 developed somewhat later and was used by radio. Table 2.1 Two Versions of the Morse Code Column A: the American Morse Code; Column B: the International Morse Code 2.3 BASIC CONCEPTS OF ELECTRICITY FOR COMMUNICATIONS A more practical telegraph system is illustrated in Figure 2.3. Note that the figure has just one metallic wire connecting the west station to the east station. The second wire is replaced with ground. The earth is a good conductor, and SO we use earth, called ground, as the second conductor (or wire). Such a telegraph system is called single-wire ground return. Such a system was widely in use when my wife and I did a stint for the ITU (International Telecommunication Union) in Ecuador in 1967-1969. This is a similar circuit as shown in Figure 2.2. In this case, when both keys are closed, a DC (direct current) circuit is traced from a battery in the west station through the key and relay at that point to the line wire, and from there it is traced through the relay and key at the east station and back through the earth (ground) to the battery. The relays at each end, in turn, control the local circuits, which include a separate battery and a sounder (e.g., buzzer or other electric sounding device). Opening and closing the key at one end, while the key at the other end is closed, causes both sounders to operate accordingly. A relay is a switch that is controlled electrically. It consists of (a) wire wrapped around an iron core and (b) a hinged metal strip, which is normally open. When current flows through the windings (i.e., the wire wrapped around the core), a magnetic field is set up, thereby drawing the hinged metal strip into a closed position and causing current to flow in the secondary circuit. It is a simple open-and-closed device such that when current flows there is a contact closure (the metal strip) and when there is no current through the windings, the circuit is open. Of course there is a spring on the metal strip, holding it open except when current flows. Twenty years after Morse first 