Britain's leading inventor, Sir Clive Sinclair, unveiled today a pocket television with a two-inch screen that he said will sell in the United States for under $100 and is far smaller in size and weight than Japanese models already on the market.

"You can consider this a transistor radio with a picture," Sinclair said at a press conference, holding the trim, black plastic unit in the palm of his hand.

Sinclair's vision is plainly of a world in which the low-cost mini-television is as common as the portable radio has become, "a one-per-person product," he said. After scoring a huge commercial success in the U.S. and elsewhere with a similarly inexpensive home computer, Sinclair's design emphasis is on simplicity, keeping maintenance as well as price to a minimum.

Sinclair, 42, who was also an innovator in the technology of Digital watches and pocket calculators, has emerged as Britain's most innovative entrepreneur. Earlier this year he sold about 10 percent of his company, Sinclair Research Limited, to private investors for $20 million, which he is now plowing into development of a cheap electric car.

The television has been six years in the making. After some frustrating delays last spring, the sets will go on sale immediately by mail order in Britain and Sinclair expects to have them available in the United States sometime after the new year. Other countries will follow. Once the factory in Dundee, Scotland is at full capacity he said, it will be churning out a million sets a year.

With few exceptions, the black and white units (color is still on the drawing boards) will accommodate transmission systems anywhere on the globe. They have slip-in flat batteries adapted by Polaroid from cameras that lasts about 15 hours, five times as long as conventional batteries and cost about $5 each. There are only two controls--on and off and tuning--and an ordinary radio-style aerial.

The sets are truly pocket-size. The weight is only 9.5 oz. compared to 16.12 oz. for the Sony "watchman" which is also about twice as large.

Sony, which beat Sinclair to the market--although he claims to have developed the flat-screen technology first--has just introduced a second generation set which will sell in the United States for $99.99. Casio, best known for calculators, is introducing a 2.5-inch screen set for about $300. And Seiko has been selling a television watch in Japan with a screen that is hardly more than an inch in diameter. But picture quality is poor, according to experts, and its price is between $350 and $450.

Although demand for those small televisions has been considerable, industry analysts say it has not as yet started to match the popularity of the omnipresent headphone stereos. So far buyers have tended to be men over 30 who use the sets for watching sports events.

By dropping the sale price so sharply--the initial cost will be about $110--Sinclair expects to spark an enormous consumer urge for a personal television.

It is because of that anticipated excitement that Sinclair has decided to use the unusual mail order approach, which he freely concedes tends to hold back demand. For the time being, he said his factory will be producing about 10,000 units a month, only half the number that Soney is turning out of its new Watchman models.