MARIANGELA LISANTI, a senior at Staples High School in Westport, sparkled among some of the brainiest students in the nation. There she was, presenting her research on quantum theory at the second annual Siemens Westinghouse Science and Technology Competition final in Washington, earlier this month.

And the panel of judges was buzzing. Even Lisa Randall, a physics professor at the Massachusetts Institute of Technology who had won the prestigious Westinghouse competition in 1980 and was the keynote speaker, heard about 17-year-old Mariangela. Scientists and university professors who were judging the students' independent research told Dr. Randall about Mariangela shortly after she arrived.

''She had been singled out as being extremely impressive,'' Dr. Randall said.

In fact, Dr. Randall recalled colleagues telling her, ''There's a girl in this competition who doesn't know how smart she is.''

Mariangela won the individual national prize -- a $100,000 scholarship to the college of her choice. A $100,000 team prize was awarded to three students at the North Carolina School of Science and Mathematics in Durham. There were about 600 entrants.

The Siemens Westinghouse competition started last year and is open to high school seniors. Siemens AG is a German electrical engineering and electronics company, which bought a division of Westinghouse that designs and builds power generators. The Siemens Foundation started the Siemens Westinghouse Science and Technology Competition in 1998 to help students who are gifted in science math and technology further their secondary education.

The Siemens contest is a separate competition from the Intel Science Talent Search, which was sponsored by Westinghouse from 1942-1998 and is more widely known.

Mariangela won Siemens' top individual prize after demonstrating the lightweight device she designed at Yale over the summer and built for about $35. Her invention measures the voltage as the electricity is conducted across a gold wire as thin as a nanometer (one billioneth of a meter).

Mariangela's best friend at Staples, Gauri Shah, said Mariangela is as warm, sweet, and bubbly as she is brilliant.

''Oh, yeah, I won the Westinghouse,'' Mariangela told Gauri after flying home from Washington. ''She's very modest,'' Gauri said.

And busy. Mariangela helped Staples place third in the region in the National Engineering Design Challenge, an annual competition that asks high school students to invent a device that meets a particular social need, such as a page turner for people who can't use their hands. Mariangela founded a team at her school so that Staples could enter that contest. She's captain of that team, a member of the National Honor Society, and the first violinist in her school's symphonic and chamber orchestras.

With all of Mariangela's activities, she often goes to bed at midnight and gets up at 5 a.m. to complete her homework. And, Mariangela has been so preoccupied with the Siemens and upcoming Intel Science Talent Search competitions that she's bouncing around like a charged atom to complete her college applications, which include Harvard, Yale, Princeton, Cornell, the University of Virginia, and the California Institute of Technology. She said she would like to be a research professor some day. Physics is her favorite subject.

Mariangela has a 1,550 combined Scholastic Aptitude Test score. She is one of 24 students in the country who received this year's Siemens Awards for Advanced Placement (a $3,000 scholarship) for posting the highest scores on several advanced placement math and science exams.

Mariangela's mother, Anna Lisanti, knew that she had a very bright and curious daughter. Mariangela asked ''Why?'' so often while growing up that her grandmother jokingly nicknamed her ''Miss Why.''

When Mariangela wanted to do scientific research outside of school, Mark A. Reed, chairman of electrical engineering at Yale, became her mentor. His wife, Elizabeth, had introduced him to Mariangela through a friend.

Mr. Reed gave Mariangela a list of 5 to 10 topics to study, from the physics of small silicone transistors to the fabrication of computer chips. Secretly, he said, he hoped that Mariangela would choose to work in the area of measuring quantum phenomena -- such as the electronic signature of the arrangement of atoms -- in a small electronic system.

Mr. Reed said he thought that quantum measurement was the most challenging topic and one where the least amount of research had done. ''You're pushing the limits of what you can measure on a small scale,'' he said.

Mariangela, who said she loves to take on challenging projects, chose quantum measurement even though she didn't know very much about the field. And, she spent the past summer at Yale building a lightweight contraption that revealed thousands of different arrangements of atoms on an electrical wire a thousand times faster than the arrangements had been shown before.

''There's got to be an easier way,'' Mariangela said she thought. And, she spent a week studying at Yale until she figured out how to do it. She even worked in the lab on her 17th birthday last Sept. 2.

''I want to finish,'' Mariangela told her mother.

She not only finished the research, but she astounded the Yale graduate students working alongside her and Mr. Reed. Measurements were spitting out so rapidly onto her oscilloscope that she was forced to learn computer programming to control and calculate the streaming data, Mr. Reed said.

''What she did is so phenomenal,'' Mr. Reed said. ''Someone might make an apparatus. Someone might take a measurement. But what she did was she developed the technique, she developed the apparatus, she did the measurements and she analyzed them in a month and a half. I would have considered doing the measurement a success. But she went on to improve on the thing and polish up the data. She did in a few months what it often takes university students years to do.''

Mariangela's parents, Anna and Anthony, are Italian immigrants. Mr. Lisanti is a salesman. Anna Lisanti was a secretary. Both have trouble understanding not only where their daughter's scientific gift came from but also the magnitude of her creation. ''But exactly what does this thing do?'' Ms. Lisanti would ask.

Mr. Reed isn't sure how Ms. Lisanti's research will be applied in technology.

''She has done some very nice, beautiful fundamental work on what happens in the quantum world,'' she said. ''Someone will learn from that and build upon it.''

Takhee Lee, a postdoctoral fellow at Yale, said he was very surprised watching Mariangela tackle the field of quantum mechanics. ''I never heard of a high school student doing this work,'' Dr. Lee said.

Photos: Mariangela Lisanti, 17, a senior at Staples High School in Westport, won a $100,000 scholarship for research on quantum theory. Her invention, which she built, measures voltage as electricity is conducted across a gold wire as thin as one billionth of a meter. (Photographs by C. M. Glover for The New York Times)