The use of vacuum tube or electron tube amplifiers is so widespread that it is unnecessary to explain their operations here in any detail. It is important that the principle of amplification be understood, however. The input or information wave causes the grid to act as a sort of faucet as shown in Figure 6. That is, it controls the flow of electrons (the current in the circuit) from cathode to anode. A weak signal can therefore cause a similar, but much stronger, signal to appear in the circuit. The larger signal is subsequently used to power a loudspeaker in the radio set. [Illustration: Figure 6 _Amplification by a three-element vacuum tube._] Power source Cathode Grid Input wave Anode Output wave The amplification principle can be applied in another equally important way. Once a signal gets started in the circuit, part of it can be _fed back into the input_ of the circuit. Thus the signal is made to go “round and round”, continuously regenerating itself. The device has become an _oscillator_, that is, a frequency generator that produces a steady and temporally coherent wave. The frequency of the wave can be rigidly controlled by suitable circuitry. The oscillator plays a vital part in radio transmission, for a transmitter beams energy continuously, not just when sound is being carried. The oscillator generates what is called a “carrier wave”. Information, such as speech or music, is carried in the form of audio (detectable-by-ear) frequencies, which ride “piggyback” on the carrier wave. In other words, the carrier wave is _modulated_, or varied, in such a way that it can carry meaningful information. The familiar expressions AM and FM, for example, stand for Amplitude Modulation and Frequency Modulation—two different ways of impressing information on the carrier wave. Figure 7 shows a basic and an amplitude- (or height-) modulated wave. [Illustration: Figure 7 _(a) Unmodulated radio wave._ _(b) Amplitude-modulated wave carries information._] The electron tube made its giant contribution to radio, television, and other electronic devices by making it possible to generate, detect, and amplify radio waves. Because radio waves are easily controlled, something useful can be done with them. Suppose we set up five radio transmitters, all beaming at the same frequency. The waves might look like those shown in Figure 8. Although the waves are temporally (or time) coherent, they are out of step, and not _spatially coherent_. But since good control is possible in radio circuits, we can force each antenna to radiate in _phase_ (that is, in step) with the others, thus producing fully coherent radiation (Figure 8). [Illustration: Figure 8 _(a) Spatially incoherent radiation._ _(b) Spatially coherent radiation._] Such a process can increase the radiation _power_ to an almost unlimited degree. But it does nothing to solve the problem of the limited total carrying capacity of the radio spectrum.