It used to be that Paul Jones, librarian at J.E.B. Stuart High School in Falls Church, felt "like a cop," he says, standing at the library door to check bookbags and under students' arms for some of the 800-odd editions that disappeared from his literary domain each year.

Then last year along came an elctronic device called the Spartan system (a trade name) that sounds an unobtrusive "beep" when a student attempts to carry off a book that has not been properly checked out.

The sophisticated security system, which is costing Fairfax schools $2,955 this year for a test at Stuart High, has kept library book losses to 49 at the Falls Church school since it was installed in March, Jones said.

Aside from improving Jones' self-image, allowing him to be "friendly and responsive instead of a watchdog," the device so far has saved the school system nearly $7,000 - the total replacement cost of books the library would have expected to lose without the system.

The savings now go to buying new books rather than replacing old ones.

Jones said the state requires a high school library to stock at least 10 books per pupil, and estimated Stuart's collection at 17,000 books.

"Most students don't take unchecked books from the library with the intention of stealing them," Jones said. "Maybe they don't want to be bothered having to return them on time because they have an assignment to finish first, or they don't want to wait in line to check them out, whatever.

"The problem is that very few follow through with their good intentions. After a while, they're too embarrassed to bring them back and they trash them. So there goes another book."

He said that recently a former Stuart student, who just graduated from college, brought in a brown paper sack full of Stuart library books he had used five years earlier.

"I just told him I was glad to get the books back," Jones said. "It's pretty hard to explain losing $8,000 a year on a library operation to the taxpayers paying for it."

The detection system, which gives results Jones is proud to explain to tax-payers, works like this:

Strips of magnetic tape are fastened inside the spine of each book in such a way that its presence is almost undetectable. When a student checks out a book, the librarian places it in a console, about the size of an electric typewriter, which "sensitizes" the tape with an electromagnetic impulse. Then the student passes through a screening device just inside the library door.

An unsenitized book triggers the screening device, which emits a hightoned "beep" and causes an aluminum bar, that serves as a gateway, to lock.

Some students said they had a field day with the system when it first went in. Many tested it to see if it really worked by deliberately trying to carry out unchecked books. Others ripped out magnetic tapes they could find in books and taped them to other student's backs to watch the alarm system work.

"It sort of became a competition to see who could get through without getting caught," said Stuart junior Gleen Hollowell. "But everyone's pretty well given in and accepted it now."

He added that his watch once caused the screener to emit a false alarm.

Several students admitted their skepticism of the system at first, because they felt it an unnecessary restriction and an affront to their honesty.

But now, like Hollowell, they agreed that if will cut down on book losses "it'll be all right."

Jones said the system now sounds only once or twice a week, mostly because students have forgotten to check out the book. Students throw joking, reproachful looks at the young people who get caught at the gate.

"We've got a high peer standard here at Stuart," Jones said. "Maybe at a school where kids were out to tear up the place this system wouldn't work."

Stuart's system was installed through a lease-purchase arrangement that allows that school to purchase it after five years at a total expense of about $10,000.

Edith Ashworth, library services supervisor for the school system, said Stuart is expected to keep the system next year, with similar systems probably being installed at two other schools.

Book losses at Fairfax high schools range from 3 to 10 percent a year, with the highest number of losses last year at Marshall High School, Ashworth said.

The decline of large book losses in Jones' library not only makes his life easier, but causes him to look forward to building a better library.

"In three or four years we'll have a significantly upgraded library," he said, "because we're finally able to concentrate on making it that way."