Part 16 (1/2)

We ht, therefore, pay no further attention to what is actually inside the box or how all these effects are brought about We ht treat the entire box as if it was for

97 If we do so, we are replacing the box by so which is equivalent so far as effects are concerned, that is we are replacing an actual audion by two circuits which together are equivalent to it

The men who first perfor that if an alternator, which has an e m f of _V_ volts, is connected to _F_ and _G_, the effect is the saer alternator is connected between _F_ and _P_ How inary alternator is depends upon the design of the audion For sons 65 or alh usually a number of times less than 40 They used a little Greek letter called ”n of the tube Then they said that the hidden alternator in the output circuit wasas the actual alternator which was applied between the grid and the fila the sound and name of the letter they used the letter [Greek:97

Noe are ready to talk about the audion as an a to notice is the fact that we have an open circuit between _F_ and _G_ This is true as long as we don't apply an e96 and thus let the grid become positive and attract electrons fro about ill happen in the circuit _G-F_, for there will be no current

As to the circuit _F-P_, we can treat it as a resistance in series hich there is a generator [Greek:as that which is connected to _F_ and _G_ The next probleenerator We call the resistance which the tube offers to the passage of electrons between _P_ and _F_ the ”internal resistance” of the plate circuit of the tube How large it is depends upon the design of tube In some tubes it may be five or six thousand ohe tubes used in high-powered trans sets it is much less Since it will be different in different cases we shall use a symbol for it and say that it is _R_{p}_ oh an audion as an aet the most out of an audion see that the telephone, or whatever circuit or piece of apparatus is connected to the output terminals, shall have a resistance of _R_{p}_ ohms When the resistance of the circuit, which an audion is supplying with current, is the same as the internal resistance of the output side of the tube, then the audion gives its greatest output That is the condition for the greatest ”areatest power” as we say

That rule is ays select the telephone receivers which we use with an audion and always ask carefully as to their resistance e buy Sometimes, however, it is not practicable to use receivers of just the right resistance Where we connect the output side of an audion to some other circuit, as where we let one audion supply another, it is usually i some special apparatus

This leads to the next rule: If the telephone receiver, or the circuit, which ish to connect to the output of an audion, does not have quite nearly a resistance of _R_{p}_ ohned transfors 94 and 95

A transforether so that an alternating current in the pri current in the secondary Of course, if the secondary is open-circuited then no current can flow but there will be induced in it an e m f which is ready to act if the circuit is closed Transfore resistance look small or vice versa To show you why, I shall have to develop some rules for transfor e m f of ten volts applied to the primary of an iron-cored transformer which has ten turns There is one volt applied to each turn Now, suppose the secondary has only one turn

That one turn has induced in it an alternating ethe secondary, then each turn has induced in it just one volt But the e ether If the secondary has twenty turns, there is induced in it a total of twenty volts So the first rule is this: In a transformer the number of volts in each turn of wire is just the sah-voltage alternating ecurrent through the primary of a transformer which has in the secondary, many times more turns of wire than it has in the prie than we impress on the primary

You can see one application of this rule at once When we use an audion as an a current we send the current which is to be a 94 We use a transformer with many times more turns on the secondary than on the pririd of the ae effect in the plate circuit of the arid circuit of an audion with a proper value of negative C-battery is really open-circuited and no current will flow in it For that case we get a real gain by using a ”step-up” transformer, that is, one with more turns in the secondary than in the primary

It looks at first as if a transfory it will not_ although we may use it, as we shall see in a minute, to permit a vacuum tube to work into an output circuit more efficiently than it could without the transfory in the secondary circuit of a transforive to the primary

Suppose we have a transformer with twice as many turns on the secondary as on the pri e m f of a certain number of volts In the secondary there will be twice as many volts because it has twice as many turns The current in the secondary, however, will be only half as large as is the current in the primary We have twice the force in the secondary but only half the electron strea and two youngsters ask you to pull them and their sleds up hill You pull one of them all the way and do a certain amount of work On the other hand suppose you pull them both at once but only half way up You pull twice as hard but only half as far and you do the sa 98]

We can't get more work out of the secondary of a transforn the transforreater pull (e m f) in the secondary the electron strealy smaller

You remember hoe measure resistance We divide the e m f (number of volts) by the current (number of amperes) to find the resistance (number of ohms) Suppose we do that for the pri 98 which we are discussing See what happens in the secondary There is only half as h the secondary had one-fourth as much resistance as the primary And so it has, but we usually call it ”iht wires resist but coils or condensers i 99]

Before we return to the question of using a transformer in an audion circuit let us turn this transforh the side with the larger nus Let's talk of ”primary” and ”secondary” just as before but, of course, remember that now the primary has twice the turns of the secondary On the secondary side we shall have only half the current, but there will be twice the e m f The resistance of the secondary then is four times that of the pri 94 and see what sort of a transformer should be between the plate circuit of the tube and the telephone receivers Suppose the internal resistance of the tube is 12,000 ohms and the resistance of the telephones is 3,000 ohms Suppose also that the resistance (really impedance) of the primary side of the transformer which we just considered is 12,000 ohms The impedance of its secondary will be a quarter of this or 3,000 ohms If we connect such a transforreatest output from the tube

In the first place the primary of the transformer has a number of ohms just equal to the internal resistance of the tube The tube, therefore, will give its best to that transformer In the second place the secondary of the transformer has a resistance just equal to the telephone receivers so it can give its best to them The effect of the transformer is to make the telephones act as if they had four times as much resistance and so were exactly suited to be connected to the audion

This whole matter of the proper use of transfor up vacuu your radio set will mean poor efficiency Whenever you have two parts of a vacuuether be sure and buy only a transforned to work between the two iether

There is one more precaution in connection with the purchase of transfor for all the important frequencies which they are to transnals will be distorted and ible

If you take the precautions which I haveset for

94 That is only one possible scheme of connections You can use any detector circuit which you wish,[10] one with a grid condenser and leak, or one arranged for feed-back In either case your aure