Letters of a Radio-Engineer to His Son

by John Mills

When our circuit L-C of Fig. 49 is tuned to the frequency of the oscillator we get in it a maximum current. There is a maximum stream of electrons, and hence a maximum number of them crowded first into one and then into the other plate of the condenser. And so the condenser is charged to a maximum voltage, first in one direction and then in the other.

Now connect the circuit L-C to the grid of an audion. If the circuit is tuned we’ll have the maximum possible voltage applied between grid and filament. In the plate circuit we’ll get an increase and then a decrease of current. You know that will happen for I prepared you for this moment by the last page of my ninth letter. I’ll tell you more about that current in the plate circuit in a later letter. I am connecting a telephone receiver in the plate circuit, and also a condenser, the latter for a reason to be explained later. The combination appears then as in Fig. 53. That figure shows a C-W transmitter and an audion detector. This is the sort of a detector 122 we would use for radio-telephony, but the transmitter is the sort we would use for radio-telegraphy. We shall make some changes in them later.

Whenever we start the oscillating current in the transmitter we get an effect in the detector circuit, of which I’ll tell you more later. For the moment I am interested in showing you how the transmitter and the detector may be separated by miles and still there will be an effect in the detector circuit every time the key in the transmitter circuit is closed.

This is how we do it. At the sending station, that is, wherever we locate the transmitter, we make a condenser using the earth, or ground, as one plate. We do the same thing at the receiving station where the detector circuit is located. To these condensers we connect inductances and these inductances we couple to our transmitter and receiver as shown in Fig. 54. The upper plate of the condenser in each 123case is a few horizontal wires. The lower plate is the moist earth of the ground and we arrange to get in contact with that in various ways. One of the simplest methods is to connect to the water pipes of the city water-system.

Now we have our radio transmitting-station and a station for receiving its signals. You remember we can make dots and dashes by the key or switch in the oscillator circuit. When we depress the key we start the oscillator going. That sets up oscillations in the circuit with the inductance and the capacity formed by the antenna. If we want a real-sized stream of electrons up and down this antenna lead (the vertical wire), we must tune that circuit. That is why I have shown a variable inductance in the circuit of the transmitting antenna.

What happens when these electrons surge back and forth between the horizontal wires and the ground, I don’t know. I do know, however, that if we tune the antenna circuit at the receiving station there will be a small stream of electrons surging back and forth in that circuit.

Usually scientists explain what happens by saying that the transmitting station sends out waves in the ether and that these waves are received by the antenna system at the distant station. Wherever you put up a receiving station you will get the effect. It will be much smaller, however, the farther the two stations are apart.

I am not going to tell you anything about wave motion in the ether because I don’t believe we know 124enough about the ether to try to explain, but I shall tell you what we mean by “wave length.”