International projectionist (Jan-Dec 1935)

"7 Relation of Electron Flow to Current Flow DO THESE STATEMENTS CONFUSE YOU? "Electron flow and current flow are the same; therefore, current flows from negative to positive." * "The direction of current and the direction of electrons are opposite." -k "The movement of the electrons through the circuit is current flow. The direction of the electron flow is from negative to positive . . . The directon of current from a D.C. generator is from the positive brush through the external circuit to the negative brush . . . When the direction of the current is known, the direction of the resulting flux may be determined by the right-hand rule." ~k "Although we speak of the current as flowing from positive to negative of the circuit, the electrons (which really are the current) flow from negative to positive." THE substance of each of the accompanying introductory statements was taken from as many text and reference books on radio theory. To the technically trained they are understood and accepted from the viewpoint adopted by the author of the book; but to the person just starting out to master the principles of electricity and radio, and studying from the numerous references at hand, this apparent contradiction is often confusing. Of course, there is always an explanation added stating that the mix-up is due to the fact that the direction of current was assumed to be from positive to negative before the action of electrons was known. But does this statement clear up the matter to the new student? Experience has shown that in the majority of cases it does not, and the question that almost invariably comes back is: If the electron flow is the same as the current flow, and electrons flow from negative to positive, how can the current flow from positive to negative? The following explanation and analogy has proven in class work to almost always settle the question in the student's mind, and it is given here for precisely the same reason. Direction of Current Flow When the assumption that current flowed from positive to negative was first made, electricity was used primarily for such purposes as lighting, heating, ringing bells and running motors. The effects produced by electricity were what we were especially interested in, and since no means were at hand to definitely prove which way the electricity was flowing, it was assumed to be from positive to negative, and this flow was called an electric current. This direction was probably decided on because the prevailing theory at that time likened the electric current flowing in the wire to the flow of water through a pipe. And since water flows from a high level to a low level, and we generally assume positive to be high compared to negative, the assumption for the direction of the current was the most logical one to make. From I.P., May, 1933 By N. H. RANDALL When electrons were discovered, however, it was soon proved that they were the only things actually traveling in the conductor, and that they did not move from positive to negative but from negative to positive. But this discovery did not change the effects produced by the electricity as it flowed through the various circuits. Lights still burned as before, heaters still got hot, bells continued to ring and motors continued tc rotate without any difference in their action whatever. Apparently, then, from a practical standpoint, the knowledge of electron flow made no difference. But it did help from a theoretical point of view, because it enabled us to more accurately predict how certain circuits would act. During the time that the fluid theory was in use, many rules and formulas were evolved to fix, for example, the relative direction of magnetic flux and the direction of the current causing it; or the direction of rotation of a motor when the direction of the currents through the armature and field circuits is known. All of these rules were based on the assumption that current flowed from positive to negative, and by the time the electron flow was found these rules had become so thoroughly fixed in electrical science that it was not practical to change them. How, then, shall we retain these rules and at the same time state that electron flow is opposite to the direction of current? By continuing to assume that current flows from positive to negative, of course. So far, so good. But now comes the statement that the electron flow and the current flow are one and the same thing, but opposite in direction to each other. To help to visualize what is going on in the circuit let us consider this illustration. When it is said that a current flows along a wire from positive to negative, what is really happening is that the electrons are moving from atom to atom of the wire in a direction from negative to positive. They move in this direction because the electrons are themselves a negative charge and therefore the positive charge toward which they move atttracts them, while the negative charge from which they move repels them. Now, as they jump from atom to atom they leave a positive charge on the atoms they just left, while the positive charges on the atoms to which they have just jumped have been neutralized by their own negative charges. In other words, the position of the positive charge in the circuit moves from in front of the electron to behind it as the electron moves along, and the change of the position of the positive charge is caused by the motion of the electron. Now, a positive charge on an atom is caused, not by adding anything to it, but by taking an electron away from it. Therefore, nothing has moved in the direction of the positive charge except a condition, which condition is caused by the movement of the electron in the opposite direction. To further illustrate this point take the following analogy, which may appear rather silly but nevertheless usually gets the idea over. Suppose we have a row of rocks with a frog sitting on each. If the frog on rock number one jumps to the bank his rock becomes vacant. Frog number two then jumps to rock number one and as he does so the vacant condition moves from rock number one to rock number two. This is in the opposite direction to that taken by the frog. If number three frog now jumps to rock number two, the vacant condition moves in the opposite direction to rock number three. In the whole picture what has moved? Nothing, actually, except the frogs. But due to the motion of the frogs the vacant condition moved in the opposite direction. Yet the vacant condition is nothing but a lack of a frog on a rock. Now to tie this up with the electrons and the atoms. Let the frogs represent the electrons moving from negative to positive, and let the vacant condition of the rocks represent the positive charge on the atom moving from positive to negative, or opposite in direction to the (Continued on next page) [25]