The Patent and Trademark Office of the Commerce Department wants to increase its fees to such an extent that the average inventor may no longer be able to afford the cost of a patent.

Patent and Trademark Commissioner Gerald J. Mossinghoff has proposed fees that could cost an inventor $5,150 to obtain a patent and keep it for 17 years.

The small inventor will not be the only one to suffer if the proposed fee increase is approved by Congress. So will the registered patent attorneys and agents who depend on inventors for a living, as well the many typists, patent searchers and drafters of patent legislation.

The small inventor--that is, the man with an idea but little money--has often been the founder of large corporations.

Will inventors following in the steps of Cyrus McCormick, who invented the reaper, Elisha Otis, who invented the safety elevator, Willis Carrier, the air conditioner, Kellogg and his corn flakes, and Evinrude with his outboard motor, disappear from the American scene, simply because they could not afford the cost of a patent?

The proposed $5,150 fee includes a so-called "maintenance" fee that will be imposed for the first time starting Oct. 1. If an inventor fails to pay this fee he may lose his patent. Not many people can afford this with our ever-increasing cost of living. The "maintenance" fee will be due in installments; the first three years and a half years after the granting of the patent, then again after seven and a half years, and again after 11 1/2 years. The total maintenance fee is estimated to be at least $2,400.

The minimum cost of obtaining a patent today is $2,125. With the increased fees the commissioner is seeking, including the maintenance fees, it will cost an inventor at least $5,150.

If Congress permits such an increase, no doubt there will be a big drop in the number of applications for patents filed.

Congress rejected a PTO request in 1980 for a 60 percent cost recovery increase because even that would have been too burdensome on small business and individuals. It follows that an increase of more than 100 percent today is even more burdensome.

At present, 22 percent of all patents issued go to individual inventors. If Congress grants the Patent and Trademark Office its request, the patent system will become virtually an exclusive club for wealthy inventors and domestic and foreign companies.

The patent office said Friday that inventors no longer will receive hand-written, pen-and-ink letters telling them whether their inventions are acceptable.

Instead, patent examiners will compose their replies on electronic word processors equipped with a memory bank of 50,000 terms and stock phrases. About 80,000 letters a year are involved.

The patent office said that the handwritten replies actually were a fairly recent development in the history of the patent system, which dates from 1790 when it was in the State Department and Thomas Jefferson was a patent examiner.

A spokesman said handwritten replies were begun in 1970 as a cost-saving measure. Before that, typewriters were used.

Recent U.S. patents granted to residents of Maryland and Virginia:Maryland

Barbara S. Zashoff of Rockville. Controlling Ink Diffusion in Watercolor copying. Patent No. 4,321,870. 9 Claims.

Stephen B. Leighton of Silver Spring and William H. Boyd of Hyattsville. Everting Tube Device with Relative Advance Control. Patent No. 4,321,915. 11 Claims.

Anthony Chaconas of Greenbelt. Nail Polish Remover. Patent No. 4,321,936. 6 Claims.

Paul L. Fleming of Rockville and Thane Smith of Middletown. Planar Transmission Line Attenulator and Switch. Patent No. 4,322,695. 5 Claims.

John S. Snyder Jr. of Monrovia. Reliability-Weighted Analog Threshold Decoder. Patent No. 4,322,848. 14 Claims.

Nedim Savas of Wheaton. Napkin Ring Display Unit. Design Patent No. D263,535. Term of 14 years.

Joseph E. Adkinson of Chevy Chase. Cocktail Table or Similar Article. Design Patent No. D263,536. Term of 14 years. Virginia

Peter J. Danko of Alexandria. Supporting Structure. Patent No. 4,321,875. 18 Claims.

Clemens A. Iten of Staunton. Package for Securing Slotted Safety Razors. Patent No. 4,322,002. 5 Claims.

Everett C. Grollimund of Midlothian. Web Transport Apparatus. Patent No. 4,322,043. 12 Claims.

William T. Hynes of Falls Church. Batting Practice Device. Patent No. 4,322,075. 12 Claims.

Edward D. Furrow of Crimora. Replaceable Reinker for Inked Ribbon Cartridge. Patent No. 4,322,172. 10 Claims.

Larry H. Capots of Annandale et al. Identification of Materials Using Their Complex Dielectric Response. Patent No. 4,322,678. 10 Claims.

Charles M. Davis Jr. of McLean and Thomas G. Giallorenzi of Washington. Fiber Optic Accelerometer and Method of Measuring Internal Force. Patent No. 4,322,829. 9 Claims.

Claim: Defines the invention and sets the bounds of the property rights granted the inventor.

Copies of any of the above patents can be obtained from the Patent and Trademark Office for 50 cents each bvy addressing orders to the Commissioner of Patents and Trademarks, Washington, D.C. 20231.