It is not necessary to use a grid condenser in a feed-back circuit but it is perhaps the usual method of detecting where the regenerative circuit is used. The whole value of the regenerative circuit so far as receiving is concerned is in the high efficiency which it permits. One tube can do the work of two. We can get just as loud signals by using another tube instead of making one do all the work. In the regenerative circuit the tube is performing two jobs at once. It is detecting but it is also amplifying.[9] By "amplifying" we mean making an e. m. f. larger than it is without changing the shape of its picture, that is without changing its "wave form." To show just what we mean by amplifying we must look again at the audion and see how it acts. You know that a change in the grid potential makes a change in the plate current. Let us arrange an audion in a circuit which will tell us a little more of what happens. Fig. 93 shows the circuit. This circuit is the same as we used to find the audion characteristic except that there is a clip for varying the number of batteries in the plate circuit and a voltmeter for measuring their e. m. f. We start with the grid at zero potential and the usual number of batteries in the plate circuit. The voltmeter tells us the e. m. f. We read the ammeter in the plate circuit and note what that current is. Then we shift the slider in the grid circuit so as to give the grid a small potential. The current in the plate circuit changes. We can now move the clip on the B-batteries so as to bring the current in this circuit back to its original value. Of course, if we make the grid positive we move the clip so as to use fewer cells of the B-battery. On the other hand if we make the grid negative we shall need more e. m. f. in the plate circuit. In either case we shall find that we need to make a very much larger change in the voltage of the plate circuit than we have made in the voltage of the grid circuit. [Illustration: Fig 93] Usually we perform the experiment a little differently so as to get more accurate results. We read the voltmeter in the plate circuit and the ammeter in that circuit. Then we change the number of batteries which we are using in the plate circuit. That changes the plate current. The next step is to shift the slider in the grid circuit until we have again the original value of current in the plate circuit. Suppose that the tube is ordinarily run with a plate voltage of 40 volts and we start with that e. m. f. on the plate. Suppose that we now make it 50 volts and then vary the position of the slider in the grid circuit until the ammeter reads as it did at the start. Next we read the voltage impressed on the grid by reading the voltmeter in the grid circuit. Suppose it reads 2 volts. What does that mean? [Illustration: Fig 94]