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. [Illustration: FIG. 23] [Illustration: FIG. 26] [Illustration: FIG. 27] [Illustration: FIG. 24] [Illustration: 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.