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. [Illustration: FIG. 19 FIG. 20 FIG. 21 FIG. 22 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.