Suppose we had concealed in a box the audion and circuit of Fig. 96 and that only the terminals which are shown came through the box. We are given a battery and an ammeter and asked to find out all we can as to what is between the terminals F and G. We connect the battery and ammeter in series with these terminals. No current flows through the circuit. We reverse the battery but no current flows in the opposite direction. Then we reason that there is an open-circuit between F and G.
As long as we do not use a higher voltage than 188that of the C-battery which is in the box no current can flow. Even if we do use a higher voltage than the “negative C-battery” of the hidden grid-circuit there will be a current only when the external battery is connected so as to make the grid positive with respect to the filament.
Now suppose we take several cells of battery and try in the same way to find what is hidden between the terminals P and F. We start with one battery and the ammeter as before and find that if this battery is connected so as to make P positive with respect to F, there is a feeble current. We increase the battery and find that the current is increased. Two cells, however, do not give exactly twice the current that one cell does, nor do three give three times as much. The current does not increase proportionately to the applied voltage. Therefore we reason that whatever is between P and F acts like a resistance but not like a wire resistance.
Then, we try another experiment with this hidden audion. We connect a battery to G and F, and note what effect it has on the current which our other battery is sending through the box between P and F. There is a change of current in this circuit, just as if our act of connecting a battery to G-F had resulted in connecting a battery in series with the P-F circuit. The effect is exactly as if there is inside the box a battery which is connected into the hidden part of the circuit P-F. This concealed battery, which now starts to act, appears to be several times stronger than the battery which is connected to G-F.
189Sometimes this hidden battery helps the B-battery which is on the outside; and sometimes it seems to oppose, for the current in the P-F circuit either increases or decreases, depending upon how we connect the battery to G and F. The hidden battery is always larger than our battery connected to G and F. If we arrange rapidly to reverse the battery connected to G-F it appears as if there is inside the box in the P-F circuit an alternator, that is, something which can produce an alternating e. m. f.
All this, of course, is merely a review statement of what we already know. These experiments are interesting, however, because they follow somewhat those which were performed in studying the audion and finding out how to make it do all the wonderful things which it now can.
As far as we have carried our series of experiments the box might contain two separate circuits. One between G and F appears to be an open circuit. The other appears to have in it a resistance and a battery (or else an alternator). The e. m. f. of the battery, or alternator, as the case may be, depends on what source of e. m. f. is connected to G-F. Whatever that e. m. f. is, there is a corresponding kind of e. m. f. inside the box but one several times larger.
We might, therefore, pay no further attention to what is actually inside the box or how all these effects are brought about. We might treat the entire box 190as if it was formed by two separate circuits as shown in Fig. 97. If we do so, we are replacing the box by something 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.