Radio Broadcast (May-Oct 1922)

RADIO BROADCAST 153 Here again one object causes another to move when there is no apparent physical connection between them. Again the lack of connection is not real. There is a connection. Magnetic lines of force pass from the magnet to the nail and cause the motion in a manner very similar to that caused by the electrostatic lines of force. Magnetic lines of force are also present in a radio wave. A radio wave, then, is composed of magnetic lines of force and electrostatic lines of force. A radio wave is represented in figure 2. This riG.Z. a b a Amplitude of yrai^e is repref,ented by fhe density of the I i ties of -force figure shows a radio wave moving from left to right. The electrostatic lines of force are represented by lines, the magnetic lines of force are represented by little circles at the end of the lines. It must be remembered that these are lines though they cannot be shown as such in a simple drawing. They extend at right angles to the electrostatic lines of force. In other words they extend away from you and into the paper as you look at the figure. There is one other thing about a wave that we should observe. 1 n a water wave we see that part of the water in the wave is above the level of the water when it is smooth and the other part of the wave is below the level. This is true of all kinds of waves — part of the wave disturbance is on one side of the usual (waveless) condition, and the other part of the wave disturbance is on the opposite side of the usual (waveless) condition. This is true of radio waves. Look at figure 2 and note that the arrows show that the electrostatic lines of force are directed upward in one part of the wave and downward in another part. This is also true of the electromagnetic lines of force. The open circles represent those that are directed toward you; the solid circles represent those that are directed away from you. It must be clearly understood that this wave travels onward just as a water wave travels onward. This means that any point in the path of the wave is swept by lines of force, both magnetic and electrostatic, directed in one way and an instant later the same point is swept by lines of force directed in the opposite way. Between each reversal of these lines of force there is a brief instant in which no lines of force sweep the point. As we already noted, the velocity of these waves is 300,000,000 meters per second. (They may be of any length; for example, as short as 50 meters or as long as 50,000 meters.) An examination of figure 2 shows that the amplitude of the wave is shown by the density of the lines of force and that the wave length is shown by the distance between the densest part of the lines of force going in one direction to the densest part of the next series of the lines of force going in the same direction. All this can be very easily represented by a curved line as in figure 3, which is labeled the same as figure 2. Note how the curved line accurately represents the more complicated drawing of figure 2. The amplitude, which is represented in figure 2 by the density of the lines of force, is shown by the height of the wave in figure 3 (m). A radio wave is usually represented as shown in figure 3. It must always be borne in mind, however, that it is actually as shown in figure 2. ENERGY OF RADIO WAVES AS HAS been noted the density of the lines . of force determines the amplitude, that is to say, the density of the lines of force determines the energy of the wave. Thus a very — _ b powerful radio transmitting station sets up radio waves having an enormous number of lines of force, a low-powered transmitting radio station sets up waves having only a comparatively few lines of force. As these lines of force sweep a receiving station they affect the instruments therein, the magnitude of their effect being determined by the amplitude of the wave. A radio wave spreads out from a transmitting station, the front of the wave spreading over a larger and larger area as it passes outward, in much the same way as ripples spread out from a stone thrown in a pool of water. As the number of lines of force in the radio wave does not vary, this means