F. H. Richardson's bluebook of projection (1935)

CHAPTER XV] ELEMENTS OF ELECTRICITY There once was a theory that electric current moved from positive to negative. Today it is agreed that that theory was exactly 100 per cent wrong; that current actually moves from negative to positive. But the old idea, in spite of the fact that it was wrong, was useful, and still is. It helped create electric lights, dynamos, telephones, street cars, and a good many other very valuable electric appliances, and they worked; and when they stopped working men could fix them in accordance with the theory that current moved from positive to negative. It is a useful theory today. Any number of electricians and engineers,, thoroughly convinced that current flow is from negative to positive, nevertheless run down trouble in a circuit by starting at the positive end, merely as a matter of habit and because they find it handier to do so. Obviously, then, a theory doesn't have to be right to be useful. And for practical work, as apart from pure science, a theory that is useful, and can be applied successfully to practical apparatus, is all that is necessary. In many places in this book current is spoken of as moving from positive to negative, and any circuit spoken of in that way is one in which the projectionist will very probably find it handier — more convenient — to think about the action in accordance with the older idea. In the case of most electrical circuits, either theory will do. (1) But the idea that current moves from positive to negative, while convenient in the case of ordinary circuits, won't work at all in the case of vacuum tube circuits. With such apparatus it is absolutely necessary to think of current as moving from negative to positive, 428