Although Thomas Clark's radiotelegraph system used standard spark transmitters, based on the description in this article his "wireless telephone" appears to have used induction rather than radio signals, and was likely similar to what had been developed earlier by Amos Dolbear, Nathan Stubblefield and A. Frederick Collins. However, because of their limited transmission ranges, none of the wireless telephone induction systems ever proved to be commercially practical.
Detroit News Tribune, December 23, 1906, pages 1-2:




Sounds  Are  Carried  Through  the  Earth  and  Heard  With  Vivid  Distinctness.

    Wireless telephony is an accomplished fact and Thomas E. Clark, of Detroit, has achieved a notable success in his experimental work. Mr. Clark has long been recognized as one of the successful producers in wireless telegraphy who have brought it into practical commercial use.
    The public service company of which he is the main prop is doing a commercial business of gradually expanding proportions. A few days ago the country was startled, and Mr. Clark no less than others, by a letter from the government station at Key West, Fla., stating that one of the messages sent out from the Detroit station at Cass and State streets had been received there. The message was repeated, in proof, and the letter was addressed to correspond to the signature of the message, the sender being unknown to the receiver. As the Clark company has attempted nothing so ambitious but has limited its scope to the field embracing the district including the cities of Detroit, Cleveland and Buffalo, this far reaching message was a general surprise.

Faraday's  Deductions.

    But, wireless telegraphy, wonderful as it is, suggests other startling possibilities while it furnishes practical service. Wireless telephony is a natural corollary and experiment along that line has demonstrated that perfect speaking communication is possible over unknown distances without the use of metallic conductors. Present achievement and future elaboration are, and must be, somewhat like history of a coral reef. Coral is built up by myriads of tiny insects, which have the faculty of absorbing lime from seawater and depositing it upon the surface to which they are attached. Each in turn pertifies himself and dies, forming a new base upon which later generations may build until the reef reaches the surface of the ocean. A generation back, James Clerk Maxwell made certain philosophical deductions from the experiments of Michael Faraday. The basis upon, which all wireless experimenters are working to-day, traces back to this Scotch-Irish origin. Whereas our famous Dr. Franklin assumed that electricity was a mysterious, invisible fluid which could be generated under certain conditions, Maxwell decided that the phenomenon of electricity were due to a disturbance of a universal medium which pervades the atmosphere above us, the earth beneath our feet and all space, to some degree. This medium, known as the ether, pervades everything, according to the theory, yet it is susceptible to either gentle or violent agitation.

The  Laws  of  Hydrostatics.

    The laws of hydrostatics teach that pressure exerted upon any portion of a confined mass of water exerted by which a man or a machine exerting a pressure of 100 lbs. upon a single water in a strong receptacle is able to exert a power of many tons by communicating that pressure to a large mass.
    Heinrich Hertz, pupil of Helmholtz, satisfied himself that Maxwell's theory was correct and proceeded to convincing demonstrations. He found that sparks produced by an electrical machine could be received by a circular loop or spiral of wire, and from this he was able to discover the reflection, diffraction and polarization of the waves. The phenomena of electricity appear to be nothing more than vibrations of the universal medium, differing from light and heat only in wave length and frequency. Hertz paved the way for the useful adaptation of these waves to transmission.
     A stone thrown into the ocean produces not only surface ripples, which spread in all directions, but causes similar displacements throughout the entire breadth and depth and affecting in some manner, in the course of time, every atom in the mass. An electric spark produces a similar disturbance in the ether, which is intimately vast in extent as compared even with the ocean. It would require a receiving instrument of incalculable sensitivity to pick up the disturbance created at the port of Cleveland by a pebble dropped of the dock at the foot of Woodward avenue, but the ether, being a more sensitive and more diffusible body, responds quite obediently, and communicates its vibrations throughout this area, so that they can be used to excite receiving instruments of the wireless telegraph, which respond, impulse for impulse, in accordance with the Morse or any other signalling code.

How  Waves  Are  Imparted.

    To impart these waves or impulses to the atmosphere elevated poles or conductors called antennae are made use of. From the tip of a tall tower or mast ashore impulses can be sent to the receiving spar of a ship out of sight on the lakes or the ocean, but this sort of impulse will not carry the human voice or any other continuous sound because a sustained vibration is required to transmit all the waves. For wireless telegraphy each impulse is like a quick stroke on a piano key. The moment the finger is withdrawn the mechanical damper closes against the piano wire, hushing its vibration and preventing possible discord with the note immediately following. As Mr. Clark graphically expresses it: "Wireless telegraph is transmitted with the same effect as the picking of a banjo string while wireless telephony is only possible through a sustained vibration like that of a long-drawn bow upon the string of a violin." The snappy spark will not do.
    That being the case, he makes use of the earth instead of the upper air for his medium and by means of his sending instrument or transmitter subjects it to a continuous flow of varying intensity. The transmitting instrument consists of a special Clark transmitter containing an inductance coil, condenser, a transmitting induction coil, primary and secondary, and the necessary condenser capacity for the production of high tension currents. The secondary circuit leads to the earth and causes a displacement to take place in accordance with the well-known laws of dialectic strain. The primary battery is almost insignificant, consisting of two ordinary dry cells, as far as experiment has proceeded. All this combination is a separate part of the system. It has no connection with the receiving instrument except that each makes contact with the earth at its own station.

The  Receiving  Instrument.

    The receiving instrument consists of a Clark special type double headgear microphone, which holds an instrument resembling a telephone receiver to both ears of the person listening. It is connected with a receiving device which is capable of taking up the vibrations imparted to the earth mass and delivering them to the ear in a perfect reproduction of the exciting vibrations imparted by the voice of the person speaking. As yet Mr. Clark has tried his instrument with only two dry cell batteries and with apparatus designed for short distances. On Saturday morning he gave a demonstration in his office to representatives of The News which were marvelous in the perfection of their transmission. The sound was carried without loss of volume, change of pitch and entirely devoid of the sonorous defects which are characteristic in nearly all telephones of the ordinary type using metallic circuit. There was no blurring or feathering of tone, no accompanying crackling or other acoustic complication apparent. The instrument seemed to select, out of all the myriad waves that are constantly imparted to the mass of the earth, only those of the transmitting instrument, to which it was correctly attuned by adjustment to the intervening distance and the power of the sending battery.

Merely  a  Stepping-Stone.

    This achievement, wonderful as it is, is but the stepping stone to a vast amount of research and experiment that must come, before the wireless telephone is adapted to general use. Difficulties are numerous. A wire of given metal, diameter and length, has a certain resistance with very little variation. In wireless experiment it is found that the earth and air, or the ether which permeates everything is exceedingly variable in carrying quality, according to the temperature and conditions which are not yet discovered. The barometer, thermometer, hygroscope, etc. do not tell all that is going on among the elements, and it is not yet known why wireless signals will go one day half way around the earth with apparent ease, and next day struggle painfully across a gap of perhaps 500 miles, although impelled by the same amount of power. The art of adjusting instruments to the changing condition must be discovered.
    Of equal nicety is the tuning of instruments into responsiveness. A glass tumbler on the table will ring out when a certain key of a piano is struck or note of a violin is sounded, because the two are attuned in harmony and the waves of vibration created in one set up waves of similar length in the other. Sound vibrations that are audible to the ear, range from 16 per second for the lower notes to 86,500 per second for the notes of the highest pitch. The ordinary ear is incapable of hearing vibrations outside this variation, although actual sound must have a much wider range. Musical notes range from 16 to 4,224 vibrations. Sound is simple as compared with other phenomena, for the vibrations of heat begin at 134 trillions per second, and those of light at 483 trillions. Electricity has its nearest analogy to light. Its speed of travel is almost identical according to some authorities.
    In order to perfect the wireless telephone so that a user in Detroit can call up a particular phone out of perhaps hundreds in a distant city, the sending instrument or transmitter must be given the right degree of power to overcome both the distance and the varying accidental conditions which affect transmission, and must be attuned to set up vibrations in the one particular phone desired, that will call the attention of the owner and afterward enable him to hold converse over it. This must be regarded as a large contract, but Mr. Clark has secured enough of a start in both voice transmission and in the art of attuning to make the achievement seem practically in his hand.
Thomas E. Clark