The Wiegand effect, a phenomenon discovered in the 1970s, is the unusually useful behavior of magnetic fields in specially designed wire that outputs voltage.

This Wiegand wire, as it's called, is magnetic iron-alloy wire treated so that it forms a hard outer shell around a soft inner core.

External fields easily magnetize the outer shell, which also resists demagnetization, even when external fields are removed — a characteristic called higher coercivity. The soft wire filling behaves differently: It's not magnetized until after the shell gets its fill of magnetization.

Herein lies the magic: At the very moment that the wire's shell becomes fully magnetized, and the core is finally allowed to collect its own portion of magnetization, poof — both core and shell switch polarity. The switch generates significant voltage that can be harnessed for all kinds of sensing and motion applications.

His ladies sung to him

Before he had an oscilloscope to see pulses, it was Wiegand's perfect pitch that enabled him to listen through a loudspeaker to the magnetic pulses produced by his wire. John always referred to each wire as “she” and said that the wires sung to him.

Wiegand later met Milton Velinsky and together they formed Wiegand Electronics to develop product applications for the Wiegand Effect. Wiegand did the inventing and Velinsky did the promoting and selling.

How Wiegand wire is made

Ferromagnetic Vicalloy wire (made of cobalt, iron, and vanadium) is twisted and untwisted during increasingly aggressive coldworking under tension. Then the wire is hardened to hold its crystalline structure.

John R. Wiegand: The man

John Wiegand, inventor of his namesake wire was neither engineer nor physicist, but a musician by training. Born in Germany in 1912, Wiegand came to the U.S. in the 1930s and studied piano and choral conducting at the Juilliard School of Music, New York.

There, he became interested in audio amplifiers — and later, became an engineering assistant for magnetic amplifiers at the Bell Telephone Laboratory. In 1944, he began work for Sperry Gyroscope Company in Lake Success, N.Y., and then for a government contractor as a product developer of tape recorders.

In 1965, after becoming an electronics technician at Echlin Mfg. Corp., Branford, Conn., Wiegand began the relentless pursuit of magnetic research that led to his terrific discovery: Seven years later, on June 25, 1974, his bistable ferromagnetic wire was patented.

Wiegand wire placed in alternating longitudinal magnetic fields forms a hysteresis loop, and shell and core polarity switches cause large Barkhausen discontinuities. The amplitude of a typical Wiegand pulse depends on the sensor setup — so is somewhat independent of excitation field strength and orientation — but pulse width generally remains constant: Magnetic switching of Wiegand wire induces voltage across pickup coils lasting about 10 µsec.

Pulses, switching, and magnetization

Once Wiegand wire flips magnetization, it retains that magnetization until flipped again. Mechanisms and sensors utilizing the Wiegand effect must accommodate this retention.

In symmetric switching, alternating positive and negative magnetic fields of equal strength trigger the Wiegand wire. First, a saturating magnetic field aligns the core and shell polarities. Magnet movement causes replacement of this field with an opposite field of equal strength.

As the field intensifies, the Wiegand wire core switches polarity, and outputs a large voltage pulse. As the field strengthens further, the Wiegand-wire shell follows suit and also switches polarity — producing a smaller pulse of the same polarity (often miniscule on oscilloscopes compared to the pulse associated with the core's switch.)

Applications

The Wiegand effect is used for myriad applications. The alternating positive and negative magnetic fields that magnetize and trigger the wire are produced in three ways:

1. By magnets affixed to rotating or moving equipment (paired with stationary Wiegand-wire pickups)

2. Moving Wiegand wires (paired with stationary read heads), or

3. AC-generated fields (paired with Wiegand wires in varied setups).