A Georgetown University physics professor and two collegues have invented a machine that can combat one tool in the arsenal of international terrorists: the deadly letter-bomb.According to Dr. William Gregory, the so-called CALM instrument (a scientific acronym) "is accurate 99.94 percent of the time, registering a false alarm about one time out of 10,000."

The computerized, desk-top-sized CALM detects the electrical properties of an envelope's contents, a matter of considerable interest to multinational companies, world leaders and others who attract mail from radical or crazy correspondents. In 1973, a secretary at the British embassy lost a hand to a letter bomb and Gregory tells of a judge in the state of Washington who received a letter with a string hanging from the envelope with the words "Pull Here." The judge did and died from the resultant explosion.

For Gregory, the CALM is the result of a decision three years ago to begin devising practical applications for work done Georgetown scientists.

"A lot of the stuff you do just never goes anywhere, "he says. "It stops at the laboratory door and that wasn't too soul-satisfying."

So along with two fellow researchers, Gregory began applying for grants that could be used to invent commercially viable products. The team also devised a larger machine that identifies metals sight unseen; the so-called MIDSY can tell the make and model of a gun hidden in a box. Beyond the obvious security applications, Gregory hopes to refine the inventions so they may be used to scan the body, eliminating the need to use X-rays.

Patents have been granted the individual inventors, and should a device earn back Georgetown's expenses, the university will split future royalties 50-50 with the physicists. Gregory, awed at the complex patent and licensing regulations, recently completed studies to become a patent lawyer - "I spent so much time with patent attorneys that I figured they either had to learn more physics or I had to learn the laws," he says.

Future plans: to reduce the cost of CALM from tens of thousands of dollars. Meanwhile a prototype is in operation at an undisclosed government agency.