demonstrated his telegraph on a Baltimore-Washington route, telegraph covered the country from coast to coast. It caused a revolution in com- munications. Its use is still prevalent in many parts of the world, as I discussed above. 2.3.3 What Is Frequency? To understand more advanced telecommunication concepts, we need a firm knowledge of frequency and related parameters such as band and bandwidth, wavelength, period, and phase. Let us first define frequency and relate it to everyday life. The IEEE defines frequency as "the number of complete cycles of sinusoidal varia- tion per unit time." The time unit we will use is the second. For those readers with a mathematical bent, if we plot y = sin x, where X is expressed in radians, a "sine wave" is developed as shown in Figure 2.4. Figure 2.5 shows two sine waves; the left side illustrates a lower frequency, and the right side shows a higher frequency. The amplitude, measured in this case as voltage, is the excursion, up or down, at any singular point. Amplitude expresses the intensity at that point. If we spoke of amplitude without qualifying it at some point, it would be the West Station East Station Sounder Line Wire Relay Figure 2.3 A practical elementary telegraph circuit with ground return. SIGNALS CONVEY INTELLIGENCE Figure 2.4 A sine wave. Here frequency is the number of times per second that a wave cycle (one peak and one trough) repeats at a given amplitude. A is the amplitude and a is the wavelength. IT is IT radians or 180° and 2r is the radian value at 360°. Figure 2.5 A simple sine wave. (a) Lower frequency, (b) higher frequency. Note that the wavelength is shown traditionally as a (the Greek letter lambda) and that part a has a longer wavelength than part b. maximum excursion in the negative or positive direction (up or down). In this case it is 6 volts. If it is in the "down" direction, it would be -6 volts, based on Figure 2.5; and in the "up" direction it would be +6 volts. Frequency is an important aspect of music. For example, the key of A is 440 