Electrical World and Engineer, July 11, 1905, pages 100-101:
The Murgas System of Wireless Telegraphy.
The Murgas wireless system which has been successfully tried in the vicinity of Wilkesbarre, has been confined entirely to land operations. Many of the mountain ranges rise up 1,500 ft. higher than the hill on which the Wilkesbarre station stands, and between Wilkesbarre and Scranton, where the second station has been erected, there are several peaks of this height in the range which runs at an angle across the path of the waves, but so far the contour of the land seems to have had little effect in preventing the proper transmission of signals.
The only troublesome element is the presence of inductive disturbances. The line, as it may be called for convenience sake, is situated in a particularly awkward position as regards electric railways. There are three short roads which lie about one-quarter of the distance, two lie at least half, and one, the famous "cannon ball," of the "Laurel line," and a third-rail system, not only parallels the whole from station to station, but at the Scranton end makes an entire circle around the building. Measurements taken with a ballistic galvanometer on the antennæ at all hours at Wilkesbarre show the inductive disturbance to be very great, at times the spot of light from the mirror of the galvanometer traveling by a series of jerks entirely off the scale. On other occasions when the antenna was brought into proximity to the earth there could be obtained in the air-gap sparks ¼ to ½ in. in length, and at very regular intervals, the period averaging from above five to one per second.
Such a problem as this was no easy one to solve, and Rev. Father Murgas, the inventor of the system, when he found this state of affairs to exist, after having gone to the expense of building both stations and thoroughly equipping them, taking years of planning and months to erect, learned that his work was by no means near completion. After several weeks of careful and minute investigation he at last hit upon the proper syntonization, and messages passed freely back and forth with no disturbance from external influences whatever. In arriving at this point of perfection, Father Murgas found it necessary to reconstruct the transformers, readjust the condensers and choke coils several times. Calculation could not often be depended upon, so that it was a matter of continuous experimentation to arrive at the proper tuning.
The fundamental features of the system involve the use of separate tones or tone impulses by the use of two interrupters working on the same induction coil. Such impulses being received in an ordinary telephone receiver by means of proper apparatus, tones differing in pitch will be produced, and while the number of tones which may be produced is indefinite and a variety of codes may be used, two tones are sufficient to produce a system analogous to the Morse, one tone corresponding to the dot and the other to the dash. For reasons as already stated the rapidity of transmission will be greatly increased by such a system, and, moreover, signals differing in character are more readily distinguished from each other than those which depend upon time intervals for differentiation. Tones are well distinguished in physics from other sounds, and the range of frequencies of vibration within which tones are produced is sharply defined. They are more readily distinguished than other sounds, and hence in systems employing a telephone receiver it is preferable to make the frequencies of the impulses come within the range to produce a tone. The interrupters above mentioned are running constantly and are connected through a double key similar to that used in the syphon recorder system, except that the keys are quite distinct from each other, and each operates one interrupter. The signals will, therefore, be transmitted to some extent as in long-distance cable telegraphy, but will be received by telephone or by an apparatus similar to the old needle with the differently toned bells, used by the British Post Office some twenty-five years ago.
The power for transmission is supplied at twenty-four volts and three amperes on the primary circuit, and the spark from the great induction coil on open circuit is about six inches when operating to Scranton. With this small current the common telegraph key is found to be large enough. A view of the transmitting apparatus is given in Fig. 1.
The coherer used with the telephone is of the imperfect contact type. It consists of a needle, making about one revolution in two hours, on which rests on their edges several small carbon pieces cut from a thin disc or diaphragm like those used in a telephone transmitter. It is perfectly self-restoring and increases the tones about ten times. Fig. 2 shows the receiving apparatus.
The towers are 150 ft. high, with an additional mast arm, making a total height of 200 ft. They are substantially built of good pine, with elbow and connecting plates of iron, as may be seen from the illustration, and there are no guy wires except those used between the top of the towers and the top of the mast arms, and these are of galvanized iron broken in several places by strain insulators. (See Fig. 3.)
The antenna is composed of 10 cables made of No. 10 B. & S. stranded, well-insulated copper wires. Each cable hangs from a separate insulator of particularly well-insulated construction. Each insulator is a rubber rod 30 in. long and 1.5 in. thick, with outer petticoat tubes of polished hard rubber. The insulation of the antenna cables is practically perfect, hanging as they do from a cross rope connected to the top of the mast arms. At a height of 30 ft. from the ground all of the cables are fastened to another cross rope attached at each end to a 150,000-volt insulator, and from that point they are gathered in a bunch and brought into the operating room through a hole in the center of a square plate glass window.
The earth connection, which measures 4 ft. on each side, goes through a smaller window, and is made of a flat strip of copper attached to the water pipes of the regular city water supply.
The tuning device is similar to the Slaby-Arco system, but with a peculiar arrangement of the auto-transformer by which the tuning can be adjusted very readily.
Signals have been transmitted recently with great regularity and outside interference seems to have been entirely overcome. The earlier experiments, up to the point where something really definite was arrived at were carried on entirely at the inventor's own expense. This covered a period of several years and was a heavy drain on his own private resources. Fortunately, at this point he was able to obtain the aid of the Electric Signal Company, of Philadelphia, and through the president, Mr. Joseph F. Stokes, he has been placed in a position to carry out all experiments on any scale desirable. The Wilkesbarre station is built of concrete and is very roomy and comfortable, with quarters for an operating staff. The station at Scranton is also completed.