TABLE OF CONTENTS A radio transmitter generally consists of a group of vacuum-tube amplifiers. For convenience in the control of the frequency, we usually begin our radio signal with an oscillation of very small amplitude; perhaps it is only a small fraction of a watt at its outset. Then we pass this signal through successive stages of amplification, multiplying its power at each stage until we arrive at the antenna with a power of 100 watts, or 100 kw, or whatever is required. Somewhere along the series of amplifiers, the information that is to be transmitted will be impressed upon the radio signal. We need to give serious consideration to all possible ways in which the tubes may be used to perform these various functions, so that we can apply them in the most practical and economical way that will satisfy a particular set of requirements.
The vacuum tube is an electrical valve; it is, in fact, called a valve in some countries. Unlike a switch, which can be turned "on" or "off," the vacuum tube is capable of continuous variation in the effective resistance seen between plate and cathode; and this resistance is controlled by the potential of the grid. In Fig. 56-1 we see an experimental circuit which we might connect in order to learn something about the manner in which a tube will control the flow of a current; the battery shown on the right-hand side of the illustration is supplying a fixed potential of perhaps 2,000 volts...
