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 we wish. The batteries and slide wire resistance which are connected in the grid circuit are already familiar to you.
When the slider is set as shown in Fig. 86 the grid 170is at zero potential and we are at the point 1 of the characteristic shown in Fig. 87. Now imagine an incoming signal, as shown in that same figure, but suppose that as soon as the signal has stopped making the grid positive we shift the slider a little so that the C-battery makes the grid slightly negative. We have shifted the point on the characteristic about which the tube is being worked by the incoming signal from point 1 to point 2.
Every time the incoming signal makes one complete cycle of changes we shift the slider a little further and make the grid permanently more negative. You can see what happens. As the grid becomes more negative the current in the plate circuit decreases on the average. Finally, of course, the grid will become so negative that the current in the plate circuit will be reduced to zero. Under these conditions an incoming signal finally makes a large change in the plate current and hence in the current through the telephone.
The method of shifting a slider along, every time the incoming signal makes a complete cycle, is impossible to accomplish by hand if the frequency of the signal is high. It can be done automatically, however, no matter how high the frequency if we use a condenser in the grid circuit as shown in Fig. 88.
When the incoming signal starts a stream of electrons through the coil L of Fig. 88 and draws them away from plate 1 of the condenser C it is also drawing electrons away from the 1 plate of the condenser CG which is in series with the grid. As electrons leave plate 1 of this condenser others rush away from the grid and enter plate 2. This means that the grid doesn’t have its ordinary number of electrons and so is positive.
If the grid is positive it will be pleased to get electrons; and it can do so at once, for there are lots of electrons streaming past it on their way to the plate. While the grid is positive, therefore, there is a stream of electrons to it from the filament. Fig. 89 shows this current.
All this takes place during the first half-cycle of the incoming signal. During the next half-cycle electrons are sent into plate 1 of the condenser C and also into plate 1 of the grid condenser CG. 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 came from. That is all right, but while they were away the grid got some electrons from the filament to take their places. The result is that the grid has now too many electrons, that is, it is negatively charged.
