It is possible to arrange a lamp and two switches so that it can be turned on or off at either switch independently of the other. This is a very convenient method of installing a light on a stairway or in a hall. One switch may be placed at the top of the stairs and the other at the bottom. A person going either up or down stairs can light the lamps ahead and turn it out as he passes the last switch regardless of its position at the other end. The switches must be of the type called "two point" or "double throw single pole" and the levers must always rest on one of the contacts and never be left in between. [Illustration: FIG. 161.—Double Control System.] CHAPTER XVI. COHERER OUTFITS FOR WIRELESS TELEGRAPHY. Making a Coherer. How to Adjust the Instruments. MANY experimenters desire some device by which wireless signals may be made clearly audible without the necessity of holding a telephone receiver to the ear. It is not possible for the amateur to make such an arrangement which will receive signals over long distances. It is, however, possible to arrange a "coherer" so that signals coming from a nearby station may be indicated by the ringing of a bell. [Illustration: FIG. 162.—The Coherer Details.] Such a "coherer" outfit will receive messages coming within a radius of one mile and may also be used for demonstrating the principles of wireless telegraphy when both the receiving and transmitting station are located in the same room or building. In order to secure these results it will be necessary to substitute a device called a "coherer" for the detector in the receiving apparatus. 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.