Harper's Electricity Book for Boys

by Joseph H. Adams

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.

TYPES OF GROUNDS

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].