Letters of a Radio-Engineer to His Son Part 6

[Ill.u.s.tration: Fig 21]

One member of the Council jumped up and said "But what if the grid is made negative?" The Commissioner had forgotten to see what happened so he went home to take more readings.

He s.h.i.+fted the battery clip along, starting at _c_ of Fig. 22. At the next meeting of the Council he brought in the following list of readings and hence of points on his proposed road.

Minus 1.0 Volts and Plus 0.6 Mil-ampere " 2.0 " " " 0.4 " "

" 3.0 " " " 0.2 " "

" 4.0 " " " 0.1 " "

" 5.0 " " " 0.0 " "

Then he showed the other members of the Council on the map of Fig. 23 how the Audion Characteristic would look.

[Ill.u.s.tration: Fig 22]

There was considerable discussion after that and it appeared that different designs and makes of audions would have different characteristic curves. They all had the same general form of curve but they would pa.s.s through different sets of points depending upon the design and upon the B-battery voltage. It was several meetings later, however, before they found out what effects were due to the form of the curve. Right after this they found that they could get much better results with their radio sets.

Now look at the audion characteristic. Making the grid positive, that is going on the positive side of the zero volts in our map, makes the plate current larger. You remember that I told you in Letter 6 how the grid, when positive, helped call electrons away from the filament and so made a larger stream of electrons in the plate circuit. The grid calls electrons away from the filament. It can't call them out of it; they have to come out themselves as I explained to you in the fifth letter.

[Ill.u.s.tration: Fig 23]

You can see that as we make the grid more and more positive, that is, make it call louder and louder, a condition will be reached where it won't do it any good to call any louder, for it will already be getting all the electrons away from the filament just as fast as they are emitted. Making the grid more positive after that will not increase the plate current any. That's why the characteristic flattens off as you see at high values of grid voltage.

The arrangement which we pictured in Fig. 22 for making changes in the grid voltage is simple but it doesn't let us change the voltage by less than that of a single battery cell. I want to show you a way which will.

You'll find it very useful to know and it is easily understood for it is something like the arrangement of Fig. 14 in the preceding letter.

[Ill.u.s.tration: Fig 24]

Connect the cells as in Fig. 24 to a fine wire. About the middle of this wire connect the filament. As before use a clip on the end of the wire from the grid. If the grid is connected to _a_ in the figure there is applied to the grid circuit that part of the e. m. f. of the battery which is active in the length of wire between _o_ and _a_. The point _a_ is nearer the positive plate of the battery than is the point _o_. So the grid will be positive and the filament negative.

On the other hand, if the clip is connected at _b_ the grid will be negative with respect to the filament. We can, therefore, make the grid positive or negative depending on which side of _o_ we connect the clip. How large the e. m. f. is which will be applied to the grid depends, of course, upon how far away from _o_ the clip is connected.

Suppose you took the clip in your hand and slid it along in contact with the wire, first from _o_ to _a_ and then back again through _o_ to _b_ and so on back and forth. You would be making the grid _alternately_ positive and negative, wouldn't you? That is, you would be applying to the grid an e. m. f. which increases to some positive value and then, decreasing to zero, _reverses_, and increases just as much, only to decrease to zero, where it started. If you do this over and over again, taking always the same time for one round trip of the clip you will be impressing on the grid circuit an "_alternating e. m. f._"

What's going to happen in the plate circuit? When there is no e. m. f.

applied to the grid circuit, that is when the grid potential (possibilities) is zero, there is a definite current in the plate circuit. That current we can find from our characteristic of Fig. 23 for it is where the curve crosses Zero Volts. As the grid becomes positive the current rises above this value. When the grid is made negative the current falls below this value. The current, _I_{B}_, then is made alternately greater and less than the current when _E_{C}_ is zero.

You might spend a little time thinking over this, seeing what happens when an alternating e. m. f. is applied to the grid of an audion, for that is going to be fundamental to our study of radio.

[Footnote 3: A mil-ampere is a thousandth of an ampere just as a millimeter is a thousandth of a meter.]

LETTER 10

CONDENSERS AND COILS

DEAR SON:

In the last letter we learned of an alternating e. m. f. The way of producing it, which I described, is very crude and I want to tell how to make the audion develop an alternating e. m. f. for itself. That is what the audion does in the transmitting set of a radio telephone. But an audion can't do it all alone. It must have a.s.sociated with it some coils and a condenser. You know what I mean by coils but you have yet to learn about condensers.

A condenser is merely a gap in an otherwise conducting circuit. It's a gap across which electrons cannot pa.s.s so that if there is an e. m. f.

in the circuit, electrons will be very plentiful on one side of the gap and scarce on the other side. If there are to be many electrons waiting beside the gap there must be room for them. For that reason we usually provide waiting-rooms for the electrons on each side of the gap. Metal plates or sheets of tinfoil serve nicely for this purpose. Look at Fig.

25. You see a battery and a circuit which would be conducting except for the gap at _C_. On each side of the gap there is a sheet of metal.

The metal sheets may be separated by air or mica or paraffined paper.

The combination of gap, plates, and whatever is between, provided it is not conducting, is called a condenser.

Let us see what happens when we connect a battery to a condenser as in the figure. The positive terminal of the battery calls electrons from one plate of the condenser while the negative battery-terminal drives electrons away from itself toward the other plate of the condenser. One plate of the condenser, therefore, becomes positive while the other plate becomes negative.

[Ill.u.s.tration: Fig 25]

You know that this action of the battery will go on until there are so many electrons in the negative plate of the condenser that they prevent the battery from adding any more electrons to that plate. The same thing happens at the other condenser plate. The positive terminal of the battery calls electrons away from the condenser plate which it is making positive until so many electrons have left that the protons in the atoms of the plate are calling for electrons to stay home just as loudly and effectively as the positive battery-terminal is calling them away.

When both these conditions are reached--and they are both reached at the same time--then the battery has to stop driving electrons around the circuit. The battery has not enough e. m. f. to drive any more electrons. Why? Because the condenser has now just enough e. m. f. with which to oppose the battery.

It would be well to learn at once the right words to use in describing this action. We say that the battery sends a "charging current" around its circuit and "charges the condenser" until it has the same e. m. f.

When the battery is first connected to the condenser there is lots of s.p.a.ce in the waiting-rooms so there is a great rush or surge of electrons into one plate and away from the other. Just at this first instant the charging current, therefore, is large but it decreases rapidly, for the moment electrons start to pile up on one plate of the condenser and to leave the other, an e. m. f. builds up on the condenser. This e. m. f., of course, opposes that of the battery so that the net e. m. f. acting to move electrons round the circuit is no longer that of the battery, but is the difference between the e. m. f. of the battery and that of the condenser. And so, with each added electron, the e. m. f. of the condenser increases until finally it is just equal to that of the battery and there is no net e. m. f. to act.

What would happen if we should then disconnect the battery? The condenser would be left with its extra electrons in the negative plate and with its positive plate lacking the same number of electrons. That is, the condenser would be left charged and its e. m. f. would be of the same number of volts as the battery.

[Ill.u.s.tration: Fig 26]

Now suppose we connect a short wire between the plates of the condenser as in Fig. 26. The electrons rush home from the negative plate to the positive plate. As fast as electrons get home the e. m. f. decreases.

When they are all back the e. m. f. has been reduced to zero. Sometimes we say that "the condenser discharges." The "discharge current" starts with a rush the moment the conducting path is offered between the two plates. The e. m. f. of the condenser falls, the discharge current grows smaller, and in a very short time the condenser is completely discharged.

[Ill.u.s.tration: Fig 27]

That's what happens when there is a short conducting path for the discharge current. If that were all that could happen I doubt if there would be any radio communication to-day. But if we connect a coil of wire between two plates of a charged condenser, as in Fig. 27, then something of great interest happens. To understand you must know something more about electron streams.

Suppose we should wind a few turns of wire on a cylindrical core, say on a stiff cardboard tube. We shall use insulated wire. Now start from one end of the coil, say _a_, and follow along the coiled wire for a few turns and then scratch off the insulation and solder onto the coil two wires, _b_, and _c_, as shown in Fig. 28. The further end of the coil we shall call _d_. Now let's arrange a battery and switch so that we can send a current through the part of the coil between _a_ and _b_. Arrange also a current-measuring instrument so as to show if any current is flowing in the part of the coil between _c_ and _d_. For this purpose we shall use a kind of current-measuring instrument which I have not yet explained. It is different from the hot-wire type described in Letter 7 for it will show in which direction electrons are streaming through it.

The diagram of Fig. 28 indicates the apparatus of our experiment. When we close the switch, _S_, the battery starts a stream of electrons from _a_ towards _b_. Just at that instant the needle, or pointer, of the current instrument moves. The needle moves, and thus shows a current in the coil _cd_; but it comes right back again, showing that the current is only momentary. Let's say this again in different words. The battery keeps steadily forcing electrons through the circuit _ab_ but the instrument in the circuit _cd_ shows no current in that circuit except just at the instant when current starts to flow in the neighboring circuit _ab_.

[Ill.u.s.tration: Fig 28]

One thing this current-measuring instrument tells us is the direction of the electron stream through itself. It shows that the momentary stream of electrons goes through the coil from _d_ to _c_, that is in the opposite direction to the stream in the part _ab_.

Now prepare to do a little close thinking. Read over carefully all I have told you about this experiment. You see that the moment the battery starts a stream of electrons from _a_ towards _b_, something causes a momentary, that is a temporary, movement of electrons from _d_ to _c_. We say that starting a stream of electrons from _a_ to _b_ sets up or "induces" a stream of electrons from _d_ to _c_.

What will happen then if we connect the battery between _a_ and _d_ as in Fig. 29? Electrons will start streaming away from _a_ towards _b_, that is towards _d_. But that means there will be a momentary stream from _d_ towards _c_, that is towards _a_. Our stream from the battery causes this oppositely directed stream. In the usual words we say it "induces" in the coil an opposing stream of electrons.

This opposing stream doesn't last long, as we saw, but while it does last it hinders the stream which the battery is trying to establish.

[Ill.u.s.tration: Fig 29]

The stream of electrons which the battery causes will at first meet an opposition so it takes a little time before the battery can get the full-sized stream of electrons flowing steadily. In other words a current in a coil builds up slowly, because while it is building up it induces an effect which opposes somewhat its own building up.

Did you ever see a small boy start off somewhere, perhaps where he shouldn't be going, and find his conscience starting to trouble him at once. For a time he goes a little slowly but in a moment or two his conscience stops opposing him and he goes on steadily at his full pace.