The detector, although the most important of the instruments, is perhaps the simplest. It is constructed of a hardwood base with a small brass plate fastened on by means of a binding post. On the other end of the base is fastened a double binding post which holds a brass spring, as in the drawing. On the end of this spring is fastened a copper point made by winding a few inches of No. 36 or 40 wire on it and allowing about three sixteenths of an inch to project. This completes the detector but, for use in this instrument, lead sulphide or Galena crystals must be secured.
The condenser is made of two pieces of fin-foil, four by ten, and three pieces of waxed paper a little larger than the foil. A piece of wire is twisted into the end of each piece of foil, and then one sheet of foil is laid on a sheet of paper. This is then covered by another sheet of paper upon which is laid the second sheet of foil. On top of this is laid the third sheet of paper and the whole is folded into a convenient bundle. The sheets of foil must be well insulated from each other and the wires must project from the condenser.
The ground connection is made by soldering a wire to a cold-water pipe. In the case of a portable set the ground may be made by driving a metal rod into the ground or sinking metal netting into a body of water.
The telephone receivers cannot well be made and must therefore be bought. The type of 'phones used will therefore depend entirely on the builder's purse.
The Sending Set
The same aerial and ground are used for sending as were used for receiving, and for the experimenter, it will be far cheaper to buy a spark coil for his sending set than to attempt to make one.
For a field set there will be very little need of a sending helix, as close tuning will be hardly possible; but for the stationary set this is very useful.
The helix is made by building a drum with square heads fastened together by six or eight uprights, arranged on the circumference of a circle. On this then are wound ten or twelve turns of No. 10 or 12, brass or copper wire. Binding posts are fastened to the ends of the wire and variable contact made on the turns by means of metal spring clips.
The spark gap is made of a hardwood base with two to which are fastened strips of brass. Under these strips are
