Part 11 (1/2)

[Illustration: Fig 58]

I shall not stop now to tell you much about the telephone receiver for it deserves a whole letter all to itself You know that a net attracts iron Suppose you wind a coil of insulated wire around a bar58 Send a streah the turns of the coil--a steady streaure The strength of the net is altered For one direction of the electron streaer For the opposite direction of current the59]

Fig 59 shows a sinet, a coil about it through which a current can flow, and a thin disc of iron The iron disc, or diaphrages so that it cannot net The center can m is bowed out in the form shown in the smaller sketch

Now connect a battery to the receiver winding and allow a steady streathened or weakened Suppose the streaive you the rule later Then the diaphragm is bowed out still more If we open the battery circuit and so stop the streainal position, for it is elastic The effect is veryin the botto it fly back when you renet does not pull as hard as it would if there were no current The diaphragm is therefore not bowed out sothe current by reversing the battery we arrange to send an alternating current through the coil That will have the sanet but for the other direction it is not attracted asone co current in the coil

The diaphrag current in the receiver winding As it hboring molecules of air These molecules then crowd and push the molecules of air which are just a little further away froainst those beyond them from molecule to molecule until perhaps it reaches your ear When the molecules of air next your ear receive the push they in turn push against your eardrum

In the meantime what has happened? The current in the telephone receiver has reversed its direction The diaphragnet and the adjacent molecules of air have eveneach other and follow the diaphragm in the other direction The molecules of air just beyond these, on the way toward your ear, need crowd no longer and they also o even farther than their old positions for there is nowthe line until the air ive your eardrum a chance to expand outward As they move away they make a little vacuuoes on over and over again just as often as the alternating current passes through one cycle of values And you, unless you are thinking particularly of the scientific explanations, say that you ”hear a musical note” As acurrent you will say that the ”pitch” of the note has been increased or that you hear a note higher in the musical scale

If we started with a very low-frequency alternating current, say one of fifteen or twenty cycles per second, you wouldn't say you heard a note at all You would hear a sort of a ruradually increase the frequency of the alternating current you would find that about sixty or perhaps a hundred cycles a second would give you the impression of a er you have et up into the thousands of cycles a second Then, perhaps about twenty-thousand cycles a second, you find you hear only a little sound like wind or like steah a leak A few thousand cyclesat all

You know that for radio-trans alternating currents with frequencies of several hundred-thousand cycles per second It certainly wouldn't do any good to connect a telephone receiver in the antenna circuit at the receiving station as in Fig 60 We couldn't hear so high pitched a note

[Illustration: Fig 60]

Even if we could, there are several reasons why the telephone receiver wouldn't work at such high frequencies The first is that the diaphragm can't be moved so fast It has soet started If you try to start it in one direction and, before you really get it going, change your o in the other direction, it sio at all So even if there is an alternating current in the coil around thevibration of the diaphragh, certainly not if it is above about 20,000 cycles a second

The other reason is that there will only be a very feeble current in the coil anyway, no h You reether and each has an effect on all the others which can reat unwillingness to get started and an equal unwillingness to stop Their unwillingness is ht It is also reater by the presence of the iron core An alternating e ets the electrons started at all before it's ti in the opposite direction There is very little movement to the electrons and hence only a very sh

If you want a rule for it you can re e m f the siven coil Of course, we er, that is pull and shove the electrons harder, but unless the coil has a very small inductance or unless the frequency is very loe should have to use an e et any appreciable current

Condensers are just the other way in their action If there is a condenser in a circuit, where an alternating e m f is active, there is lots of trouble if the frequency is low If, however, the frequency is high the same-sized current can be maintained by a smaller e m f

than if the frequency is low You see, when the frequency is high the electrons hardly get into the waiting-room of the condenser before it is tio toward the other rooh electrons crowded together in the waiting-roo by the e m f Because the electrons do not push back very hard a small e m f can drive them back and forth

Ordinarily we say that a condenser iher is the frequency of the current And as to inductances, we say that an inductance iher is the frequency

Noe are ready to study the receiving circuit of Fig 54 I showed you in Fig 57 how the current through, the tube will vary as tioes on

It increases and decreases with the frequency of the current in the antenna of the distant transraph, as we say, of how this plate current varies It will be necessary to study that carefully and to resolve it into its coether again will give the whole To show you what Imoney

Suppose a boy was started by his father with 50 cents of spending money

He spends that and runs 50 cents in debt The next day his father gives him a dollar Half of this he has to spend to pay up his yesterday's indebtedness This he does at once and that leaves hi for a dollar and so runs 50 cents in debt

Day after day this cycle is repeated We can shohat happens by the curve of Fig 61a

[Illustration: Fig 61a]

On the other hand, suppose he already had 60 cents which, he was saving for so to run into debt each day and to pay up the next, as shown in Fig 61a Then ould represent the story of this 60 cents by the graph of Fig 61b