The American Monthly Review of Reviews, June, 1907, pages 681-685:
WIRELESS TELEPHONY BY THE DE FOREST SYSTEM.
BY HERBERT T. WADE.
THE question of wireless telephony is by no means new and such experimenters as Preece, Ruhmer, and Arco in Europe, as well as several scientists in this country, have been at work on this problem for several years. The differences between wireless telegraphy and wireless telephony are in the main as fundamental as in the two familiar systems employing wires. While the telegraph transmits an electric current so interrupted as to correspond to signals arranged by a given code, the telephone depends upon the rapidity of interruption or variation in the intensity produced by the sound vibration.
There have been several methods proposed for telephony without wires, the earliest, perhaps, being the radiophone of Prof. Alexander Graham Bell, where a selenium cell was placed in the focus of a silvered parabolic reflector and was connected with a battery and a telephone receiver. The variation in the electric resistance of selenium with the amount of light should be a valuable principle in wireless telephony, but it was found that the radiophone of Professor Bell could be used only over short distances. The effect of telephone currents upon an arc lamp was later investigated, and it was discovered that this device could be used for the transmission of sound. Accordingly, Ernest Ruhmer, of Berlin, combined the "speaking arc" with the selenium cell, and by use of a searchlight for his transmitting apparatus and a selenium cell in a parabolic reflector for a receiver, was able to transmit sound effectively up to seven miles, but his experiments do not seem to have come to any very practical outcome. The reason for this will be apparent when it is realized that the difficulty of directing the beam of light to the distant reflector and the ease with which it could be obstructed or intercepted, not only by solid materials, but by fog, must be taken into consideration. Wireless telephony has also been attempted through ground circuits, and by induction over short distances, but for a really practical system we have to consider such modifications as have come from the development of wireless telegraphy. Count Arco, in Germany, has carried on important telephonic communication with modifications of his wireless telegraph system, while in a demonstration recently made with the Fessenden system, wireless telephony was carried on from a tall mast at Brant Rock across Duxbury Bay to Plymouth, a distance of about twelve miles.
The most systematic and important experiments, however, are those recently carried on by Dr. Lee De Forest, who, for a number of years, has been working in wireless telegraphy, and has devised methods and apparatus that have proved most successful. After almost a year of constant experiment on radio-telephony Dr. De Forest has reached the conclusion that wireless telephony on a practical and commercial scale has been realized.
The practical development of this invention now has progressed so far that sounds produced in Dr. De Forest's laboratory in New York City have been heard not only at other laboratories, several miles distant, but distinctly at Quarantine, twelve miles, on board the steamer Bermudian. In fact, Dr. De Forest has been informed by numerous amateurs in New York City and Brooklyn that their apparatus has frequently responded to the waves corresponding to music sent out from his transmitter. It might be said here by way of explanation that modern wireless telegraph systems employ almost exclusively for long distance work a telephone to receive the signals. This instrument is one of the most sensitive means of detecting the presence of an electric current or any variation in its intensity or frequency. Consequently, if an experimenter is listening for the click, click, corresponding to the wireless signals sent out at some transmitter station, and hears instead music or the human voice, he must know that his apparatus is responding not to the dots and dashes of the Morse alphabet, but to waves in unison with the original sound waves appropriate to the sound. Now, in wireless telegraphy we do not change the frequency of electric waves, but we produce and interrupt them at such frequency as we desire in order to form our dots and dashes. In wireless telephony we have a similar condition, and while we cannot vary the frequency of the waves yet we can vary their amplitude or intensity as we please. Our ability to do this very rapidly, in fact as rapidly as the vibrations of the human voice or other sound, makes wireless telephony possible, as receivers have been devised sufficiently sensitive to respond to these waves and to translate them into varying electric currents which produce corresponding vibrations in the diaphragm of a telephone receiver and thus produce sound that is audible to the human ear.
Passing from general considerations it may aid the reader to recapitulate the fundamental principles involved in wireless or wave telegraphy. Reduced to its simplest elements, there is an alternating current of high voltage surging or oscillating in a vertical wire, one end of which is earthed while the other is elevated. The current is transformed by means of a suitable oscillator or induction coil and transformer, so that the current flowing in the vertical or aerial wire or antenna is of high voltage. From this wire electric waves are sent out in all directions with a velocity equaling that of light, some 186,400 miles per second. Corresponding to the antenna at the transmitting station there is a similar one placed at the receiving station in the path of the waves, so that in the circuit of the receiving apparatus corresponding oscillating impulses may be established. These oscillations can take place directly in the antenna, or an induction coil or transformer may be employed. To detect these oscillations we can use any one of several devices that will respond to small differences of electric potential. Thus we have the coherer, where a number of fine metallic fragments group themselves differently for the passage of the waves and enable a current to pass. Then there is the electro-magnetic receiver, such as is used in the Marconi system, where the antenna is connected with the primary coil of an induction coil, and a telephone is in the circuit of the secondary, there being an iron wire which moves through the coils. The electrolytic receiver is also employed, as it is more sensitive than either of those just mentioned. When the current passes through the electrolytic cell of this instrument there is a change in its resistance to which the telephone immediately responds. Finally there is a receiver, known as the audion, invented by Dr. De Forest, and which is available for both telegraphy and telephony. It consists of a small incandescent lamp having a tantalum or other filament supplied with two small plates of platinum within the bulb, and connected with the outside by platinum leading wires. This lamp is lighted by current from the small storage battery shown in the illustration. The antenna is connected with the two platinum wings, or in the most recent forms of the device, with a grid and a wing. Now the interior of the bulb is highly exhausted, and when a current passes there is ionization, or the separation of the rarefied gas into minute particles or ions. When the waves fall on this receiver the resistance of the interior is changed and the telephone which is in the circuit will respond immediately. The audion has been found well adapted for telephonic work and has been used by Dr. De Forest in his experiments on sound transmission.
The experimental apparatus for wireless telephony, as arranged in Dr. De Forest's laboratory, is shown in the illustration. In its commercial form the transmitting apparatus is reduced in size and is of much the same character as the receiver shown in the second photograph. The primary current used can be obtained from the lighting mains or a storage battery, and after passing through inductance coils to prevent any undue variations in the circuit, goes to an oscillator, shown at the extreme left of the picture. This may be any form of high frequency interrupter, though in the apparatus, as now arranged, there is an arc which supplies an alternating current of sufficient frequency. In connection with this is a condenser and the primary of a transformer. The former is under the two large coils of the transformer, shown in the center of the picture. The secondary is connected with the antenna or aerial wire, which in Dr. De Forest's laboratory is mounted on the roof of the building. The secondary is also connected with an ordinary carbon microphone transmitter and then to the earth. Just as in the ordinary telephone transmitter the resistance of the carbon changes with the vibration of the diaphragm, so here the change in resistance affects not the frequency of the waves but their amplitude or intensity. This is in essence the transmitting apparatus, disregarding, of course, such questions as the power hat must be used, the nature of the waves (i. e., their length and frequency), and the height of the antenna, all of which affect the distance of transmission. At the receiving station we have the antenna and transformer coil connected to the earth as before, and the audion and telephone arranged in circuit, as shown in the diagram. An ordinary telephone is used, and the audion and telephone battery are contained in a single wooden case, which, with the storage cells used to light the lamps, is shown on the table by which Dr. De Forest is standing.
Wireless telephony will do away with the trained operators on vessels, so that a larger number as well as smaller steamships can be in communication with the shore or with each other during a voyage. Not only can reports be made by coastwise vessels and steamers on the Great Lakes, but the exact position or the dangerous proximity of other vessels or light-houses can be determined. This immediately suggests the use of wireless telephony in naval operations, especially with a fleet, or where a torpedo boat or other vessel is employed on a detached service. Wireless telephony between sea and land does not stop at the receiving station on the shore, since it is possible to connect the instruments so that conversations can be immediately and directly transmitted to the wire circuit of the land system. Furthermore, experiments have already been undertaken which have demonstrated the feasibility of communicating between moving trains and central offices or signal stations, and even establishing direct connection with public lines. In fact, the readiness with which farmers' telephone lines,--often using fence wires,--have been constructed in the West leads to the belief that a suitable wireless telephone system would find widespread appreciation in rural communities and mountainous districts.
The great and universal appreciation of music reproduced by graphophone, telharmonium, or other device has suggested to Dr. De Forest that radio-telephony has also a field in the distribution of music from a central station, such as an opera house. By installing a wireless telephone transmission station on the roof, the music of singers and orchestra could be supplied to all subscribers who would have aerial wires on or near their homes. The transmission stations for such music would be tuned for an entirely different wave length from that used for any other form of wave telegraph or telephone transmission, and the inventor believes that by using four different forms of wave as many classes of music can be sent out as desired by the different subscribers.
At the present time, then, it would seem that there is a distinct commercial future for wireless telephony, but a conservative judgment would indicate that its use would be supplemental to existing wire circuit systems, and that the latter are not likely to be supplanted for many years to come by any form of radio-telephonic apparatus. It will occupy a new and large field for marine work, for military lines especially during war or manoeuvers, for communication between islands, and for mountainous and sparsely settled districts, especially where temporary installations are desired, which can be installed on the shortest notice and maintained without specially trained operators.