Henry: More about inductance and magnetic permeability – Electronic Products Word of the Week

An understanding of magnetism was critical to the advancement of our understanding of electricity and, by extension, our modern understanding of electronics. The American scientist Joseph Henry (1797–1878) discovered electromagnetic induction independently of and at about the same time as Michael Faraday (1791–1867) in England in 1835. Along with Edward Davy, Henry invented the critical electrical relay (a.k.a. telegraph relay), a switch controlled by electricity that repeats a weak signal received at the end of a long wire. Davy's relay used a magnetic needle that dipped into a mercury contact when an electric current passed through the surrounding coil. This relay made the telegraph practical, allowing a weak current to operate a powerful local electromagnet over very long distances. By the time of the Civil War (1861-1865), the telegraph could be used by President Lincoln as a reliable communication tool.

The unit henry (symbol H) is today the derived unit of inductance in the Systeme Internationale. It can be at the very center of electronic calculations. Wikipedia states, “If the rate of change of current in a circuit is one ampere per second and the resulting electromotive force is one volt, then the inductance of the circuit is one henry. Other equivalent combinations of SI units are as follows:

where many of these units of measure are themselves a who’s-who of the history of electronics [this is a link to a chart of common electronics units of measure and biographies of the people for whom they are named]:

A = ampere,
C = coulomb,
F = farad,
J = joule,
kg = kilogram,
m = meter,
s = second,
Wb = weber,
T = tesla,
V = volt,
Ω = ohm.”

Inductance is squarely inside the analog world, but today, engineers have figured out how to bring the principle into the digital world as well. Texas Instruments, for example, sell an inductance to ditital converter. The device “provides a sensing technology that enables low-cost, high-resolution sensing of conductive targets in the presence of dust, dirt, oil, and moisture, making it extremely reliable in hostile environments.” Inductive sensing technology “enables precise measurement of linear/angular position, displacement, motion, compression, vibration, metal composition, ideal for applications in markets such as automotive, consumer, computer, industrial, medical, and communications.”

So Joseph Henry’s discovery of inductance has come a long way and promises to go even further. Research out of Stanford University centers on photonic crystals — materials that can confine and release photons. To fashion their device, the Stanford team created a grid of tiny cavities etched in silicon, forming the photonic crystal. By precisely applying electric current to the grid they can control – or “harmonically tune,” as the researchers say — the photonic crystal to synthesize magnetism and exert virtual force upon photons. The researchers refer to the synthetic magnetism as an effective magnetic field.

So, you see Joseph Henry’s work goes on, almost 200 years after he began.