If you happen to be one of the dozens of people who leave the downtown Martin Luther King library each day without checking out your books, prepare yourself for a shock today.

You may hear bells and encounter a locked exit turnstyle - if all goes according to schedule.

The D.C. public libraries have installed a new "tattle tape" system at the King library as part of an effort to control the loss or theft of materials.

The director of the D.C. libraries, Hardy R. Franklin, told a Senate subcommittee this week the city system has been unable to determine just how many books are missing from the libraries, but he said nationally an average of 10 to 15 per cent of all books are lost or stolen. Library staffers indicate the loss from Washington's downtown library is probably higher.

Franklin said before the Senate District Appropriations subcommittee Wednesday that the tattle tape system and a new computerized circulation control system being installed will allow the library to make an inventory of its holdings.

"To inventory the nearly 2 million pieces of material we have in the system would mean we would have to close the library several days," Franklin told the reporter later. Library staff personnel would have to go to the shelves, pull off each book and compare the titles with shelf lists, he said.

The tattle tape system is activated by a small strip of magnetic tape inserted somewhere in a book, on records, cassettes or recorders. WHen the borrower checks the material out the tape is desensitized by a device at the counter.

Franklin said the first materials being protected by the system are reference books and new books. About 122,000 of the 600,000 volumes at the Martin Luther King library have been tattle-taped, and when the job htere has been completed, Franklin said, work will begin to install the system at all the libraries' branches.

Franklin said the tattle tape system equipment cost $25,000, and he estimated it would pay for itself in several years. He said it was too costly to retrieve books by conventional methods, such as telephoning delinquent borrowers.

"We simple do not have the funds to hire people to go out and ask for the return of materials," he said.

The first phase of the computerized control system was installed at the King library in January, and the second phase - to be completed sometime in the fall - will include four branches. Within three years, the entire D.C. public library system will be computerized, Franklin said.

A $170,000 grant under Title I of the Library Services Construction Act alloweed the District to buy the mini-computer necessary to start the system, and individual cathode ray computer terminals will be added at the branches at a cost of less than $9,000 each.

A computer screen shows the librarian in seconds where any book is located, records loans and even automatically produces and overdue notice.

The computer and tattle tape systems are not unique, but when completely installed, they will represent one of the most comprehensive public library protective systems in the area, Franklin said. He noted that the Washington Technical Institute has tattle tape, and that several libraries including John Hopkins University and Baltimore County, are computerized.