Part 14 (1/2)

[Illustration: Fig 85]

Suppose for exa 63 and that its characteristic under these conditions is given by Fig 56 I've redrawn the figures to save your turning back The audion will not act as a detector because an incoe value of the current in the plate circuit If, however, we connect a C-battery so as to ative, we can shi+ft this characteristic so that the incorid sufficiently negative to reduce the plate current to the value shown by the line _oa_ in Fig 85 Then the signal will be detected because, while it er and se, in a higher value of the plate current

[Illustration: Fig 86]

You see that e have done is to arrange the point on the audion characteristic about which the tube is to work by properly choosing the value of the grid voltage _E_{C}_

There is an i an audion for a detector where we arrange to have the grid voltage change steadily, gettingin Before I tell how it is done I want to show you ill happen

Suppose we start with an audion detector, for which the characteristic is that of Fig 56, but arranged as in Fig 86 to give the grid any potential which ish The batteries and slide wire resistance which are connected in the grid circuit are already fa 86 the grid is at zero potential and we are at the point 1 of the characteristic shown in Fig 87 Now iure, but suppose that as soon as the signal has stopped rid positive we shi+ft the slider a little so that the C-battery ative We have shi+fted the point on the characteristic about which the tube is being worked by the inconal fro 87]

Every ties we shi+ft the slider a little further and ative You can see what happens As the grid becoative the current in the plate circuit decreases on the average Finally, of course, the grid will becoative that the current in the plate circuit will be reduced to zero Under these conditions an incoe in the plate current and hence in the current through the telephone

Thesignal makes a complete cycle, is inal is high It can be done autoh the frequency if we use a condenser in the grid circuit as shown in Fig 88

[Illustration: Fig 88]

When the incoh the coil _L_ of Fig 88 and draws the electrons away fro}_ which is in series with the grid As electrons leave plate 1 of this condenser others rush away frorid doesn't have its ordinary nurid is positive it will be pleased to get electrons; and it can do so at once, for there are lots of electrons strearid is positive, therefore, there is a strea 89 shows this current

All this takes place during the first half-cycle of the inco the next half-cycle electrons are sent into plate 1 of the condenser _C_ and also into plate 1 of the grid condenser _C_{g}_ As electrons are forced into plate 1 of the grid condenser those in plate 2 of that condenser have to leave and go back to the grid where they carid got some electrons frorid has now too ed

[Illustration: Fig 89]

An instant later the signal e rid condenser Again electrons frorid is left without its proper nuain it receives electrons from the filarid circuit which is forrid condenser and the connecting wire

These electrons can't get across the gap of the condenser _C_{g}_ and they can't go back to the filament any other way So there they are, trapped Finally there are so ative all the time as almost entirely to oppose the efforts of the plate to draw electrons away from the filament

[Illustration: Pl VIII--To Illustrate the Mechanism for the Production of the Human Voice]

Then the plate current is reduced practically to zero

That's the way to arrange an audion so that the incoe in plate current We can tell if there is an inconal because it will ”block” the tube, as we say The plate-circuit current will be changed from its ordinary value to almost zero in the short tinal

We can detect one signal that way, but only one because the first signal ative and blocks the tube so that there isn't any current in the plate circuit and can't be any If ant to put the tube in condition to receive another signal we inally caet out of their trapped position and go back to the fila 90]

To do so we connect a very fine wire between plates 1 and 2 of the grid condenser We call that wire a ”grid-condenser leak” because it lets the electrons slip around past the gap By using a very high resistance, we can ap that notin In that case we can leave the leak per 90 Of course, the leak must offer so easy a path for the electrons that all the trapped electrons can get honal and the next

One way of h resistance like this is to draw a heavy pencil line on a piece of paper, or better a line with India ink, that is ink round particles of carbon The leak should have a very high resistance, usually one or two million ohms if the condenser is about 0002 oh with a leaky grid-condenser and an audion is very efficient so far as telling the listener whether or not a signal is co radio-telephone signals although it is best adapted to receiving the telegraph signals from a spark set

I don't propose to stop to tell you how a spark-set transmitter works