On pages 77 and 85 of the 1924 edition of Robison’s Manual of Radio Telegraphy and Telephony there appear statements of Fleming’s “Right Hand Rule” and “Left Hand Rule” for use as methods of remembering the directional relations that exist between the mutually perpendicular flux, motion of conductor and induced e.m.f.
These rules are handed down from so eminent an authority and their use has extended over such a long time that any criticism of them amounts almost to heresy. In lieu of criticism, therefore, let it be stated that there are those who have absolutely no memory for detail but who can learn only by acquiring a sort of mental picture of what happens. As for these particular rules: in that part of Naval Academy history in which the writer participated, the one that could remember those rules—and get them straight— was promptly crowned King of the May; the rest helped decorate the Pole. Almost everybody used the wrong hand or the wrong fingers!
Without attempting to write a thesis for a doctor’s degree, I ask the indulgence of my readers in allowing me to present a mental picture of inductive phenomena which has assisted me in determining the direction of flow of induced current. It is submitted that the idea has the merit of being a fairly close approximation of what actually happens.
In the beginning, it must be remembered that the so-called lines of force emerge from the north pole of a magnet into the air gap, and re-enter the magnet at the south pole. See Fig. 1. Also, when a uni-directional current is flowing in a conductor, the direction of flow of this current and the direction of the circular lines of force around the conductor bear the same mutual relation as exists between the threads of an ordinary right-handed screw and the direction of motion of the screw when turned or “driven.” See Fig. 2.
If, then, the lines of force are pictured, as Fig. 1, as a collection of arrows, the actual motion involved, whether it is the conductor or the flux itself that moves, may readily be fitted into the picture as it really exists. If a good stiff conductor moves quickly across a space already occupied by arrows or lines of force it is reasonable to suppose that the latter would be bent around the conductor before being actually cut. Picture them as bent, then, and the bent arrow and the direction of current flow will bear precisely the same relation as indicated in Fig. 2.
Suppose, for example, that in Fig. 3 a conductor is moved up ward, bending the arrows as indicated. The current in the conductor would be indicated as flowing into the paper. If the poles were reversed, the ux arrows would be reversed, and the current would then ow out of the paper.
It does not matter whether the flux moves or whether the conductor moves, for arrows will be bent in either case, and the direction in which they are curled around the conductor before being cut will in either case determine the direction of flow of the induced current, as indicated in Fig. 2. The inductive effects of intermittent direct currents are likewise apparent if it is remembered that, as the primary current increases, its magnetic field swells up, and the arrow while maintaining their unity of direction, move laterally outward from the primary conductor bending around the secondary conductor as they hit it. Conversely, the primary field collapses upon decrease of primary current, and the arrows are pulled in laterally toward the primary, bending the other way around the secondary conductor. Thus opposite e.m.f.’s are induced in the secondary as the primary current increases and decreases.
As a final test, let us apply this method in confirmation of Lenz’s Law. Suppose the primary circuit in Fig. 4 to be suddenly closed so that the primary current is momentarily increasing. The lines of force thus generated are as indicated by the solid arrows. Arrows A and B have a lateral motion and curl around the secondary as indicated. Following the principle shown in Fig. 2, the resulting induced current and its own flux are as indicated by the dotted arrows. Lenz’s Law states that when there is relative motion between a conductor and a magnetic field, the current induced in the conductor is such as to oppose the motion. Electrical inertia.