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.
When wireless signals pass through the relay they will cause the resistance to drop and allow sufficient current from the dry cell to pass through the coherer to energize the magnets of the relay and draw the armature in, thus closing the decoherer circuit. When the armature of the relay is drawn in towards the magnets the relay contacts are brought together. This closes the decoherer circuit and starts the hammer to tapping the coherer, thus restoring the filings to their former loose state. The decoherer will tap the coherer as long as the incoming signals continue. By breaking the signals up into dots and dashes of the telegraph code a message may be easily transmitted and read from the tapping of the decoherer.
[Illustration: FIG. 166.—Coherer, Decoherer and Relay Connections.]