Before delving into the mysteries of receiving and sending messages without wires, a word as to the history of the art and its present day applications may be of service. While popular interest in the subject has gone forward by leaps and bounds within the last two or three years, it has been a matter of scientific experiment for more than a quarter of a century.

The wireless telegraph was invented by William Marconi, at Bologna, Italy, in 1896, and in his first experiments he sent dot and dash signals to a distance of 200 or 300 feet. The wireless telephone was invented by the author of this book at Narberth, Penn., in 1899, and in his first experiments the human voice was transmitted to a distance of three blocks.

The first vital experiments that led up to the invention of the wireless telegraph were made by Heinrich Hertz, of Germany, in 1888 when he showed that the spark of an induction coil set up electric oscillations in an open circuit, and that the energy of these waves was, in turn, sent out in the form of electric waves. He also showed how they could be received at a distance by means of a ring detector, which he called a resonator.

In 1890, Edward Branly, of France, showed that metal filings in a tube cohered when electric waves acted on them, and this device he termed a radio conductor; this was improved upon by Sir Oliver Lodge, who called it a coherer. In 1895, Alexander Popoff, of Russia, constructed a receiving set for the study of atmospheric electricity, and this arrangement was the earliest on record of the use of a detector connected with an aerial and the earth.

Marconi was the first to connect an aerial to one side of a spark gap and a ground to the other side of it. He used an induction coil to energize the spark gap, and a telegraph key in the primary circuit to break up the current into signals. Adding a Morse register, which printed the dot and dash messages on a tape, to the Popoff receptor he produced the first system for sending and receiving wireless telegraph messages.

After Marconi had shown the world how to telegraph without connecting wires it would seem, on first thought, to be an easy matter to telephone without wires, but not so, for the electric spark sets up damped and periodic oscillations and these cannot be used for transmitting speech. Instead, the oscillations must be of constant amplitude and continuous. That a direct current arc light transforms a part of its energy into electric oscillations was shown by Firth and Rogers, of England, in 1893.

The author was the first to connect an arc lamp with an aerial and a ground, and to use a microphone transmitter to modulate the sustained oscillations so set up. The receiving apparatus consisted of a variable contact, known as a pill-box detector, which Sir Oliver Lodge had devised, and to this was connected an Ericsson telephone receiver, then the most sensitive made. A later improvement for setting up sustained oscillations was the author's rotating oscillation arc.

Since those memorable days of more than two decades ago, wonderful advances have been made in both of these methods of transmitting intelligence, and the end is as yet nowhere in sight. Twelve or fifteen years ago the boys began to get fun out of listening-in to what the ship and shore stations were sending and, further, they began to do a little sending on their own account. These youngsters, who caused the professional operators many a pang, were the first wireless amateurs, and among them experts were developed who are foremost in the practice of the art today.

Away back there, the spark coil and the arc lamp were the only known means for setting up oscillations at the sending end, while the electrolytic and crystal detectors were the only available means for the amateur to receive them. As it was next to impossible for a boy to get a current having a high enough voltage for operating an oscillation arc lamp, wireless telephony was out of the question for him, so he had to stick to the spark coil transmitter which needed only a battery current to energize it, and this, of course, limited him to sending Morse signals. As the electrolytic detector was cumbersome and required a liquid, the crystal detector which came into being shortly after was just as sensitive and soon displaced the former, even as this had displaced the coherer.

A few years ahead of these amateurs, that is to say in 1905, J. A. Fleming, of England, invented the vacuum tube detector, but ten more years elapsed before it was perfected to a point where it could compete with the crystal detector. Then its use became general and workers everywhere sought to, and did improve it. Further, they found that the vacuum tube would not only act as a detector, but that if energized by a direct current of high voltage it would set up sustained oscillations like the arc lamp, and the value of sustained oscillations for wireless telegraphy as well as wireless telephony had already been discovered.

The fact that the vacuum tube oscillator requires no adjustment of its elements, that its initial cost is much less than the oscillation arc, besides other considerations, is the reason that it popularized wireless telephony; and because continuous waves have many advantages over periodic oscillations is the reason the vacuum tube oscillator is replacing the spark coil as a wireless telegraph transmitter. Moreover, by using a number of large tubes in parallel, powerful oscillations can be set up and, hence, the waves sent out are radiated to enormous distances.

While oscillator tubes were being experimented with in the research laboratories of the General Electric, the Westinghouse, the Radio Corporation of America, and other big companies, all the youthful amateurs in the country had learned that by using a vacuum tube as a detector they could easily get messages 500 miles away. The use of these tubes as amplifiers also made it possible to employ a loud speaker, so that a room, a hall, or an out-of-door audience could hear clearly and distinctly everything that was being sent out.

The boy amateur had only to let father or mother listen-in, and they were duly impressed when he told them they were getting it from KDKA (the Pittsburgh station of the Westinghouse Co.), for was not Pittsburgh 500 miles away! And so they, too, became enthusiastic wireless amateurs. This new interest of the grown-ups was at once met not only by the manufacturers of apparatus with complete receiving and sending sets, but also by the big companies which began broadcasting regular programs consisting of music and talks on all sorts of interesting subjects.

This is the wireless, or radio, as the average amateur knows it today. But it is by no means the limit of its possibilities. On the contrary, we are just beginning to realize what it may mean to the human race. The Government is now utilizing it to send out weather, crop and market reports. Foreign trade conditions are being reported. The Naval Observatory at Arlington is wirelessing time signals.

Department stores are beginning to issue programs and advertise by radio! Cities are also taking up such programs, and they will doubtless be included soon among the regular privileges of the tax-payers. Politicians address their constituents. Preachers reach the stay-at-homes. Great singers thrill thousands instead of hundreds. Soon it will be possible to hear the finest musical programs, entertainers, and orators, without budging from one's easy chair.

In the World War wireless proved of inestimable value. Airplanes, instead of flying aimlessly, kept in constant touch with headquarters. Bodies of troops moved alertly and intelligently. Ships at sea talked freely, over hundreds of miles. Scouts reported. Everywhere its invisible aid was invoked.

In time of peace, however, it has proved and will prove the greatest servant of mankind. Wireless messages now go daily from continent to continent, and soon will go around the world with the same facility. Ships in distress at sea can summon aid. Vessels everywhere get the day's news, even to baseball scores. Daily new tasks are being assigned this tireless, wireless messenger.

Messages have been sent and received by moving trains, the Lackawanna and the Rock Island railroads being pioneers in this field. Messages have also been received by automobiles, and one inventor has successfully demonstrated a motor car controlled entirely by wireless. This method of communication is being employed more and more by newspapers. It is also of great service in reporting forest fires.

Colleges are beginning to take up the subject, some of the first being Tufts College, Hunter College, Princeton, Yale, Harvard, and Columbia, which have regularly organized departments for students in wireless.

Instead of the unwieldy and formidable looking apparatus of a short time ago, experimenters are now vying with each other in making small or novel equipment. Portable sets of all sorts are being fashioned, from one which will go into an ordinary suitcase, to one so small it will easily slip into a Brownie camera. One receiver depicted in a newspaper was one inch square! Another was a ring for the finger, with a setting one inch by five-eighths of an inch, and an umbrella as a "ground." Walking sets with receivers fastened to one's belt are also common. Daily new novelties and marvels are announced.

Meanwhile, the radio amateur to whom this book is addressed may have his share in the joys of wireless. To get all of these good things out of the ether one does not need a rod or a gun--only a copper wire made fast at either end and a receiving set of some kind. If you are a sheer beginner, then you must be very careful in buying your apparatus, for since the great wave of popularity has washed wireless into the hearts of the people, numerous companies have sprung up and some of these are selling the veriest kinds of junk.

And how, you may ask, are you going to be able to know the good from the indifferent and bad sets? By buying a make of a firm with an established reputation. I have given a few offhand at the end of this book. Obviously there are many others of merit--so many, indeed, that it would be quite impossible to get them all in such a list, but these will serve as a guide until you can choose intelligently for yourself.

F. C.