Cut two pieces of the brass rod one and three-quarters inches long and face off the ends perfectly square and smooth. Slip these rods through the upper holes in the binding posts and into the glass tube as shown in Figure 163. Be sure that everything lines up perfectly and then remove one of the plugs. Before putting the plug back in place put some nickel and silver filings in the tube so that when the rods are pushed almost together, with only a distance of three-sixty-fourths to one-sixteenth of an inch between, the filings will about half fill the space. [Illustration: FIG. 164.—Pony Type Relay.] The filings are a mixture of silver and nickel and must be very carefully prepared. The mixture should be nine-tenths nickel and one-tenth silver. The filings may be secured by holding a five cent piece over a clean sheet of paper and filing it with a coarse file. The silver filings are made in the same way by using a ten cent piece or a quarter in place of the nickel. Do not use the fine dust and powder which is formed, but only the coarse filings. The success of the coherer will depend upon its adjustment and it will probably be necessary for the beginner to experiment considerably to find out just the right amount of filings to place in the tube and how far apart the brass plugs should be placed. The decoherer is made by removing the gong from an old electric bell and mounting the bell on the base in the position shown in Figure 163. The hammer should be bent so that it will tap the coherer lightly when a battery is connected to the bell binding posts. The only thing required to complete the apparatus is a relay. A relay of the "Pony" type such as that shown in Figure 164 will serve for ordinary work. It should be wound to at least 150 ohms, for to a certain extent, the higher the resistance, the more sensitive it will be. When it is desired to secure the maximum possible distance with the coherer a "polarized" relay will be necessary. A polarized relay is much more sensitive than the ordinary sort. The polarized relay should be wound to 1000 ohms. [Illustration: FIG. 165.—Connections for the Receiving Set.] Figure 165 illustrates how the coherer, decoherer and relay should be connected. Two wires connected to the coherer posts lead to the magnets of the relay in series with a single dry cell. The contact posits of the relay are connected to the decoherer in series with another dry cell.