Yale Scientific Monthly, October, 1903, pages 3-9:
DEFOREST SYSTEM OF WIRELESS TELEGRAPHY.
A. S. MCLEAN.
THE problem of telegraphing without wires, about which so much has been said and written during the last few years, has at last assumed such tangible form that there seems no longer any doubt of the future success of this mode of communication between distant points, especially along particular lines. The stage of experiment is now passed. One by one the obstacles in the way of the successful transmission of aerographic messages have been removed or overcome, and now it seems reasonable to accept wireless telegraphy as a fixed factor in the commercial world. This conclusion is strengthened by the fact that at least four capitalized companies, each in competition with the others here in the United States, are supporting as many different systems.
The first real American system is that known as the de Forest System. Dr. Lee de Forest, the inventor, is a graduate of Yale, '96S., and it may be of interest to note that much of the work on which his system is based was directly carried on in the laboratories of the Sheffield Scientific School. Dr. de Forest's work at Yale dealt primarily with Hertzian wave phenomena, and his invention is the outcome of his investigations and observations made while engaged in this study.
The astonishing results obtained in connection with Wireless Telegraphy, coupled with the general ignorance of the principles of electricity, have served to impress the average reader with the idea that the theory of operation is incomprehensible except to the scientific mind. Certainly, the first visit to a wireless station in full operation in no way tends to dispel this conception. But omitting the technicalities of construction, which are necessarily different in different systems, it will be found that the general idea is quite simple. All systems, of course, are based on Hertz's theory of electrical waves. In simple English, this theory may be stated thus: Any periodic disturbance, such as an oscillatory discharge from a condenser, gives rise to a system of waves in the surrounding electrostatic field which are propagated in all directions. It has been shown, in theory, that the velocity of propagation of these waves in air is the same as the velocity of light, which would make wireless telegraphy practically instantaneous for all terrestrial distances. While all systems use the same kind of waves, yet there are numerous ways of creating and recording the original
disturbance, and it is in these particulars that the many systems in use differ.
The de Forest system uses the alternating current exclusively, as against the direct current of most other systems. The advantage claimed for this is that a greater frequency of oscillation is obtained at the discharge, and this in turn allows of more rapid use of the key in sending. The original circuit is of comparatively low potential, say of 500 volts, and it is in this circuit that the key is operated. From the diagram it will be seen that the current then passes through a one-to-one choke temperature, and then through a step-up transformer, the ratio of which depends upon the distance the waves have to travel. After passing through spiral choke coils, which act as safety plugs on the system, the current enters the condenser. The condenser is supplied with a mushroom spark gap, and a variable induction coil. One pole of the sparker is connected with the antenna, and the other is grounded. The other wire from the antenna, which, by the way, is the apparatus from which the waves radiate, is also grounded, thus completing the transmitting circuit.
The receiver, which is connected as shown in diagram, is thrown into the circuit by the switch. It is in this departure from the "coherer" plan that the de Forest system differs radically from the Marconi and all other similar systems. In the place of the coherer is an instrument called the responder. It will be remembered that the coherer consists essentially of a tube containing metal filings which close and break the circuit as the impulse is felt at the antenna. The iron filings readily arrange themselves to transmit a wave, but must be disarranged by mechanical means to break the circuit. The limitation of the operation of "tapping" the coherer with regard to speed is such that about fifteen words a minute constitutes the maximum speed of a system using a coherer. However, in the case of the de Forest responder, the action, while depending as in the coherer on a sudden decrease of resistance, is practically instantaneous, requiring no intermediate step to be gone through between each impulse. The advantage of this is very evident, since it allows much greater speed of manipulation. The impression in the responder is simply a slight click, which is modified to an audible sound in an ordinary telephone earpiece. The antenna consists of two parallel wires, connected at their ends to an apparatus producing electrical oscillations at high frequency, the wires being of the same length, and each one quarter, or any multiple, of the wave length of the electrical oscillation. Electromagnetic waves are then set up along the wires. If the corresponding nodes and loops of electrical oscillations in the two wires be connected together, the waves will not be interrupted; and, further, if these bridges be grounded, they lead off waves of different length than those to which the system is adjusted or attuned. The responder is placed at half way between the nodes or at a loop where all the energy of the system is electro-static, and the instrument will receive oscillations of the greatest intensity.
A variable induction coil, shown in the plan connected with the condenser, is another very important feature of the apparatus. Up to this time it has been practically impossible to obtain a continuous stream of electro-magnetic radiation adapted to this class of work. In this device, by cutting off more or less resistance in the induction coil, any number of discharges may be secured. In other words, the apparatus can be tuned for practically any wave frequency. The mushroom type of discharge gap has been found best suited to this work.
The operation of transmitting is quite simple. The key, whose connections, for the purpose of safety, are insulated in oil, is operated in the same manner as in the ordinary line telegraph. The continuity of the discharge, allowing a steady stream of sparks while the key is closed, makes it possible to transmit messages with great distinctness, a brief depression of the key signifying a dot and a long one a dash. In receiving, the waves striking the antenna are led into the responder, where they set up electrical disturbance which is the cause of audible clicks. The diagram shows clearly the path of the current and impulses both in sending and in receiving messages.
For different distances and different instruments the De Forest system has an apparatus which is introduced into the circuit by which particular waves only affect particular instruments. That is, by adjusting the instrument to any determined tuning, messages coming from another station of the same adjustment would be received clearly, while all others would be indistinct. Another improvement about to come out is the wireless call, by means of which a wireless message will register its presence and thus obviate the necessity of having some one at the receiver all the time.
The speed of the De Forest system is also worthy of note. The average speed is thirty-five to forty words per minute, but the record to date is fifty-two words. It is said that the maximum speed by any other known system is only fourteen words per minute, with an average of even less. This speaks well for the American system, and from a business standpoint is a strong argument in favor of its application to commerce.
The business successes of the De Forest company are already many and important. The system has been officially adopted by the United States government, as well as by a number of railroads and steamboat lines. About thirty government and private stations have been installed in different parts of the country, and a number of others are in construction. During the recent cup races the system was in constant use for communication with the land, and the Publisher's Press received all its news through the De Forest operators. Sir Thomas Lipton's yacht Erin was equipped with the De Forest system before leaving England. During his stay in America his ship was at all times in communication with the various De Forest stations around New York harbor, and Sir Thomas stated that the convenience added much to his pleasure and comfort. Perhaps the most interesting use to which the wireless has been put is the transmitting of dispatches for a newspaper. The Providence Journal has established the "Block Island Wireless," which is published daily at Block Island, R. I. All of its telegraphic news is received from Point Judith, R. I., by De Forest wireless telegraphy. The complete success of the experiment, which is now a permanent enterprise, is shown by the record for ten successive nights, which was fifty thousand words without a slip.
The outlook for wireless telegraphy is indeed bright. The field for business is practically limitless. Especially in navigation it is an indespensable factor, and during the next few years great development can be expected in this direction. If it even diminishes the nnmber of disasters on the seas it will have done a great work. It may reasonably be expected that Wireless Telegraphy will in time undertake all the tasks now performed by the outlook, the fog-whistle, the light-house and danger-signal, and will accomplish them with greater reliability than seems to be attainable at present. It is destined to play a very important part in the railroad world. By the use of wireless telegraphy the company can keep in constant communication with its passenger trains, enabling it, in case of danger, to stop the trains at a moment's notice, thus averting accident and possible disaster.
The fact that there are so many systems in the field to-day leads one to wonder which one or ones will survive. Time alone can tell. But basing our opinions on the results of earlier Yale efforts, it is not without reason to conjecture that the American De Forest system of Wireless Telegraphy--the outcome of Yale genius--will stand at the head among its competitors. *
* The illustrations in this article are by courtesy of "Marine Engineering."