A coherer consists essentially of a few metal filings between two metal plugs. The filings are made part of a circuit which includes a relay and a battery. Under ordinary conditions the filings lie loosely together and their resistance to an electric current is so great that they will not allow enough energy to flow to operate the relay. If, however, the two plugs are connected to the aerial and ground so that the currents of the incoming wireless signals also pass through the filings, they will suddenly "cohere" or stick together and permit sufficient current to pass to operate the relay.
[Illustration: FIG. 163.—The Complete Coherer.]
A "tapper," or as it is also sometimes called a "decoherer," is also arranged in the circuit so as to automatically shake the filings up and restore them to their loose state as soon as each signal is received. The decoherer consists simply of an ordinary bell mounted so that the hammer will strike the coherer and jar it.
The first thing required in order to make a coherer is a pair of double binding posts. These should be mounted on a wooden base six inches long and four inches wide as shown in Figure 163.
A piece of glass tubing about one and one-half inches long and one-eighth of an inch in diameter inside together with some brass rod which will just slip into the tube tightly will be required.
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.