The Freeman-DeForest-Smythe system was innovative in that it used audio reception, but also quickly obsolete. In his autobiography, Lee DeForest wrote that at the 1901 International Yacht Races he found the Freeman transmitter to be so unreliable that "I threw the useless device overboard". Moreover, the Responder receiver was little better, and eventually DeForest quietly replaced it with an infringing version of Reginald Fessenden's electrolytic detector.
Western Electrician, July 27, 1901, pages 49-50:
A New System of Space Telegraphy.
So numerous of late have been the new systems and methods of space telegraphy, each heralded as "novel" and proving, on investigation to represent but slight modifications of, and questionable improvement upon, the Marconi system, that it is refreshing to come upon a system operating with radically different apparatus, both at the receiving and sending ends. Such a system has been developed recently in Chicago.
The receiver of this new system is the joint invention of Lee De Forest, a graduate of Yale University of the class of '96S, and Edwin H. Smythe of the engineering department of the Western Electric company of this city. Mr. Smythe's 10 years' work in the field of telephony has given him experience that has proved especially valuable in dealing with the problem in hand, while during his three years of graduate work at Yale Mr. De Forest made a specialty of the subject of the Hertzian waves, taking the degree of Ph. D. for work along that line. Readers of the Western Electrician will also be interested in knowing that for a time Mr. De Forest was connected with the editorial staff of this journal, resigning to prosecute work on this invention. The sending apparatus has been developed by Professor Clarence E. Freeman, E. E., associate professor in electrical engineering at the Armour Institute of Technology; and, exemplifying the liberal spirit of that institution in relation to original scientific research, its laboratories were thrown open for the development of the new system.
The ideas embodied in the receiver and the sender were originally worked out independently by the inventors, and it was found that by their combination a new system was possible, which did away entirely with the coherer at the receiving station, and the induction coil at the sending end.
In the coherer, heretofore almost invariably used as the receiving instrument, in space telegraphy, the great drawback is the necessity for a tapping, or decohering device, with the complication of apparatus this involves, its uncertainty of action, and, most important of all, the time lag, with limited speed of signal transmission thus inevitable.
To the new receiving device its inventors have applied the name "responder," a term first suggested by the London Electrician as one suitable for any resistance device sensitive to electric radiations, regardless of any theory of the particular action involved. It would be well to state that the device is not an "anti-coherer," for although the action of the electric waves upon the new receiver is to cause a sudden increase in its resistance, similar to that observed with the arsenic or lead-peroxide "coherer," yet this effect is due to an entirely different action. Just what this action is and the mode of operation, as well as the full theory involved and the conditions governing have been the subject of careful and painstaking scientific research on the part of the inventors. Until pending patents are granted, however, they are not free to publish the full results of these investigations to the scientific world. The receiving device, in its case, is illustrated in Fig. 2.
The responder appears to be extremely sensitive in its action, and is instantly and automatically restored to its normal condition of high electrical resistance at the cessation of the Hertz impulse. This automatic restoring quality of the responder allows an ordinary telephone receiver to be used to the best advantage as the signal-receiving device. In this one hears, as it were, the sound of the spark at the sending instrument, for, however rapidly these sparks follow one another the responder restores itself to its original condition after each, and is thus able to respond to the succeeding impulse. Thus, by employing an induction coil and Wehnelt interrupter, with an adjustable platinum anode, for the sending instrument, a sort of crude musical scale or siren can be operated, and the same note is heard, ascending or descending, by one listening at the telephone connected in the responder circuit.
In telegraphing the receiving operator hears the dots and dashes of the Morse code in short or long "ripples," very much as the sending operator hears it in the sound of the induction coil's spark.
This automatic restoring quality of the new responder, therefore, allows a speed of signal transmission limited only by the ability of the receiving operator to read. If desired, an ordinary relay may be substituted for the telephone and the signals received on the Morse sounder. The telephone is preferred, however, on account of the greater simplicity of the receiving circuit thus made possible, comprising, as it then does, besides the responder, only choke coils, the necessary resistance, a cell of battery and the telephone.
The responder has been found to work best on a small current. Thus, with one ordinary dry cell several thousand ohms' resistance is included in circuit. The normal resistance of the device is about 50 ohms, increasing to a thousand or more under the influence of the electric impulse.
Operating on normally closed circuit, as this responder does, it is interesting to note that the conditions in the conducting path over which the Hertz waves oscillate are not exactly the same as when a coherer is substituted. Ordinarily the coherer, being of extremely high resistance, acts as a condenser to the electric oscillations, so that a loop of the electrostatic standing waves tends to form at its upper electrode. Thus when the coherer is placed, as is customary, at the foot of the upright wire, this antenna at first vibrates with a static loop at either end; that is, its length represents a one-half wave length. In other words, its natural period of vibration is just twice that holding after the coherer has been broken down, and a node formed at the earthed connection.
Using a closed-circuit responsive device at the foot of the antenna, however; this first harmonic of the fundamental vibration is not called into existence until the work of the received impulse is accomplished, and the responder completely opened.
Theory and experiment have alike shown that the action of the Hertzian oscillation on the coherer is largely influenced by the potentials produced across its terminals. Thus, with the same amount of energy, an oscillation having a steeper wave front will operate a receiving instrument, at a greater distance than another of more gradual rise, because, in the case of the first wave the cutting of the antenna by the lines of force is at a greater rate, and the potentials of the currents therein induced, and transmitted to the coherer, are correspondingly raised.
Starting out with this premise, Professor Freeman has sought to produce a sending apparatus calculated to deliver its energy in the most efficient form, and, by securing the most sudden possible breakdown of the insulating medium in the spark gap, produce a form of electric oscillation far more effective than any which can be generated by the devices ordinarily used at the sending station. How well he has succeeded in accomplishing this result the record of the tests given below shows.
From the very nature of the induction coil the rise of potential at the terminals of its secondary winding must be comparatively slow, while the high resistance of the winding causes a loss of energy, which may amount, especially in the larger coils now being introduced for space telegraphy, to a considerable percentage of the total amount delivered. In fact, it appears very questionable whether the induction coil can be made an efficient machine for the generation of these electric oscillations representing any considerable amount of energy.
It is well known among physicists that the potential and nature of a spark discharge depend not only on the width of the spark gap, but on the manner in which this potential is produced. Thus a sudden application at the terminals causes a breakdown at less potential, but of a more violent nature, than if the rise there were gradual. Losses from brush discharge are decidedly less, and inequalities on the surfaces of the spark terminals are of less importance. This seems to indicate that it may be possible to secure a much greater regularity in the spark discharge, each single spark being alike effective. Professor Freeman's sending instrument is said to be constructed so as to embody the above-mentioned advantageous features, and, in addition, it does away with the necessity for the usual circuit breaker, Wehnelt or hammer interrupter, with all the irregularities and losses involved in their operation. The illustration (Fig. 1) shows the sending apparatus with the double "toadstool" spark gap, a form which has been found especially effective for the work in hand.
The tests which the new system has thus far undergone, while not covering a great distance in miles, have been severe, and have demonstrated the possibilities of the new instruments. Several successful trials have been made with the sending station located at the Armour Institute, and the receiving instruments placed upon some city structure, such as the Lexington Hotel (a distance of 1½ miles) and on the roof of the Auditorium (a distance of, approximately, four miles). In every case the range lay over a district of tall buildings, steel structures, metal chimneys, elevated railroad, and densely threaded with telegraph wires and feeder cables. A range more unfavorable to the transmission of signals would be hard to find. From the mast of the steam yacht Pathfinder, using an antenna 50 feet in height, signals were transmitted perfectly to a similar antenna connected to the responder placed on the city's four-mile crib, in the lake, over a space of five miles. A small induction coil was used as the sender in this latter test.
In the before-mentioned trial from the Armour Institute to the tower of the Auditorium some rather startling results were obtained, showing the sensitiveness of the receiver, as well as the remarkable efficiency of the new sending device. With a 12-inch induction coil and Wehnelt interrupter, a spark gap of three-fourths inch was required to transmit the signals over this range, but when the new sending device was employed a spark gap of one-quarter this length, or but three-sixteenths inch, sufficed to give the signals perfectly at the receiver. That is to say, the form of wave generated by the sender appeared to be, roughly, 400 per cent. more effective than that delivered by the ordinary induction coil.
The feat of transmitting signals over this distance with three-sixteenths-inch spark gap would be remarkable, even over water, and when the nature of the ground traversed is taken into consideration, together with the fact that the flagstaff supporting the sending antenna was copper-sheathed and earth-connected, the inventors seem well justified in regarding the result as something of a triumph.
In this experiment the receiving wires were hung over the west side of the Auditorium tower, so that there was a clear field behind the wires. When, however, they were hung over the south side of the building, yet still facing Armour Institute, so that the great stone mass of the tower lay back of the wires, and but a few feet behind them, scarcely an impulse was recorded. The only plausible explanation of this fact seems to be that reflection of the waves by the building produced an interference in the wires with the impulses directly received.
Further perfections and developments of the new system are to be pushed by the inventors as rapidly as possible; for both the receiving and sending devices seem to afford many possibilities not in the scope of the systems heretofore employed.