from positive to negative. Even today there are some scientists who insist that current flows from positive to negative, even though they admit that electron flow is from negative to positive. This confusion won’t disturb us at all if we realize that circuits can be traced either way. Starting at the positive pole of the battery in Fig. 8 and tracing through the circuit we can see that all the current from the source will have to pass through and divide between filaments before it reaches the rheostat. Tracing through from the negative to positive we observe that all the current must pass through the resistance before passing through the filaments. It doesn’t make any difference which we consider the correct procedure--the effect is always the same, that is, the rheostat provides a control over all the current flowing tin the filaments.
Now let us suppose that the rheostat is set to a mid-position, meaning that just half its resistance is included in the circuit, and that the filaments are glowing normally. What will happen if we place another tube in the circuit in parallel ? All the tubes will become dim--we have increased the load. But if we adjust the rheostat so that it offers less resistance to current flow we can bring all the filament s up to normal brilliancy. What we are really doing is re-distributing the energy in the circuit, decreasing part of the load (the rheostat load) to compensate for the added load of another filament.
In a Radio receiver anywhere from three to five circuits meet at the vacuum tube. In later lessons we shall study each of these circuits by itself, the plate circuit, the grid circuit, the screen grid circuit in the case of screen grid tubes, and two screen grid circuits in the Pentode. This may seem difficult at first sight--and it is true that a complete Radio circuit is by no means a simple matter--but if you bear in mind that no matter how complicated any network is, it can be resolved into simple circuits, each of which has its load, its source of e.m.f., and its connecting wires, we shall never have any trouble getting the full significance of a Radio “circuit” diagram.
There is just one more thing we must consider before we leave these very simple circuits. Look at Fig. 8 again. Suppose a metal bar or a piece of bare wire fell across the connecting point A and B. What would happen ? Naturally a path would be provided for current flow through the resistance and through A and B back to the source of e.m.f. And current would flow along this path because current always takes the shortest path.
