Part 12 (1/2)

Every tihborhood 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 incoto listen, too, if these changes are frequent enough but not so frequent that the receiver diaphrag theth of the incoe to decrease the current in the antenna of the transnal and a s of the telephone receiver at the other station On the other hand if the signal strength is increased there is64]

Suppose we connect a fine wire in the antenna circuit as in Fig 64 and have a sliding contact as shown Suppose that e 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 ill happen We alternately er and sinally 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 s streanal When the slider is at _a_ there is less resistance in the antenna circuit and a larger alternating current

[Illustration: Fig 65]

[Illustration: Fig 66]

A picture of what happens would be like that of Fig 65 The signal varies in intensity, therefore, becoer and se ier and s of the telephone receiver is alternately larger and sm moves back and forth in just the time it takes to move the slider back and forth

Instead of the slider we rains 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 diaphrag makes a telephone transmitter such as you have often talked to

[Illustration: Fig 67a]

Wires connect to the carbon discs as shown in Fig 66 A strearain through the ”carbon button,” as we call it The electrons have less difficulty if the grains are compressed, that is the button then offers less resistance to the flow of current If the diaphragrains to have more room, the electron strea67b]

You can see what happens Suppose soo back and forth as shown in Fig 67a Then the current in the antenna varies, being greater or less, depending upon whether the button offers less orvariations 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 That h just the sam When these air molecules affect your eardruht there beside the trans 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 eneral ideas from this method and learn so of radio-telephones

In any system of radio-telephony you will always find that there is produced at the trans current and that this current flows in a tuned circuit one part of which is the condenser for 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 confor 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 th of the radio-frequency current

That is e say the radio-current is ” 68]

The signal which radiates out fro antenna carries all the little variations in pitch and loudness of the hunal reaches the distant antenna it establishes in that antenna circuit a current of high frequency which has just the sa station The human voice isn't there It is not trans the radio-signal, ” the carrier current,” as we sonificance hidden in the variations in strength of the received signal

If a telephone-receiver diaphragm can be nal 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 theradio-telephone signals area current which will affect the telephone receiver in confor 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 platinu else they are made of molecules and these rouped around nuclei which, in turn, are forreat 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 ed in some orderly manner In other substances, substances without special form, arouped together in a haphazard way

[Illustration: Fig 69]

For some crystals we know very closely indeed how their ed 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 ether 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 balls of each du the other unpainted to stand for a chlorine atom Now try to pile theht 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 o 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 haveup 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

[Illustration: Fig 70]

The one which interests us at present is that across the contact points of soet 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