Part 6 (2/2)

In the last letter we learned of an alternating eit, which I described, is very crude and I want to tell how toe m f for itself That is what the audion does in the trans set of a radio telephone But an audion can't do it all alone It must have associated with it some coils and a condenser You knohat I mean by coils but you have yet to learn about condensers

A condenser is ap across which electrons cannot pass 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 ap there must be roo-rooap 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 ap, plates, and whatever is between, provided it is not conducting, is called a condenser

Let us see what happens e connect a battery to a condenser as in the figure The positive terminal of the battery calls electrons froative battery-terminal drives electrons away from itself toward the other plate of the condenser One plate of the condenser, therefore, becoative

[Illustration: Fig 25]

You know that this action of the battery will go on until there are so ative plate of the condenser that they prevent the battery fro happens at the other condenser plate The positive terminal of the battery calls electrons away fro positive until so many electrons have left that the protons in the ato for electrons to stay home just as loudly and effectively as the positive battery-ter them away

When both these conditions are reached--and they are both reached at the sa electrons around the circuit The battery has not enough e m f to drive any h e m f hich 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 space in the waiting-rooe of electrons into one plate and away fro 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 er 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 nued and its e m f would be of the sa 26]

Now suppose we connect a short wire between the plates of the condenser as in Fig 26 The electrons rush hoative 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 Soe current” starts with a rush thepath is offered between the two plates The e rows smaller, and in a very short tied

[Illustration: 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 soreat interest happens To understand youmore 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 fro the coiled wire for a few turns and then scratch off the insulation and solder onto the coil tires, _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- in the part of the coil between _c_ and _d_ For this purpose we shall use a kind of current- instrument which I have not yet explained It is different from the hot-wire type described in Letter 7 for it will shohich direction electrons are strea 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 co that the current is only ain 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_

[Illustration: Fig 28]

One thing this current- instruh itself It shows that the h 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 strea causes a momentary, that is a temporary,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 strea 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 strea, as , but while it does last it hinders the strea to establish

[Illustration: Fig 29]

The stream of electrons which the battery causes will at first meet an opposition so it takes a little tiet the full-sized strea steadily In other words a current in a coil builds up slowly, because while it is building up it induces an effect which opposes so up

Did you ever see a soing, and find his conscience starting to trouble hioes a little slowly but in a oes on steadily at his full pace