The author of this book, H. LaVerne Twining, was the Head of Physics and Electrical Engineering at the Los Angeles, California Polytechnic High School.
 
Wireless Telegraphy and High Frequency Electricity, H. LaVerne Twining, 1909, pages 105-116:

CHAPTER  VII.

AMATEUR  STATIONS  AND  SELECTIVE  TUNING.

    Many boys and amateurs are establishing wireless telegraph stations in every section of the United States. These stations are a source of never-ending delight to the boy who has scientific tastes. Plate X
    He finds here a means within his reach, by means of which he can study electricity from both a theoretical and a practical standpoint. The mere theoretical study of scientific subjects is unsatisfactory. One reads of course of the things that others have done, with great interest; but the subject remains a mystery, unless one can become familiar with it by actual contact.
    Wireless telegraphy furnishes a means for that contact, and that is why it is becoming so popular with many people. It furnishes an avenue along which a boy may expend his leisure time, much to his profit and enlightenment.
    Los Angeles has its share of boys whose minds turn toward the natural phenomena of nature. Electricity is fascinating to all boys, and wireless telegraphy gives them the opportunity to become familiar with alternating current phenomena at a small cost. This is of immense practical value to them, because the phenomena of wireless telegraphy is the phenomena of the commercial alternating current, and the knowledge they obtain here will be of great value to them in the electrical field of the commercial world.
    When so many stations are operating, interference becomes serious; and this problem of interference must be solved before wireless telegraphy or telephony can possibly become a commercial success.
    If the wireless companies now in the field were doing a large business, it would be necessary for each company to occupy the field all of the time, night and day. Under present conditions, only one sending station can operate at a time. When one station is sending, all stations far and near are affected by the waves sent out by that station, and, if the station is a strong one, no other station can receive any one except the sending station.
    If five stations are sending in the same region, then every station that is not sending hears in his telephone a confused buzz, the result of the mixture of the waves of all of the sending stations. If one of the sending stations has a tone to its buzz radically different from all of the others, the skillful operator can pick out that particular one and read it.
    If some of the sending stations are weaker than others, owing to distance or to low power, then the strongest signal can be read. If a dozen people are talking all at once in a room, one can pay attention to one of them and understand what he is saying, even though the others are talking.
    The solution of interference is not to be found by driving the amateurs out of the field. It would be very unfortunate, indeed, if our Congress were to take action giving a monopoly of the space above and about us to any corporation or set of corporations. The development of aerial navigation and wireless telegraphy demands that the air, the same as the surface of the ocean, be kept a public highway. Plate XI
    Selective tuning offers a complete solution of this problem.
    For selective tuning, undamped continuous waves are necessary. Condenser discharges at the frequencies now in use give a train of damped oscillations as shown in Fig. 79. No selective tuning can be effected on a wave of this nature.
    Continuous undamped waves, like those in Fig. 60, are necessary. These undamped waves would give an effect like that in Fig. 84. If these waves could be rectified as in Fig. 69, a still better effect might be produced.
    It will be observed that in Fig. 79 there is considerable distance between the maximum value of each discharge. This leaves a blank between the discharges. The higher the frequency, the smaller this blank.
    Referring to Fig. 84, it will be seen that there is a blank interval between the maximum value of the continuously sustained oscillations. If these oscillations could be rectified, a greater effect would be produced.
    The higher the frequency, the closer the maximum values. This high frequency produces a short wave length. Fessenden has solved the problem of selective tuning by the use of a high frequency alternator. Poulsen and The Collins Wireless Telegraph Company have solved the problem by use of the direct current arc.
    I do not maintain that the amateur should be allowed to obstruct the growth of wireless telegraphy. He is not doing so. He is in fact advancing its interests. If no amateurs were in the field, the problem would be just as serious, because only one station of only one company could work in the same region at a time, and this fact alone would render wireless telegraphy useless commercially.
    The amateur is then only bringing to the front, more forcibly, the necessity for selective tuning. If selective tuning is impossible, the only remedy is government ownership of wireless, not for commercial use, but for the uses to which it is now putting it.
    Wireless telegraphy and telephony have no commercial use unless selective tuning is possible. Theoretically, selective tuning is possible, and there are two commercial companies in the field today in the United States, who claim to have solved it.
    The Collins Wireless Telephone Company claims to have solved selective tuning to such an extent that they can work beside the most powerful wireless telegraph stations, absolutely without interference.
    The company that owns the Fessenden patents claims to have accomplished the same thing for wireless telegraphy. The time is close at hand when these companies will demonstrate what they can do in this line. Selective tuning thus solves the difficulty without driving any one off the public domain.
    Tuning is possible with the circuits given in this book. If stations differ radically in their wave length, they can be received on different parts of the closed oscillation circuit, and in this case, when one is coming in loud, the other is coming in weak.
    Fig. 59 is an excellent circuit for this purpose. Mr. Farran tried this circuit for the first time, some few days ago, and found it to be excellent, not only for tuning out near-by stations with short wave lengths, but also for bringing in distant long wave stations strongly. We had been using the Shoemaker connections shown in Fig. 32 with excellent results, but it is not very selective. Fig. 59
    In the connections shown in Fig. 59, the closed oscillation circuit AOCP is joined to the looped aerial EIB by a movable contact E and a fixed point B. B is attached at the junction of the condenser and the inductance. D is the detector around which the telephone should be shunted. F is a movable contact for the ground. The condenser C is adjustable.
    When E and F are in the middle of the tuning coil, all short waves ground and become very weak or silent entirely. All long waves set up oscillations in the closed circuit. With this arrangement we were able to tune out the boys in near-by stations and read TM, the government station at Point Loma, Cal., 100 miles away, or PI, the United Wireless station at Catalina, fifty miles away.
    The station on the Polytechnic High School was established in the fall of 1908. It is stretched horizontally, between the science hall on 20th St., and the main building on Washington St. It is about 35 feet from the ground at one end and 50 feet at the other, being 20 feet above the roof of the building part of the way, and 30 feet the rest of the way. It points north and south. The aerial is composed of four strands of No. 12 aluminum bare wire, 200 feet long. They are joined together at the Washington St. end and are brought, side by side, about 2 feet apart, to a pole on top of the science hall. Here two leads are brought down into the office through a skylight. One lead comes from two of the wires on one side, and the other lead comes from the two wires on the other side. Plate XII
    The 20th St. end is shown in Plate XI, and the interior is shown in Plate X. The aerial thus has 800 feet of wire in parallel. There are two leads 60 feet long. This makes 720 feet of wire in the aerial. The aerial can thus be used as a looped one or otherwise, as desired.
    The receiving instruments are similar to those described in this book, and they are connected as shown in Fig. 59 or Fig. 32. The detectors used are silicon, iron pyrite or perikon. The best work has been done with the pyrite. The perikon is very sensitive, but not as reliable as the pyrite. The Collins 2,000-ohm receivers are used with no potentiometer or battery. A 75-ohm Bell telephone is also used that is very sensitive.
    In the sending, a 1-kilowatt transformer is used, giving 20,000 volts on the high side. The Massie connections are used for a hook-up. With this outfit, Mr. Farran has been able to send as far south as San Diego and out to sea far north of Santa Barbara, covering 100 miles south and at least 180 miles north. This was done on 2½ amperes in the primary of the transformer. The current was cut down with a water rheostat and only a 3/16 inch spark was used.
    The ships, communicated with, read us with ease and said that the station came in strong and clear. This means, of course, that the station was reaching much farther, but we have not been able to test to farther distances.
    In receiving we have been able to hear the U. S. warships in Magdalena Bay, 725 miles in an air line south, and Table Bluff, 560 miles north.
    We have thus been able to work with ships at sea both receiving and sending, to a distance of at least 180 miles, using in sending only 2½ amperes. The aerial tunes on this many amperes. We are able to get the Farallones and the San Francisco stations at night, 368 miles north in an air line. Magdalena Bay was received in the daytime, but Table Bluff at night.
    Mr. Roy Zoll was the first boy to install a sending and receiving outfit in the city, so far as I know. Mr. J. T. LaDu and myself had installed small sending outfits some time previously.
    Prior to this time there were some receiving stations, but none equipped with sending instruments. We used power transformers from the first.
    Mr. Zoll's first aerial consisted of two baskets, 27 feet long, strung up on a 75-foot pole. Each basket had four No. 18 copper wires in it, arranged around circular hoops 1 foot in diameter. The two baskets were 2½ feet apart. With this vertical aerial and a carborundum detector, he was able to hear the fleet of sixteen battleships on their trip up from Magdelena Bay in April, 1908, long before they reached San Diego. Plate XIII
    Mr. Zoll's station is located on the top of a hill. His pole is made of a eucalyptus tree, 75 feet high. He used ordinary Bell telephone receivers. Point Loma and the United Wireless station in San Diego, the San Francisco stations and the Farallones were all picked up by him before any of the rest of us heard them. Some time later he added to these baskets two loops each, containing three wires 70 feet long. These branches were joined together at the top, and they spread apart 64 feet at the bottom. With this aerial his range was considerably increased. He was able to receive the following stations, besides those already heard: SV, Tatoosh Island, Washington; and SP, Navy Yard, Puget Sound, 1,000 miles north.
    Later he stretched three wires 160 feet horizontally from the top of his pole to another pole, and stretched two radiating wires from this, one 140 feet long and the other 180 feet long, making in all 1,320 feet of wire.
    After the first change iron sulphide was used as a detector. With this aerial he heard the West Virginia at Magdalena Bay, 725 miles south.
    Mr. A. E. Abrams was among the first to install a receiving and transformer sending outfit. His pole is 71 feet high. It is on the top of his house. The aerial consists of four wires 75 feet long, of No. 18 bare copper wire. The spreaders are 10½ feet long.
    His transformer is a 350-watt. The primary consists of four layers containing 166 turns, tapped at five points. His secondary has 75,000 turns of No. 36 D.S.C. The coils are 1/8 inch thick with 1/8 inch space between them. The iron core is 16½ inches by 5½ inches, with a cross section of 2½ square inches.
    In receiving he has heard as far north as Cape Blanco and south of San Diego.
    Mr. D. Whiting of 627 St. Paul St., located on the hills, has a pole 120 feet high. The aerial is 140 feet long, containing six wires, arranged in a loop system. The leads are 60 feet long. He uses a 2-kilowatt transformer of 40,000 volts. The iron in the transformer is ordinary sheet iron. DeForest connections are used in the sending.
    His receiving sets are connected according to the Shoemaker and Massie systems. He has a pair of Collins Wireless telephones and uses silicon, iron pyrites, perikon and electrolytic detectors.
    He has been able to receive as far north as Tatoosh, 1,000 miles, and as far south as Magdalena Bay, 725 miles.
    The Southern Pacific Telegraph School, 542 Central Ave., has a wireless station. The aerial is a horizontal one, 65 feet high at one end and 45 on the other. It is composed of 10 strands of No. 12 copper wire. They are all connected together at the upper end and brought in to the instruments at the other end by one lead. They have a 1-kilowatt transformer and apparatus made by a Los Angeles boy. Plate XII is a view of their sending and receiving outfits. This school is owned and managed by Mr. F. D. Mackay. Facilities are presented here for a training in railroad telegraphy, commercial telegraphy and wireless telegraphy.
    Plate XIII is a photograph of the author's station. The house is 35 feet high and the pole is 40 feet high, thus giving 75 feet above the ground. Aerials of various kinds have been tried. The one now in operation is as it appears in the cut, with the exception of a small wire which connects all of the guy wires together. This wire was too small to photograph. There are in all twelve guy wires, although only ten can be seen in the cut. Two of them are so nearly in line with the pole as to be invisible.
    Three of them are 85 feet long, four of them 35 feet and five of them 50 feet long. This gives a total length of 645 feet. They are all connected together at the base and a lead is brought in to the instruments. They are all thoroughly insulated from the pole, the house and the ground and are unconnected at the top. Provision is made also for swinging up any kind of an aerial besides. The lead wire goes in at the rear of the building.
    The guy wires themselves, however, form such a good aerial as to make another unnecessary. With these guy wires and the use of 2½ amperes, we have been able to send to Catalina, 50 miles away, and San Diego, 100 miles away. In receiving we have heard all of the San Francisco stations at night, 365 miles away. We have also heard the warships in San Francisco Bay.
    We have used a 166-watt transformer and a kilowatt transformer at this station. We used a Collins 2,000-ohm telephone and a very sensitive Bell telephone receiver of 75 ohms resistance. The detectors used were carborundum, silicon and iron pyrites. Our best work was done with iron pyrites.