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COMPONENT PARTS OF A SOUND SYSTEM 443
take time at this point to explain just what an amplifier
does not do, as well as what it does.
We have just seen what it docs nut do. It does not make a weak current stronger. 'It docs use a weak current to operate a very delicate valve by means of which a much greater amperage is turned on or off and increased or decreased. The result is that the fluctuations thus introduced into the flow of the heavier amperage represent an exact or nearly exact reproduction of the current fluctuations of the original or input circuit.
The details of how this is done belong in another place. We merely note here (12) that the sound current flowing into an amplifier completes a normal electrical circuit back to its source just as any other current does. (13) Only, while in transmission, it actuates a valve apparatus in which it is enabled to exercise a very delicate control over the flow of current in a circuit of greater power.
Let us agree to call the input circuit P and the heavier circuit Q. Then if further amplification is needed Q can be used to operate another valve and thus control a third, still more powerful circuit, which we will call R. And R, in turn, can be used to operate the trigger controlling a yet heavier current, and so on. The process can be and often is repeated six or seven times. Nevertheless P, Q, R and all the other speech currents in an amplifier each return to their own source unchanged, in the normal electrical way. The only essential difference between amplifier wiring and any other electrical wiring is that each speech current, as it flows around its circuit, is made to work against a valve in which it controls a stronger current.
(14) The valve, as it is called in England, is, of course, the tube. Our own word "tube" really means nothing except that the first devices of the kind, about twenty years ago, were built into long, slender tubes of glass instead of the bulbous glass or metal envelopes of the present day. If the reader cares to make a habit of thinking of amplifying tubes as valves, his understanding of their action may perhaps be clarified.
A modern amplifier contains many other appliances