 2.3 BASIC CONCEPTS OF ELECTRICITY FOR COMMUNICATIONS a' b' c' d' e' f' g' h' i' j' k l' m' n' o' p' a" Figure 2.7 Graphical construction of a sine wave. Figure 2.8 Two signals of the same frequency: (a) with different amplitudes and in-phase and (b) with the same amplitudes but 180 degrees out of phase. current or voltage. The complete curve shows values of current (or voltage) for all instants during one complete cycle. It is convenient and customary to divide the time scale into units of degrees rather than seconds, considering one complete cycle as being completed always in 360 degrees or units of time (regardless of the actual time taken in seconds). The reason for this convention becomes obvious from the method of constructing the sine SIGNALS CONVEY INTELLIGENCE wave as shown in Figure 2.7, where, to plot the complete curve, we take points around the circumference of the circle through 360 angular degrees. It needs to be kept in mind that in the sense now used, the degree is a measure of time in terms of the frequency, not of an angle. We must understand phase and phase angle because it will be used in our discussions of modulation and of certain types of distortion that can limit the rate of transmitting information and/or corrupt a wanted signal (Ref. 2). An example of two signals of the same frequency, in phase and with different ampli- tudes, is illustrated in Figure 2.8a, and another example of two signals of the same frequency and amplitude, but 180 degrees out of phase, is shown in Figure 2.8b. Note the use of IT in the figure, meaning IT radians or 180°, 2 radians or 360°. See Appendix A, Section A.9. 2.4 ELECTRICAL SIGNALS 2.4.1 Introduction to Transmission Transmission may be defined as the electrical transfer of a signal, message, or other form of intelligence from one location to another. Traditionally, transmission has been one of the two major disciplines of telecommunication. Switching is the other principal specialty. Switching establishes a connection from user X to some distant user Y. Si