APS News | This Month in Physics History

May 1962: James West and Gerhard Sessler Patent the Electret Microphone

The duo’s invention is found in everything today from hearing aids to recording equipment.

Published Apr 13, 2023
Gerhard Sessler and James West
Gerhard Sessler (left) and James West in their lab holding Teflon foil, with an electret microphone in the foreground, 1976. At left and right: Snippets from the duo’s electret microphone patent, filed in May 1962.
Nokia Bell Laboratories (Sessler and West); U.S. Patent Office, Patent 3118022 (patent)

It’s possible you’ve never heard of the electret microphone — but if you’ve talked on a telephone, you’ve definitely heard because of an electret microphone. Over 60 years after it was invented, it’s now found in nearly every consumer product that has a microphone, be it a recording device, talking toy, smartphone, or hearing aid.

The electret microphone traces its roots to 1957, when scientist and inventor James “Jim” West completed a summer internship at Bell Labs. Born in Virginia in 1931, his parents raised him to be “either a preacher, teacher, lawyer, or doctor” — not a scientist, West, now 92, recalls. Opportunities for Black scientists were slim, and to drive home the point, West’s father introduced him to several Black men who had doctoral degrees in chemistry but who were working at the post office.

Still, West wanted to know how the world ticked. This curiosity got him into some trouble along the way, like when he took apart his grandfather’s pocket watch and couldn’t put it back together. But it also led him to the physics department at Philadelphia’s Temple University, where on a bulletin board one day he spotted an advertisement for an internship at Bell Labs, widely regarded as a research powerhouse.

He joined a project in the acoustics department, where researchers were trying to determine how long it took the human ear to recognize two sounds. In other words, if you played two “clicks” very close together, “when do you hear the separate clicks?” West explains.

The project needed a new microphone and earphone to transmit these clicks, because the standard condenser microphones of the time did not produce enough sound pressure to generate such precise sounds at a hearable volume. These mics consisted of a capacitor made of a fixed plate and thin membrane diaphragm that vibrated when it encountered sound pressure. The vibrations then changed the connected circuit’s capacitance, creating an electrical current that could be amplified and heard. A bulky power source created the circuit’s polarizing voltage.

West got to work on a better version. He came across a paper in the journal Acustica that detailed how to build microphones and earphones using polystyrene as a dielectric (a material that stores rather than conducts charge), connected to a battery that was also attached to a piece of metal. West used these principles to create an earphone using metal-coated Mylar to which he applied voltage. The design solved for the limitations of the standard earphone — and the earphones worked great. West completed his internship with high marks. “I got a gold star,” he laughs.

But just a few months later, he got word that the earphones stopped working — they had only held a charge for about six months. He returned to Bell Labs and paired up with Gerhard Sessler, a German scientist at the company, to revisit the earphone and build a microphone along the same lines. They determined that, to maintain the earphones’ charge, they should have reversed the polarity of the battery over time — “our biggest mistake,” West says.

One day, the microphone shorted out as West and Sessler tested reversing the battery. Yet the mic somehow still produced sound. The duo had inadvertently turned the dielectric into an electret — a material with a quasi-permanent electric charge.

Though new to West, electrets were not new to science. The term, a portmanteau of “electricity” and “magnet,” was coined by Oliver Heaviside in the late 1800s, and Japanese scientist Mototarô Eguchi experimented with waxes and resins as electrets in the 1920s. West and Sessler set about searching for a durable material that could hold a charge for years.

James West speaks with a student.
Now in his 90s, James West teaches at Johns Hopkins University. “I love working with students and young people,” he says.
Will Kirk/Johns Hopkins University

“One of the benefits of Bell Labs was that you couldn’t lock your door,” West says — the place was intensely collaborative. The duo asked around the other departments, and as it so happened, “the invention of Teflon was only a few years earlier,” West says. The polymer seemed to be a good fit, and the two scientists painstakingly worked on setting up the right conditions for the Teflon to retain a charge. “We spent a long time figuring out how to trap these charges,” West says. Once they got it to work, they had a microphone that could remain charged likely for hundreds of years, no battery required.

West and Sessler’s patent for the microphone was filed on May 22, 1962, and electret microphones began rolling out in telephones and other consumer products later that decade. Today, they are popular for their compact size (smaller than a dime), low cost, and simple construction. The sheer ubiquity of the technology is staggering. As a rough estimate, more than two billion electret mics are made each year, West says. He only earned $1 from the patent and all the others he filed while at Bell Labs combined — researchers at the company didn’t profit from inventions they created as employees. But he doesn’t have regret. “I can’t go back and say, ‘Gee whiz,’” West says.

Some of Sessler and West’s colleagues at Bell Labs assumed they would quit and start their own company after the invention. But this was never in the cards for West. The research facility was a bona fide playground for scientists and inventors, West says. “Why would we leave?” In the 1970s, Sessler moved back to Germany and became a professor at Darmstadt University of Technology, and in the 1980s pioneered the creation of microphones using silicon micromachining.

West retired from the company in 2001 and became a professor of engineering at Johns Hopkins University. “I love working with students and young people,” he says. In addition to being a professor and researcher, West has long been an advocate for diversity in science and academia. Among other honors, he is a recipient of the National Medal of Technology, a member of the National Academy of Engineering, and an inductee in the National Inventors Hall of Fame. His name can be found on over 250 U.S. and international patents, and he’s still creating — one recent invention to come out of his research group is a digital stethoscope that can detect and monitor lung diseases like pneumonia.

“I do what I love to do,” West told The Baltimore Sun in 2018. “Curiosity has always been my motivation.”

Tess Joosse

Tess Joosse is a science writer based in the Midwest.

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