For a few hours the other day Scott Widen of Bentonville Arkansas was a millionaire, at least on his computer screen. His 7,000 shares in Digital were quoted at $209 (€225) a share instead of $2.09. "I joked with my wife that we made $1.5 million," he said.

The mistake occurred as Wall Street underwent a revolution. For the first time since it was founded by 24 brokers under a buttonwood tree in 1792, the New York Stock Exchange has dispensed with fractions and moved to decimals.

"When we go to the grocery store, we buy things in pennies, nickels, and dimes, not eighths, sixteenths, and thirty-seconds," said NYSE chairman Richard Grasso, explaining the move. "It simply made no sense for the stock markets to still be operating on the premise of the Spanish doubloon."

The transition, which brings the NYSE, the American Exchange and the Nasdaq into line with the rest of the world's markets, went smoothly, though there were some embarrassing moments for the computerised Nasdaq as it took the first steps of a phased transition to be completed by the end of March. Mr Widen and several other small time investors became millionaires for a day because Nasdaq stock data vendors had difficulty switching to decimals.

The move is not just a matter of recording numbers in a different way. It shrinks the spread between a seller's asking price and a buyer's bid. The smallest difference allowed between bids and offers before now was one-sixteenth of a dollar (6.25 cents), a fraction referred to by Wall Street traders as a "teenie". Now the spread on the most actively traded stocks can be as low as a cent. The professionals responsible for matching buyers and sellers keep the difference, so theoretically investors should benefit from a smaller spread.

One of the most successful investors on Wall Street is Michael Mathie (33) who claims to have traded more than $8 million in securities in the last three years. His gross annual income approaches $1 million. As revealed in yesterday's New York Times, Mathie is inmate No 90T1282 in Elmira prison, New York. He trades from his cell where he is in the 12th year of a 10- to 30-year sentence for manslaughter.

Mathie is allowed to receive several publications like Barron's and the Investor's Business Daily, but does not have a telephone or cell phone, and has no access to CNBC, the cable TV network which serious Wall Street investors watch around the clock.

Inmates are forbidden to run a business from prison but retain a right to free speech, which Mathie exercises by calling his father collect from a prison pay phone to make the trades on his behalf.

Mathie began investing on Wall Street with $75,000 from a settlement won against Suffolk County jail after he was raped and sexually abused by the chief of internal security. One of his biggest killings (on the stock market that is) came when he noticed that a company called China Coinverge was rising sharply at a time when China's attempt to join the World Trade Organisation was in the news.

"It went from $1 to $10 and the next day it was $18, so I threw $50,000 at it and called back at 10.30 or 11 and it was at $70. So I cleared $150,000 and got my $50,000 back in two hours," he said.

"Our resident millionaire" as the guards call him has taken some bad hits, losing $750,000 in the value of his stocks when the market plunged in one day in August 1998. He helps other prisoners by paying for legal advice and DNA testing and giving hot market tips.

His favourite stocks include Corning, Juniper Networks, JDS Uniphase, AT&T and Yahoo - which he said paid for his $97,000 Dodge Viper car, which awaits him when he is released on parole possibly this year. He believes the market is still in good shape. "Definitely we're not headed for no recession, and if we do go into a recession, it's because of the media," he said.

When he gets out Mathie at least won't have to rely on Who Wants To Be A Millionaire to make him rich. This hit TV programme is however making money for Disney, which owns ABC television. Along with improvements in its theme park business, Disney was able to report a higher-than-expected rise in fiscal first quarter earnings yesterday, bringing some cheer back to Wall Street. Disney is one of the four most actively traded stocks (after Pfizer, America Online and General Electric). Its shares had fallen 44 per cent since last May. The film division went from loss to profit with a 15 per cent increase in revenues, helped by Remember The Titans and Unbreakable. A prisoner playing the stock market and making millions might be a good storyline for a movie, but who would believe it?