Every time the key is pressed, an electric current goes through the line, causing the magnets to draw the armature downward and produce a click. When the key lever is released, the current is shut off and the lever flies up and clicks against the top of the anvil. The time between the first and second clicks may be easily varied at will, by changing the length of time that the key lever is held down. Two clicks very close together, made by pressing the key and immediately releasing it, are called a "dot". A "dash" is made by pressing the key down and holding it down for a fraction of a second so that the interval between the first and second clicks is about three times as long as that between the clicks of a dot. [Illustration: FIG. 85.—The Electro Magnets.] Any boy can make a simple telegraph set quite easily. The only drawback of such an apparatus, however, is that it is usually not practical for long lines but may only be used for ticking messages from one room to another. It is equally possible for the young experimenter to make a perfectly practical telegraph outfit which will operate over a line a mile long, if he is willing to give the time and labor necessary to the work. The telegraph set-described below is one which, because of its simplicity, particularly lends itself to construction by the young experimenter whose tools are rather limited. In order to make a really practical telegraph outfit, it is almost essential that the key and sounder frame be made out of cast iron so as to give the instruments the requisite stiffness. It is easier to use castings than to attempt to bend the parts out of metal. In order to secure the proper castings it will be necessary to first make a set of "patterns". This is not very difficult. The key frame is shown in Figure 83 and the Sounder frame in Figure 84. The patterns should be cut out of wood according to the exact shape and dimensions shown in the illustrations. Omit all the holes, however, because no holes should appear in the patterns but only in the finished castings. [Illustration: FIG. 86—The Sounder Armature.] The patterns should be made perfectly smooth and be a good, clean job throughout. After they are finished, give them each a coat of shellac and they are ready for the foundry. Almost any iron foundry will make up the castings if supplied with the patterns. If a number of experimenters "chip in" and make up a set of patterns from which several sets of castings are ordered at the same time, the cost will be very slight. After the castings are secured, they should be cleaned up with a file and drilled according to the illustration in Figures 83 and 84. Several of the holes should be tapped to receive an 8-30 machine screw. Such holes are marked in the illustrations. The holes not marked "Tap" should be just large enough to allow an 8-32 screw to slip through. [Illustration: FIG. 87.—Sounder Frame with Lever in Position.]