HYSTERESIS: HYSTERICAL AND OTHER

To all those who write in that "the technical parts of AtE are over my head" I promise not to explain how an electrical transformer works; I will Just liken it to a gearbox in an automobile. The power train transmitting power from the engine to the rear wheels goes through the gear box; in high gear, a small force makes the wheels spin fast, in low gear they exert a strong force as they turn slowly.

Power not only equals work per second, but also force times velocity. In high gear or low gear, the power remains the same (if, as usual, we throw out the finer points as an irritating nuisance): going up a steep hill, you sacrifice velocity for a stronger force; cruising on the freeway you maintain high speed with little force. A gearbox is a way to change the factors of a constant product to adapt to the load.

Which is exactly what an electric transformer does. The factors are voltage and current, the constant product, as in the gearbox, is the transmitted power. The linkage from "in" (engine) to "out" (power shaft) is not done by cogwheels, but by a magnetic field. The "in" side could be the transmission line from the power plant (high voltage), the "out" side a residential district, in which case the transformer lowers the voltage of the transmitted power.

But a gear box does not transmit all the power it gets from the en-gine; it uses (loses) some of it to heat the lubricant and the box. In the gearbox, the reason is plain old friction. In the transformer, it is a special kind of loss in the iron core transmitting the magnetic field, and if you need to drop an impressive word at a cocktail party, the name of the phenomenon is hysteresis.

The hyster comes from huster, the Greek for womb, as in hyster-ectomy, and I suppose the y stayed because that letter is a Greek upsilon, pronounced u as in full, but somebody must have goofed in transcribing it (as when the Russians write and pronounce Texas with a Russian x, which is kh as in Mikhail). Hysteria is "suffering in the womb," perhaps because it was thought an inherited disease, and what I am finally coming to is hysteresis, Greek for "a shortcoming" (more genetic damage?).

Magnetic hysteresis occurs in iron and other magnetic materials. When an electric current flows through a coil wound on an iron core, the iron is magnetized (becomes a magnet). But when the current is switched off, the iron is not demagnetized; some residual magnetism remains. To demagnetize it again, one has to pass a small amount of current in the opposite direction through the coil.

The process is shown in more detail in the figure. Tesla is the unit of something proportional to the magnetic field, and oersted is the unit of something proportional to the current. (I would not have brought up the units, but they were on the drawing, mis-spelled with capital letters.) We start with no current, no field, at the zero-zero origin, with the iron never before magnetized. Then as the current is increased, the magnetic field increases as shown by the dotted line: at first linearly, then more slowly, until the satu-ration point is reached at the upper right. Here all the atoms of the material are magnetized, and further increases of current have no more effect on the iron. This dotted "virgin curve" is not, however, retraced backward when the current is decreased again. The mag-netic field remains almost constant down to about 0.4 oersteds, then it decreases, but when the current decreases to zero, 0.8 tesla of the field remain. This residual magnetism or "remanence" is what makes permanent magnets (they are made of special alloys with high remanence). To demagnetize the metal the current has to be reversed and increased: near minus 0.2 oersteds the field has disappeared. This value is known as the "coercive force." When the reverse current is further increased, the reverse field increases as before, until saturation with the opposite magnetic polarity sets in. Reversing the current once again, the process is repeated along the full curve.

Transformers work exclusively with alternating current, so that the state of the iron moves round the curve, called a "hysteresis loop," 60 times every second (50 times on the other continents). If the current alternates between the same two levels in each direc-tion, as it normally does in AC operation, virginity is never restored, nor is any other point reached inside the loop.

For reasons to be given in a moment, hysteresis causes energy losses; but last month a way of curbing it was commercially released.