long. It was plugged at each end with silver, and the narrow space between the plugs was filled with finely powdered fragments of nickel and silver, which possess the strange property of being alternately very good and very bad electrical conductors. The waves in Marconi's first experiments were received on a suspended kite wire, exactly similar to the wire used in the transmitter, but they were so weak that they could not of themselves operate an ordinary telegraph instrument. They possessed strength enough, however, to draw the little particles of silver and nickel in the coherer together in a continuous metal path. In other words, they made these particles "cohere," and the moment they cohered they became a good conductor for electricity, and a current from a battery near at hand rushed through the connection, operated the Morse instrument, and caused it to print a dot or a dash; then a little tapper, actuated by the same current, struck against the coherer, the particles of metal were broken apart, becoming a poor conductor, and cutting off the current from the home battery.