Part 9 (1/2)

[Illustration: Fig 41]

All this leads to two very si condensers If you have a condenser with too se, you can either use twice as large plates or bring the plates you already have twice as close together; that is, ap is pretty well fixed For exa two pieces ofthe the mica So we increase the size of the plates We can do that either by using larger plates or other plates and connecting it as in Fig 41 so that the total waiting-room space for electrons is increased

[Illustration: Pl VI--Loer Trans Tube, U V 202 (Courtesy of Radio Corporation of Aot these ideas you can understand hoe use both sides of the sa 42 There are two plates connected together and a third between them Suppose electrons are pulled from the outside plates and crowded into the o on one side and sons indicate electrons and the plus signs their old hoer capacity

[Illustration: Fig 43]

[Illustration: Fig 44]

What if we have two plates which are not directly opposite one another, like those of Fig 44? What does the capacity depend upon? Iative plate Look off toward the positive plate and see how big it seeer it looks the ht opposite one another the positive plate looe But if they slide apart you don't see so much of it; and if it is off to one side about all you see is the edge If you can't see lots of atoood ho around on that side of the plate; youhich you originally came

That's why in a variable plate condenser there is very little capacity when no parts of the plates are opposite each other, and there is the greatest capacity when they are exactly opposite one another

[Illustration: Fig 45]

While we are at it we ht just as well clean up this whole business of variable capacities and inductances by considering tays in which to45 shows the sies which I won't try now to explain Wecoil and then take off taps We can make connections between one end of the coil and any of the taps The more turns there are included in the part of the coil which we are using the greater is the inductance If ant to do a real job we can bring each of these taps to a little stud and arrange a sliding or rotating contact with them Then we have an inductance the value of which we can vary ”step-by-step” in a convenient manner

Another way to make a variable inductance is to make what is called a ”vario If properly calibrated it would of course ”meter” inductance, but then it should be called an ”inducto- of boys that fellow had to drive off his property? What if there had been two different gangs playing there? Howin co in different parts of his property and so act just as if the other croasn't also trespassing

He could just add the trouble of starting one gang to the trouble of starting the other

It would be very different if the gangs have anything in coe the other e each other He would have a lot more trouble And this extra trouble would be because of the relations between gangs, that is, because of their ”s came from different parts of the town and disliked each other He wouldn't have nearly the trouble Each gang would be yelling at the other as they went along: ”You'd better beat it He knows all right, all right, who broke that bush down by the gate Just wait till he catches you” They'd get out a little easier, each in the hope the other croould catch it from the owner There's a case where their mutual relations, their mutual inductance, makes the job easier

That's true of coils with inductance Suppose you wind two inductance coils and connect theles to each other as in Fig 46a they have no effect on each other There is no mutual inductance But if they are parallel and wound the sale coil of greater inductance If the coils are parallel but wound in opposite directions as in Fig 46c they will have less inductance because of their mutual inductance You can check these statements for yourself if you'll refer back to Letter 10 and see what happens in the sa 28

[Illustration: Fig 46a]

[Illustration: Fig 46b]

If the coils are neither parallel nor at right angles there will be some mutual inductance but not as et all the variations in46c That's e arrange to do in a variable inductance of the vario 46c]

There is another way of varying the mutual inductance We can make one coil slide inside another If it is way inside, the total inductance which the two coils offer is either larger than the su upon whether the windings are in the same direction or opposite As we pull the coil out the mutual effect becomes less and finally when it is well outside the mutual inductance is very s capacity and inductance and therefore we are ready to vary the frequency of our audion oscillator; that is, ”tune” it, as we say In my next letter I shall show you e tune

Now for the rule which I promised The frequency to which a circuit is tuned depends upon the product of the number of mil-henries in the coil and the nue the coil and the condenser as much as you want but keep this product the same and the frequency will be the same

[Footnote 5: More accurately the number is 6,286,000,000,000]

LETTER 13