Every time the diaphragm moves it affects the air in the immediate neighborhood of itself and that air in turn affects the air farther away and so the ear of the listener. Therefore if there are changes in the intensity or strength of the incoming signal there are going to be corresponding motions of the receiver diaphragm. And something to listen, too, if these changes are frequent enough but not so frequent that the receiver diaphragm has difficulty in following them.
There are many ways of affecting the strength of the incoming signal.
Suppose, for example, that we arrange to decrease the current in the antenna of the transmitting station. That will mean a weaker signal and a smaller increase in current through the winding of the telephone receiver at the other station. On the other hand if the signal strength is increased there is more current in this winding.
[Ill.u.s.tration: Fig 64]
Suppose we connect a fine wire in the antenna circuit as in Fig. 64 and have a sliding contact as shown. Suppose that when we depress the switch in the oscillator circuit and so start the oscillations that the sliding contact is at _o_ as shown. Corresponding to that strength of signal there is a certain value of current through the receiver winding at the other station. Now let us move the slider, first to _a_ and then back to _b_ and so on, back and forth. You see what will happen. We alternately make the current in the antenna larger and smaller than it originally was. When the slider is at _b_ there is more of the fine wire in series with the antenna, hence more resistance to the oscillations of the electrons, and hence a smaller oscillating stream of electrons. That means a weaker outgoing signal. When the slider is at _a_ there is less resistance in the antenna circuit and a larger alternating current.
[Ill.u.s.tration: Fig 65]
[Ill.u.s.tration: Fig 66]
A picture of what happens would be like that of Fig. 65. The signal varies in intensity, therefore, becoming larger and smaller alternately.
That means the voltage impressed on the grid of the detector is alternately larger and smaller. And hence the stream of electrons through the winding of the telephone receiver is alternately larger and smaller. And that means that the diaphragm moves back and forth in just the time it takes to move the slider back and forth.
Instead of the slider we might use a little cup almost full of grains of carbon. The carbon grains lie between two flat discs of carbon. One of these discs is held fixed. The other is connected to the center of a thin diaphragm of steel and moves back and forth as this diaphragm is moved. The whole thing makes a telephone transmitter such as you have often talked to.
[Ill.u.s.tration: Fig 67a]
Wires connect to the carbon discs as shown in Fig. 66. A stream of electrons can flow through the wires and from grain to grain through the "carbon b.u.t.ton," as we call it. The electrons have less difficulty if the grains are compressed, that is the b.u.t.ton then offers less resistance to the flow of current. If the diaphragm moves back, allowing the grains to have more room, the electron stream is smaller and we say the b.u.t.ton is offering more resistance to the current.
[Ill.u.s.tration: Fig 67b]
You can see what happens. Suppose some one talks into the transmitter and makes its diaphragm go back and forth as shown in Fig. 67a. Then the current in the antenna varies, being greater or less, depending upon whether the b.u.t.ton offers less or more resistance. The corresponding variations in the antenna current are shown in Fig. 67b.
In the antenna at the receiving station there are corresponding variations in the strength of the signal and hence corresponding variations in the strength of the current through the telephone receiver. I shall show graphically what happens in Fig. 68. You see that the telephone receiver diaphragm does just the same motions as does the transmitter diaphragm. That means that the molecules of air near the receiver diaphragm are going through just the same kind of motions as are those near the transmitter diaphragm. When these air molecules affect your eardrum you hear just what you would have heard if you had been right there beside the transmitter.
That"s one way of making a radio-telephone. It is not a very efficient method but it has been used in the past. Before we look at any of the more recent methods we can draw some general ideas from this method and learn some words that are used almost always in speaking of radio-telephones.
In any system of radio-telephony you will always find that there is produced at the transmitting station a high-frequency alternating current and that this current flows in a tuned circuit one part of which is the condenser formed by the antenna and the ground (or something which acts like a ground). This high-frequency current, or radio-current, as we usually say, is varied in its strength. It is varied in conformity with the human voice. If the human voice speaking into the transmitter is low pitched there are slow variations in the intensity of the radio current. If the voice is high pitched there are more rapid variations in the strength of the radio-frequency current.
That is why we say the radio-current is "modulated" by the human voice.
[Ill.u.s.tration: Fig 68]
The signal which radiates out from the transmitting antenna carries all the little variations in pitch and loudness of the human voice. When this signal reaches the distant antenna it establishes in that antenna circuit a current of high frequency which has just the same variations as did the current in the antenna at the sending station. The human voice isn"t there. It is not transmitted. It did its work at the sending station by modulating the radio-signal, "modulating the carrier current," as we sometimes say. But there is speech significance hidden in the variations in strength of the received signal.
If a telephone-receiver diaphragm can be made to vibrate in accordance with the variations in signal intensity then the air adjacent to that diaphragm will be set into vibration and these vibrations will be just like those which the human voice set up in the air molecules near the mouth of the speaker. All the different systems of receiving radio-telephone signals are merely different methods of getting a current which will affect the telephone receiver in conformity with the variations in signal strength. Getting such a current is called "detecting." There are many different kinds of detectors but the vacuum tube is much to be preferred.
The cheapest detector, but not the most sensitive, is the crystal. If you understand how the audion works as a detector you will have no difficulty in understanding the crystal detector.
The crystal detector consists of some mineral crystal and a fine-wire point, usually platinum. Crystals are peculiar things. Like everything else they are made of molecules and these molecules of atoms. The atoms are made of electrons grouped around nuclei which, in turn, are formed by close groupings of protons and electrons. The great difference between crystals and substances which are not crystalline, that is, substances which don"t have a special natural shape, is this: In crystals the molecules and atoms are all arranged in some orderly manner. In other substances, substances without special form, amorphous substances, as we call them, the molecules are just grouped together in a haphazard way.
[Ill.u.s.tration: Fig 69]
For some crystals we know very closely indeed how their molecules or rather their individual atoms are arranged. Sometime you may wish to read how this was found out by the use of X-rays.[6] Take the crystal of common salt for example. That is well known. Each molecule of salt is formed by an atom of sodium and one of chlorine. In a crystal of salt the molecules are grouped together so that a sodium atom always has chlorine atoms on every side of it, and the other way around, of course.
Suppose you took a lot of wood dumb-bells and painted one of the b.a.l.l.s of each dumb-bell black to stand for a sodium atom, leaving the other unpainted to stand for a chlorine atom. Now try to pile them up so that above and below each black ball, to the right and left of it, and also in front and behind it, there shall be a white ball. The pile which you would probably get would look like that of Fig. 69. I have omitted the gripping part of each dumbell because I don"t believe it is there. In my picture each circle represents the nucleus of an atom. I haven"t attempted to show the planetary electrons. Other crystals have more complex arrangements for piling up their molecules.
Now suppose we put two different kinds of substances close together, that is, make contact between them. How their electrons will behave will depend entirely upon what the atoms are and how they are piled up. Some very curious effects can be obtained.
[Ill.u.s.tration: Fig 70]
The one which interests us at present is that across the contact points of some combinations of substances it is easier to get a stream of electrons to flow one way than the other. The contact doesn"t have the same resistance in the two directions. Usually also the resistance depends upon what voltage we are applying to force the electron stream across the point of contact.
The one way to find out is to take the voltage-current characteristic of the combination. To do so we use the same general method as we did for the audion. And when we get through we plot another curve and call it, for example, a "platinum-galena characteristic." Fig. 70 shows the set-up for making the measurements. There is a group of batteries arranged so that we can vary the e. m. f. applied across the contact point of the crystal and platinum. A voltmeter shows the value of this e. m. f. and an ammeter tells the strength of the electron stream. Each time we move the slider we get a new pair of values for volts and amperes. As a matter of fact we don"t get amperes or even mil-amperes; we get millionths of an ampere or "microamperes," as we say. We can plot the pairs of values which we measure and make a curve like that of Fig. 71.
[Ill.u.s.tration: Fig 71]
When the voltage across the contact is reversed, of course, the current reverses. Part of the curve looks something like the lower part of an audion characteristic.
[Ill.u.s.tration: Fig 72]
Now connect this crystal in a receiving circuit as in Fig. 72. We use an antenna just as we did for the audion and we tune the antenna circuit to the frequency of the incoming signal. The receiving circuit is coupled to the antenna circuit and is tuned to the same frequency. Whatever voltage there may be across the condenser of this circuit is applied to the crystal detector. We haven"t put the telephone receiver in the circuit yet. I want to wait until you have seen what the crystal does when an alternating voltage is applied to it.
[Ill.u.s.tration: Fig 73]
We can draw a familiar form of sketch as in Fig. 73 to show how the current in the crystal varies. You see that there flows through the crystal a current very much like that of Fig. 62a. And you know that such a current is really equivalent to two electron streams, one steady and the other alternating. The crystal detector gives us much the same sort of a current as does the vacuum tube detector of Fig. 54. The current isn"t anywhere near as large, however, for it is microamperes instead of mil-amperes.
Our crystal detector produces the same results so far as giving us a steady component of current to send through a telephone receiver. So we can connect a receiver in series with the crystal as shown in Fig. 74.
Because the receiver would offer a large impedance to the high-frequency current, that is, seriously impede and so reduce the high-frequency current, we connect a condenser around the receiver.
[Ill.u.s.tration: Fig 74]
There is a simple crystal detector circuit. If the signal intensity varies then the current which pa.s.ses through the receiver will vary. If these variations are caused by a human voice at the sending station the crystal will permit one to hear from the telephone receiver what the speaker is saying. That is just what the audion detector does very many times better.
In the letter on how to experiment you"ll find details as to the construction of a crystal-detector set. Excellent instructions for an inexpensive set are contained in Bull. No. 120 of the Bureau of Standards. A copy can be obtained by sending ten cents to the Commissioner of Public Doc.u.ments, Washington, D. C.
[Footnote 6: Cf. "Within the Atom," Chapter X.]
LETTER 16
THE HUMAN VOICE
DEAR SIR:
The radio-telephone does not transmit the human voice. It reproduces near the ears of the listener similar motions of the air molecules and hence causes in the ears of the listener the same sensations of sound as if he were listening directly to the speaker. This reproduction takes place almost instantaneously so great is the speed with which the electrical effects travel outward from the sending antenna. If you wish to understand radio-telephony you must know something of the mechanism by which the voice is produced and something of the peculiar or characteristic properties of voice sounds.
[Ill.u.s.tration: Fig 75]
The human voice is produced by a sort of organ pipe. Imagine a long pipe connected at one end to a pair of fire-bellows, and closed at the other end by two stretched sheets of rubber. Fig. 75 is a sketch of what I mean. Corresponding to the bellows there is the human diaphragm, the muscular membrane separating the thorax and abdomen, which expands or contracts as one breathes. Corresponding to the pipe is the windpipe.
Corresponding to the two stretched pieces of rubber are the vocal cords, L and R, shown in cross section in Fig. 77. They are part of the larynx and do not show in Fig. 76 (Pl. viii) which shows the wind pipe and an outside view of the larynx.
[Ill.u.s.tration: Fig 77]