"Trading was orderly, and the markets handled the record volume in stride," the Securities and Exchange Commission said in a statement.
Until Monday, the NYSE had never been halted because of market conditions. It was suspended on such occasions as a 1981 power failure, the 1981 assassination attempt on President Reagan and the 1963 assassination of President Kennedy, Johnston said.
Johnston would not conjecture about how the market would have performed Monday had there been no circuit breakers.
Some market professionals thought the halts might have worsened the situation, however, by creating a sense of urgency among sellers.
"I think [they] accelerated the decline," said Charles Bass, manager of the Nationwide Fund, a growth-stock mutual fund.
"When the market was down 300, people were accelerating their selling because they were afraid that the market was going to close before they got their trades in," he said.
Volume on the NYSE surged after the half-hour break, to around 800,000 shares per minute from less than half that amount.
However, the increase in trading doesn't prove that there was a selling panic, said Lawrence Harris, finance professor at the University of Southern California's Marshall School of Business.
"Remember, there was 30 minutes with no trading, but orders continued to flow in," he said. The orders that built up during the trading halt had to be executed along with the ones that arrived after trading resumed, boosting the volume, he said.
During the first halt in trading on the Pacific Exchange, many traders took time to order sandwiches, eat lunch, put their feet up and relax for 30 minutes.
But as the market bell sounded again, some traders became edgier and more frantic, resulting in some shouting, uncharacteristic for the usually quiet exchange.
"It's a meltdown, baby," yelled one trader, as the blinking lights signaling dropping stock prices seemed to fill the electronic screens at his post.
A loudspeaker warned traders: "Please be advised that if the Dow drops 550 points the exchange will be closed for the rest of the day." Most traders seemed relieved when that level was crossed, the final bell rang and they could go home early.
"We're in uncharted waters as far as these kinds of circuit breakers," said Rad Artukovic, a Pacific Exchange trader, who calmly crunched on cut carrots and celery as the market dropped. "So far, it seems they worked in that they gave people time to calm down."
