chapter iv.)

The sending apparatus is practically the same in all outfits, and consists of a source of electrical energy, such as a battery, or dynamo, the essential induction-coil and adjustable spark-gap between the brass balls on terminal rods, and the make-and-break switch, or telegraph-key.

It is in the various forms of coherers and receiving apparatus that the different inventors claim superiority and originality. The systems also differ in their theory of harmonic tuning or vibratory sympathy. This is accomplished by means of coils and condensers, so that the messages sent out on one set of instruments will not be picked up or recorded by the receiving apparatus of competitors.

Having made or purchased an induction-coil of proper and adequate size, it will now be necessary to construct the parts so that an adjustable spark-gap may be secured.

Make a hollow wooden base for the induction-coil to rest on. It should be a trifle longer than the length of the coil and about seven inches wide. This may be made from wood half an inch thick. The base should be two inches high, so that it will be easy and convenient to make wire connections under it. Mount the induction-coil on the base and make it fast with screws, arranging it so that the binding-posts are on the side rather than at the top of the coil, as shown in Fig. 13.

Cut a thin board and mount it across the top of the induction-coil on two short blocks, and to this attach two double-pole binding-posts (P P). The fine wires from the induction-coil are made fast to the foot of each post, and from the posts the aerial wire (A W) and ground wire (G W) lead out.

Fasten two binding-posts at the forward corners of the base, and to them make connection-wires fast to the heavy or primary wires of the coil. Wires B and C lead out from these posts to the battery and key, and to complete this part of the sending, or transmitting apparatus it will be necessary to have two terminal rods and balls attached to the top of the binding-posts (P P). This part of the apparatus is generally called the oscillator, and the rods are balanced on the posts, so that they can be moved in order to increase or diminish the space (S G), or spark-gap, between the brass balls.

When, after experiment, the proper space has been determined, the set screw at the top of the posts will hold the terminal rods securely in place.

Obtain a piece of brass, copper, or German-silver rod three-sixteenths of an inch in diameter. Now cut two short rods, each six inches long, and two inches from one end flatten the rods with a hammer, as shown at A in Fig. 14. Flatten the rod in two places at the other end, as shown at B B in Fig. 14; then bore holes through the flattened parts (A), so that the binding-screws at the top of the posts (P P) will pass through them.

Obtain two brass balls from one to one inch and a half in diameter. If they are solid or cast brass they may be attached to the ends of the terminal rods by threading, so that it will be easy to remove them. If the balls are of spun sheet-metal it will be necessary to solder them fast to the ends of the rods, and, when polishing the balls, the rods will have to be removed from the binding-posts. It is imperative that the balls should be kept polished and in bright condition at all times, to facilitate the action of the impulsive sparks.

To counterbalance these balls there should be handles at the long ends of the rods. These handles may be of wood, or made of composition molded directly on the rods. A good composition that can be easily made and molded is composed of eight parts plaster of Paris and two parts of dextrin made into a thick paste with water. The dextrin may be purchased at a paint-store, and is the color of light-brown sugar. Mix the dry plaster and dextrin together, so that they are homogeneous; then add water to make the pasty mass. Use an old table-knife to apply the wet composition to the bars. The flattened parts will help to hold the mass in place until it sets. It is best to make two mixtures of the paste and put one on first, leaving it rough on the surface, so that the last coat will stick to it. When the last coat is nearly dry it may be rubbed smooth with the fingers and a little water, or allowed to dry hard, and then smoothed down with an old file and sand-paper.

If solid brass balls are used for the terminals the composition handles may be made heavier; but in any event the proper amount of composition should be used, so that when the rod is balanced on a nail or piece of wire passed through the hole it will not tip down at one end or the other, but will remain in a horizontal position.

The overhead part of the apparatus employed to collect the electric waves is called the antennæ, and in the various commercial forms of wireless apparatus this feature differs. The general principle, however, is the same, and in Figs. 15, 16, 17, and 18 some simple forms of construction are shown.

Great care must be taken to properly insulate the rod, wire, or fingers of these antennæ, so that the full force of the vibration is carried directly down to the coherer and sounder or receiver. For this purpose, porcelain, glass, or gutta-percha knobs must be employed.

In Fig. 15 the apparatus consists of an upright stick, a cross-stick, and a brace, or bracket, to hold them in proper place.

Porcelain knobs are made fast to the sticks with linen string or stout cotton line. Then an insulated copper wire is run through the holes in the knobs, and from the outer knob a rod of brass, copper, or German-silver, or even a piece of galvanized-iron lightning-rod, is suspended. Care should be taken to see that the joint between rod and wire is soldered so as to make perfect contact. Otherwise rust or corrosion will cause imperfect contact of metals, and interrupted vibrations would be the result. The upright stick should be ten or fifteen feet high, and may be attached to a house-top, a chimney, or on the corner of a barn roof.

Another form of single antenna is shown in Fig. 16. This is a rod held fast in a porcelain insulator with cement. The insulator, in turn, is slipped over the end of a staff, or pole, which is erected on a building top or out in the open, the same as a flag-pole. Near the foot of the rod, and just above the insulator, a conducting-wire is made fast and soldered. This is run down through porcelain insulators to the apparatus.

If the pole is erected on a house-top it may be braced with wires, to stay it, but care must be taken not to have these wires come into contact with the rod, or conducting-wire.

Another form of antennas is shown in Fig. 17, where rods are suspended from a wire which, in turn, is drawn taut between two insulators. The insulators are held in a framework composed of two uprights and a cross-piece of wood.

This frame may be nailed fast to a chimney and to the gable of a roof, as shown in the drawing; and to steady the rods, so that they will not swing in a high wind, the lower ends should be tied together with cotton string, the ends of which should be fastened to the uprights. The leading-in wire is made fast to the top wire, from which the rods are suspended, and all the exposed joints should be soldered to insure perfect contact and conductivity. A modified form of the Marconi antennæ is shown in Fig. 18. This is made of a metal hoop three of four feet in diameter held in shape by cross-sticks of wood, which can be lashed fast to the ring. Leading down from it are numerous copper wires which terminate in a single wire, the whole apparatus resembling a funnel. The upper unions where the wires join the ring need not be soldered, but at the bottom, where they all come together and join the leading-in wire, it is quite necessary that a good soldered joint be made. This funnel may be hung between two upright poles on a house-top, or suspended from the towers or chimneys.

Almost any metal plate will do for the ground, or the ground-wire (G W in Fig. 13) may be bound to a gas or water pipe which goes down deep in the ground, where it is moist. Rust or white lead in the joints of gas-mains sometimes prevent perfect contact, but in water-pipes the current will flow readily through either the metal or the water. To insure the most perfect results, it is best to have an independent ground composed of metal, and connected directly with the oscillator, or coherer, by an insulated copper wire. A simple and easily constructed ground is a sheet of metal, preferably copper, brass, or zinc, to the upper edge of which two wires are soldered, as shown in Fig. 19. This is embedded in the ground three or four feet below the surface. Another ground-plate is a sheet of metal bent in [V] shape and then inverted. Two wires are soldered to the angle, and the ends brought together and soldered. This ground is buried three or four feet deep, and stands in a vertical position, as shown at Fig. 20. At Fig. 21 a flat ground is shown. This is a sheet of metal cut with pointed ends. The ground-wire is soldered to the middle of it, and it is then buried deep enough to be embedded in moist earth.

One of the best grounds is an old broiler with a copper wire soldered to the ends of the handles, as shown at Fig. 22. This is buried deep in the ground in a vertical position, and the insulated copper wire is carried up to the instruments.

The most important part of the wireless telegraphic apparatus is now to be constructed, and this requires some care and patience. The coherer is the delicate, sensitive part of the apparatus on which hinges success or failure. There are various kinds of coherers designed and used by different inventors, but while the materials differ and the construction takes various forms, the same basic principle applies to all.

The coherer can best be explained as a short glass tube in which iron or other metallic filings are enclosed. Corks are placed in both ends of the tube, and through these corks the ends of wire are passed, so that they occupy the position shown in Fig. 23, the ends being separated a quarter of an inch. Metal filings will not conduct an electric current the same as a solid rod or bar of the same metal, but resist the passage of current.

After long periods of experimenting with various devices to detect the presence of feeble currents, or oscillations, in the ether, the coherer of metal filings was adopted. When the oscillations surge through the resonator, the pressure, or potential, finally breaks down the air film separating the little particles of metal, and then gently welds their sharp edges and corners together so as to form a conductor for the current. Before this process of cohesion takes place these fine particles offer a very high resistance to the electrical energy generated by a dry cell or battery--so much so that no current is permitted to pass. But once the oscillations in the ether cause them to cohere--presto! the resistance drops from thousands of ohms to hundreds, and the current from the dry cell now flows easily through the coherer and deflects the needle of a galvanometer. This is the common principle of all coherers of the granulated metal type, although there are many modifications of the idea.

The action of the electric and oscillatory currents on particles of metal can best be understood by placing some fine iron filings on a board, as shown at Fig. 24, and then inserting the aerial and ground wires in the filings, but separated by an eighth or a quarter of an inch. A temporary connection may be made as shown in Fig. 25.

A A are aerials on both instruments; C is the open coherer, or board with iron filings, in which the ends of the aerial and ground wires are embedded; D C is a dry cell; and R is a telegraphic relay, or sounder. If the wire across C was not parted and covered with filings, the dry cell would operate R, but the high resistance of the particles of metal holds back the current.

On the opposite side, I C is the induction-coil; K is the telegraphic key, or switch, which makes and breaks the current; S B is the storage-batteries, or source of electric energy; and S G the spark-gap between the brass balls on the terminal rods. By closing the circuit at K the current flows through the primary of the induction-coil, affects the secondary coil, and causes a spark to leap across the gap between the brass balls. This instantly sets the ether in motion from A on the right, and the impulse is picked up by A on the left. This oscillation breaks down the resistance of the filings at C, and the current from battery, or dry cell (D C), flows through the filings and operates the sounder, or relay (R). This operation takes place instantly, and the particles of metal are seen to cohere, or shift, so that better contact is established. But as soon as the spark has jumped across the gap the action of cohesion ceases until the key (K) is again operated to close the circuit and cause another spark to leap across the gap. The shifting of the metal particles on the board (C) is what takes place in the glass tube of the coherer, Fig. 23, but in this confined space the particles will not drop apart again as on the flat surface, but will continue to cohere. A de-coherer is necessary, therefore, to knock the particles apart, so that the next oscillatory impulse will have a strong and individual effect. There are several forms of de-coherers in use, but for the amateur telegrapher an electric-bell movement without the bell, or, in other words, a buzzer with a knocker on the armature, will answer every purpose. (See description of buzzer on page 64.) It must be properly mounted, so that on its back stroke, or rebound, the knocker will strike the glass tube and shake the particles of metal apart. For this purpose the vibrations of the armature should be so regulated as to obtain the greatest possible speed, in order that the dots and dashes (or short and long periods) will be accurately recorded through the coherer and made audible by the sounder or telephone receiver.

Another form of coherer is shown in Fig. 26. This is made of a small piece of glass tube, two rods that will accurately fit in the tube, some nickel filings, two binding-posts, and a base-block three inches and a half long. The two binding-posts are mounted on the block, and through the holes in the body of the posts the rods are slipped. They pass into the tube, and the blunt ends press the small mass of filings together, as shown in the drawing. By means of the binding-posts these coherer-rods may be held in place and the proper pressure against the filings adjusted; then maintained by the set-screws. The nickel filings may be procured by filing the edge of a five-cent piece. Obtain a few filings from the edge of a dime and add them to the nickel, so that the mixture will be in the proportion of one part silver to nine parts nickel. This mixture will be found to work better than the iron filings alone. The aerial and ground wires are made fast to the foot-screws of the binding-posts, and the base on which the coherer is mounted may be attached to a table or ledge on which the other parts of the receiving and recording apparatus are also installed.

Another form of coherer is shown at Fig. 27. This is constructed in a somewhat similar manner to the one just described. A glass tube is provided with two corks having holes in them to receive the coherer-rods. Two plugs of silver are arranged to accurately fit within the tube, and into these the ends of the coherer-rods are screwed or soldered. Between these silver plugs, or terminals, the filings of nickel and silver are placed, and the rods are pushed together and caught in the binding-posts. The aerial and ground wires are made fast to the foot-screws of the posts.

For long-distance communication it is necessary to have a condenser placed in series with the sparking or sending-out apparatus. (See the type of condenser described and illustrated in chapter iv., page 72.)

An astatic galvanometer is also a valuable part of the receiving apparatus, and the one described on page 111 will show clearly the presence of oscillatory currents by the rapid and sensitive deflections of the needle.

For local service, where a moderately powerful battery is employed, a telegraph-key, such as described on page 190, will answer very well, but for high-tension work, where a powerful storage-battery or small dynamo is employed, it will be necessary to have a non-sparking key, so that the direct current will not form an arc between the terminals of a key. Most of the keys used for wireless telegraphy have high insulated pressure-knobs, or the make and break is done in oil, so that the spark or arc cannot jump or be formed between the points.

The plan of a simple non-sparking dry switch is shown at Fig. 28. This is built up on a block three inches wide and five inches long. It consists of a bar (A), two spring interrupters (B and C), a spring (D), and the binding-posts (E E). They are arranged as shown in Fig. 28, and a front elevation is given in Fig. 29. The strip (B) lies flat on the block, and is connected with one binding-post by a wire attached under one screw-head and run along the under side of the base in a groove to the foot of the post. Strip C is of spring-brass, and is made fast to the base with screws. This is “dead,” as no current passes through it, and its only use is to interrupt. The bar (A) is arranged as explained for the line telegraph-key, and the remaining binding-post is connected to it by a wire run under the base and brought up to one of the angle-pieces forming the hinge. A high wood or porcelain knob is made fast at the forward end of the bar, so that when high-tension current is employed the spark will not jump from the bar to the operator’s hand. The complete key ready for operation is shown at Fig. 30, and to make it permanent it should be screwed fast to the table, or cabinet, on which the coil and condenser rest. The plan of a “wet” key is shown in Fig. 31, and the complete key in Fig. 32.

A base of wood three by five inches is made and given several coats of shellac. Obtain a small rubber or composition pill or salve box, and make it fast to the front end of the base with an oval-headed brass screw driven down through the centre of the box. A wire leading to one binding-post is arranged to come into contact with the screw, and the other post is connected by wire to one hinge-plate supporting the bar. The long machine screw, or rivet, passed down through the knob and into the bar, extends down below the bar for half an inch or more, so that when the knob is pressed down the end of the screw, or rivet, will strike the top of the screw at the bottom of the box without the bar coming in contact with the edge of the box. When in operation the composition box is filled with olive oil or thin machinery oil, so that when contact is made by pressing the knob down the circuit will be instantly broken, the spring at the rear end of the bar drawing it back to rest. The oil prevents any sparks jumping across; and also breaks an arc, should one form between the contact-points. With the addition of a good storage-battery (the strength of which must be governed by the size of the induction-coil and the distance the messages are sent) and a dry-cell or two for the receiving apparatus, the parts of the wireless apparatus are now ready for assembling. Full directions for making storage-cells is given in chapter ii., page 21, and for dry-cells in