Walter Jack Cunningham, professor emeritus and former chairman of the Department of Electrical Engineering, died Jan. 7 of an illness of several months at his home in Hamden, Conn. He was 86.

Cunningham taught at the University for 42 years and authored the textbook “Introduction to Nonlinear Analysis” and a history of engineering at Yale. His textbook has been translated into several languages.

Paul Fleury, dean of the Faculty of Engineering, said Cunningham’s work remains essential to the field of engineering.

“His own research on nonlinear analysis and nonlinear equations served as a model for generations of engineering students, not only here but around the world,” Fleury said.

Born on Aug. 21, 1917, in Comanche, Tex., Cunningham led a life of service. He helped train military officers in radar theory during World War II, volunteered to give talks at the Eli Whitney Museum, and judged Connecticut high school science fairs.

Cunningham was famous among his students for dealing with “abstract-sounding” topics in demonstrations that illustrated the results of equations in a simple way, Peter Schultheiss, professor emeritus of electrical engineering, said in a University press release.

“He preserved the connection between theory and reality which is the essence of engineering,” Schultheiss said.

Richard Barker, professor emeritus of electrical engineering and applied physics, said Cunningham taught him before his own appointment as Yale faculty member. Barker said his office was located three doors from Cunningham’s, and he would often stop to talk with his former professor.

“With Jack you didn’t want to carry a nothing-conversation,” Barker said. “You needed to have something to say because you didn’t want to waste his time.”

Barker, who enrolled at Yale after serving in the Navy, said he was grateful to Cunningham for helping him improve his writing style, which at first reflected his “standard Navy English.” Barker said he remembered giving Cunningham a two-page thesis introduction that Cunningham returned two days later filled with red ink corrections.

“I spent two or three months doing nothing but bringing myself up to date on his complaints of my English. For that I praise him tremendously,” Parker said. “He really built a fine background in his students.”

Barker said he admired Cunningham for his extensive knowledge and his ability to keep his teaching to the point.

“Jack was a guy who knew everything that he taught all the way to the bone,” Barker said.