Dear Sir:
In the last letter I didn’t stop to draw you a picture of the action of the audion oscillator which I described. I am going to do it now and you are to imagine me as using two pencils and drawing simultaneously two curves. One curve shows what happens to the current in the plate circuit. The other shows how the voltage of the grid changes. Both curves start from the instant when the switch is closed; and the two taken together show just what happens in the tube from instant to instant.
Fig. 37 shows the two curves. You will notice how I have drawn them beside and below the audion characteristic. The grid voltage and the plate current are related, as I have told you, and the audion characteristic is just a convenient way of showing the relationship. If we know the current in the plate circuit we can find the voltage of the grid and vice versa.
As time goes on, the plate current grows to its maximum and decreases to zero and then goes on climbing up and down between these two extremes. The grid voltage meanwhile is varying alternately, having its maximum positive value when the plate current is a maximum and its maximum negative 98value when the plate current is zero. Look at the two curves and see this for yourself.
Now I want to tell you something about how fast these oscillations occur. We start by learning two words. One is “cycle” with which you are already partly familiar and the other is “frequency.” Take cycle first. Starting from zero the current increases to a maximum, decreases to zero, and is ready again for the same series of changes. We say the current has passed through “a cycle of values.” It doesn’t make any difference where we start from. If we follow the current through all its different values until we are back at the same value as we started with and ready to start all over, then we have followed through a cycle of values.
99Once you get the idea of a cycle, and the markings on the curves in Fig. 31 will help you to understand, then the other idea is easy. By “frequency” we mean the number of cycles each second. The electric current which we use in lighting our house goes through sixty cycles a second. That means the current reverses its direction 120 times a second.
In radio we use alternating currents which have very high frequencies. In ship sets the frequency is either 500,000 or 1,000,000 cycles per second. Amateur transmitting sets usually have oscillators which run at well over a million cycles per second. The longer range stations use lower frequencies.
You’ll find, however, that the newspaper announcements of the various broadcast stations do not tell the frequency but instead tell the “wave length.” I am not going to stop now to explain what that means but I am going to give you a simple rule. Divide 300,000,000 by the “wave length” and you’ll have the frequency. For example, ships are supposed to use wave lengths of 300 meters or 600 meters. Dividing three hundred million by three hundred gives one million and that is one of the frequencies which I told you were used by ship sets. Dividing by six hundred gives 500,000 or just half the frequency. You can remember that sets transmitting with long waves have low frequencies, but sets with short waves have high frequencies. The frequency and the wave length don’t change in the same way. They change in opposite ways or inversely, 100as we say. The higher the frequency the shorter the wave length.
I’ll tell you about wave lengths later. First let’s see how to control the frequency of an audion oscillator like that of Fig. 38.