Part 10 (1/2)

If ant the electron strea effect on the stream in coil _L-C_ we must see that both circuits have the same tune, that is the same natural frequency of oscillation

[Illustration: Fig 51]

This can be shown very easily by a simple experiment Suppose we set up our circuit _L-C_ with an ae an electron strea in that circuit Let us also e the natural frequency or tune of the circuit Now let's see what happens to the current as we vary this condenser, changing the capacity and thus changing the tune of the circuit If we use a variable plate condenser it will have a scale on top graduated in degrees and we can note the reading of the ammeter for each position of the movable plates If we do, we find one position of these plates, that is one setting, corresponding to one value of capacity in the condenser, where the current in the circuit is aof the condenser for which the circuit has the same tune or natural frequency as the circuit _cd_ Sometimes we say that the circuits are now in resonance We also refer to the curve of values of current and condenser positions as a ”tuning curve” Such a curve is shown in Fig 51

[Illustration: Fig 52]

That's all there is to tuning--adjusting the capacity and inductance of a circuit until it has the same natural frequency as some other circuit hich ant it to work We can either adjust the capacity as we just did, or we can adjust the inductance In that case we use a variable inductance as in Fig 52

If ant to be able to tune to any of a large range of frequencies we usually have to take out or put into the circuit a whole lot of et thesecoil” That's why your friends add a loading coil when they want to tune for the long wave-length stations, that is, those with a low frequency

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 e, first in one direction and then in the other

[Illustration: Fig 53]

Now connect the circuit _L-C_ to the grid of an audion If the circuit is tuned we'll have the rid and filaet an increase and then a decrease of current You know that will happen for I prepared you for this e of my ninth letter I'll tell you more about that current in the plate circuit in a later letter I a a telephone receiver in the plate circuit, and also a condenser, the latter for a reason to be explained later The coure shows a C-W transmitter and an audion detector This is the sort of a detector ould use for radio-telephony, but the transraphy We shall54]

Whenever we start the oscillating current in the transet an effect in the detector circuit, of which I'll tell youyou 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 hoe do it At the sending station, that is, wherever we locate the transround, as one plate We do the sa station where the detector circuit is located To these condensers we connect inductances and these inductances we couple to our trans 54 The upper plate of the condenser in each case is a few horizontal wires The lower plate is the et in contact with that in various ways One of the simplest methods is to connect to the water pipes of the city water-syste-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 ant 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 transe back and forth between the horizontal wires and the ground, I don't know I do knoever, that if we tune the antenna circuit at the receiving station there will be a s back and forth in that circuit

Usually scientists explain what happens by saying that the trans station sends out waves in the ether and that these waves are received by the antenna syste station you will get the effect It will be much smaller, however, the farther the two stations are apart

I a about wave h about the ether to try to explain, but I shall tell you e y, the ability to do work, travels out fro antenna in all directions Wherever you put up your receiving station you getsent out only while the key is depressed and the oscillator going This energy travels just as fast as light, that is at the enormous speed of 186,000 miles a second If you use meters instead of miles the speed is 300,000,000 y which is sent out fro the time it takes for one oscillation of the current in the antenna? Suppose the current is oscillating one million times a second

Then it takes one-millionth of a second for one oscillation In that tiy will have traveled away from the antenna one-millionth part of the distance it will travel in a whole second That is one-millionth of 300 y will go in the tiy is the wave length In the case just given that distance is 300 th, then, of 300 meters corresponds to a frequency of one million In fact if we divide 300 th, and that's the saave you in the last letter

In further letters I'll tell you how the audion works as a detector and hoe connect a telephone transy with a speech significance instead of a nificance We shall have to learn quite a little about the telephone itself and about the human voice

LETTER 14

WHY AND HOW TO USE A DETECTOR

DEAR SON:

In the last letter we got far enough to sketch, in Fig 54, a radio trans station We should never, however, use just this co station is intended to send telegraph signals and the receiving station is best suited to receiving telephonic transmission But let us see what happens

[Illustration: Fig 54]

When the key in the plate circuit of the audion at the sending station is depressed an alternating current is started This induces an alternating current in the neighboring antenna circuit If this antenna circuit, which is formed by a coil and a condenser, is tuned to the frequency of oscillations which are being produced in the audion circuit then there is a maximum current induced in the antenna