Sometimes 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. [Illustration: Fig 97] 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 as 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. The men who first performed such experiments wanted some convenient way of saying that if an alternator, which has an e. m. f. of _V_ volts, is connected to _F_ and _G_, the effect is the same as if a much stronger alternator is connected between _F_ and _P_. How much stronger this imaginary alternator is depends upon the design of the audion. For some audions it might be five times as strong, for other designs 6.5 or almost any other number, although usually a number of times less than 40. They used a little Greek letter called "mu" to stand for this number which depends on the design of the tube. Then they said that the hidden alternator in the output circuit was mu times as strong as the actual alternator which was applied between the grid and the filament. Of course, instead of writing the sound and name of the letter they used the letter [Greek: m] itself. And that is what I have done in the sketch of Fig. 97.