Part 5 (2/2)

Second: Electrons -force, that is an electromotive force or e m f We measure e m f's in volts To measure an e m f we connect a voltmeter to the two points bethich the e m f is active

Third: What current any particular e m f will cause depends upon the circuit in which it is active Circuits differ in the resistance which they offer to e iven e reater the resistance of the circuit We measure resistance in ohms To measure it we find the quotient of the number of volts applied to the circuit by the number of amperes which flow

Inof how the audion works It would be worth while to read again that letter You remember that the current in the plate circuit can be controlled by the e rid circuit There is a relationshi+p between the plate current and the grid voltage which is peculiar or characteristic to the tube So we call such a relationshi+p ”a characteristic” Let us see how it may be found and what it will be

Connect an ammeter in the plate- or B-circuit, of the tube so as to measure the plate-circuit current You will find that almost all books use the letter ”_I_” to stand for current The reason is that scientists used to speak of the ”intensity of an electric current” so that ”_I_” really stands for intensity We use _I_ to stand for so more than the word ”current” It is our symbol for whatever an ammeter would read, that is for the a 18]

Another convenience in symbols is this: We shall frequently want to speak of the currents in several different circuits It saves ti with the letter _I_ to show the circuit to which we refer For exa to talk about the current in the B-circuit of the audion, so we call that current _I_{B}_ We write the letter _B_ below the line on which _I_ stands That is e say the _B_ is subscript,to yourself be sure to read _I_{B}_ as ”eye-bee” or else as ”eye-subscript-bee” _I_{B}_ therefore will stand for the number of amperes in the plate circuit of the audion In the same way _I_{a}_ would stand for the current in the fila to talk about e m f's also The letter ”_E_” stands for the number of volts of e m f in a circuit In the filament circuit the battery has _E_{A}_ volts In the plate circuit the e

rid circuit we can let _E_{C}_ represent the nurid-filament or C-circuit

The characteristic relation which we are after is one between grid voltage, that is _E_{C}_, and plate current, that is _I_{B}_

So we call it the _E_{C}_--_I_{B}_ characteristic The dash between the letters is not a subtraction sign but merely a dash to separate the letters Noe'll find the ”ee-see-eye-bee”

characteristic

Connect sorid circuit Then connect the fila 19 Take the hich corid and put a battery clip on it, then you can connect the grid anywhere you want along this series of batteries See Fig 18 In the figure this movable clip is represented by an arrow head You can see that if it is at _a_ the battery will rid will be more positive On the other hand if the clip is at _o_ there will be no e rid will be rid and filament there is placed a voltrid, that is, tell the value of _E_{C}_, for any position whatever of the clip

We shall start with the filament heated to a deep red The manufacturers of the audion tell the purchaser what current should flow through the filament so that there will be the proper e out for one's self but we shall not stop to describe them The makers also tell how many volts to apply to the plate, that is what value _E_{B}_ should have We could find this out also for ourselves but we shall not stop to do so

[Illustration: Fig 19]

Noe set the battery clip so that there is no voltage applied to the grid; that is, we start with _E_{C}_ equal to zero Then we read the ammeter in the plate circuit to find the value of _I_{B}_ which corresponds to this condition of the grid

Next we rid as positive as one battery will19 We now have a different value of _E_{C}_ and will find a different value of _I_{B}_ e read the arid We get a new pair of values for _E_{C}_ and _I_{B}_, getting _E_{C}_ from the voltmeter and _I_{B}_ fro _E_{C}_, we find that the current _I_{B}_ increases for a while and then after we have reached a certain value of _E_{C}_ the current _I_{B}_ stops increasing Adding rid more positive doesn't have any effect on the plate current

[Illustration: Fig 20]

Before I tell you why this happens I want to show you how to make a picture of the pairs of values of _E_{C}_ and _I_{B}_ which we have been reading on the voltine a city where all the streets are at right angles and the north and south streets are called streets and numbered while the east and west thorofares are called avenues I'll draw the h the center of the city goes Main Street But the people who laid out the roads wereit Main Street they called it ”Zero Street” The first street east of Zero St we should have called ”East First Street” but they called it ”Positive 1 St” and the next beyond ”Positive 2 St,” and so on West of the ative 1 St” and so on

When they came to name the avenues they were just as precise and mathematical They called the main avenue ”Zero Ave” and those north of it ”Positive 1 Ave,” ”Positive 2 Ave” and so on Of course, the avenues south of Zero Ave they called Negative

The Town Council went al; they nu The silent policeman which stood at the corner of ”Positive 2 St” and ”Positive 1 Ave” was marked that way Half way between Positive 2 St and Positive 3 St there was a garage which set back about two-tenths of a block from Positive 1 Ave The Council numbered it and called it ”Positive 25 St and Positive 12 Ave” Most of the people spoke of it as ”Plus 25 St and Plus 12 Ave”

Sometime later there was an election in the city and a new Council was elected The hway Co the Council changed the names of all the avenues to ”Mil-amperes”[3] and of all the streets to ”Volts”

Then the Highway Co a set of volts on an audion moved that there should be a new road known as ”Audion Characteristic” He said the road should pass through the following points:

Zero Volt and Plus 10 Mil-ampere Plus 20 Volts and Plus 17 Mil-amperes Plus 40 Volts and Plus 26 Mil-amperes Plus 60 Volts and Plus 34 Mil-amperes Plus 80 Volts and Plus 43 Mil-a 21 shows the new road