The temperature was soaring. So was demand for electricity.

At 2:55 p.m., Florek picked up a telephone and broadcast an urgent message to the control rooms of 11 power companies from New Jersey to Washington:

"We are now at maximum emergency generation," said Florek, a dispatcher for the PJM Interconnection Association, the power pool that controls the flow of electricity to 22 million people in five mid-Atlantic states.

The message to utilities: Give us everything you've got; the next step is to start shutting off customers.

In downtown Philadelphia, at the Peco Energy Co. headquarters, Florek's voice crackled over a loudspeaker. An order went out to Peco's Conowingo Hydroelectric Station on the Susquehanna River to let more water flow through its generators.

In New Jersey, Public Service Electric & Gas Co. fired up the last of its combustion turbines, the jet-powered generators that act as the utility's last emergency power reserve.

At PJM headquarters in Lower Providence Township, dispatcher Bill Fox was on the telephone to the Potomac Electric Power Co. in Washington, which told him that two turbines were not yet fired up.

"How many megawatts are they?" he asked brusquely. "Bring them on."

The region's electrical utilities were pushing their limits on Wednesday afternoon as millions of air conditioners strained under the sweltering heat. By early afternoon, the 11 utilities in the power pool had already surpassed their forecast output for the day.

They had to sweat out the last hours before reaching the moment of truth, 5 p.m., when electrical demand typically peaks during a summer day.

"We don't know how much demand is out there," Jeff Williams, a PJM strategist, said at 3 p.m. "Is the whole load on now? There's a bunch of stuff out there that could kick on if the temperature goes up one degree more."

In the end, records fell. At 5 p.m., PJM utilities sold more power - 48,660 megawatts - than at any other time. But with the help of massive inflows of electricity from the Midwest and New York, the region's power system stayed intact, though a few nerves were frayed.

"It's a new high load; you're not sure what will happen," said Bruce M. Balmat, an engineer who manages PJM's performance department.

To most consumers, it was as if nothing happened. No transmission lines melted down. No brownouts occurred. There were no major failures among the 540 generating units that produced power in the crunch.

No customers were shut off on Wednesday, though a few large customers were curtailed yesterday as the system once again flirted with record output.

Most customers will not realize how much energy was consumed this week until their electric bills arrive in the mail later in the month, reflecting tens of millions of dollars spent mostly to cool hot air.

At 5 a.m. Wednesday, while the temperature in Philadelphia was 79 degrees, the PJM network was experiencing its quietest hour of the day.

As the sky grew light, the 11 utilities were generating 28,180 megawatts of power for their slumbering customers - enough power to keep 22 million window air-conditioners running.

Over the next 12 hours, the temperature would rise to 98 degrees in Philadelphia, and the PJM utilities would nearly double their output, adding on 20,000 megawatts, the equivalent of bringing 20 giant nuclear generating units on-line in less than half a day.

By 10 a.m., much of the system's largest power plants were operating at full tilt. PJM also was buying 5,300 megawatts from neighboring systems, primarily Allegheny Power Systems to the west and the New York Power Pool to the north.

At 11 a.m., Peco began opening the valves at its Muddy Run hydroelectric plant, a reservoir on the bluffs along the Susquehanna that it had filled overnight with water pumped from the river. Three hours later, Muddy Run was generating its maximum output, 860 megawatts.

By midday, Peco was turning on most of the 33 combustion turbines throughout its service territory. Eight turbines at its Croydon Station in Bristol went on between noon and 2 p.m., supplying about 304 megawatts. At its old Richmond plant, two turbines went on at noon and would remain on until 8 p.m., when demand fell off.

A good portion of the bill that electric customers pay each month is to finance the cost of generators to meet peak demand. PJM requires its members to provide 20 percent capacity above their forecast peak demand.

PJM is the world's fourth-largest power pool, dwarfed only by the national systems in France and England and the Tokyo Electric system. It was formed in 1927 when Peco, PSE&G and Pennsylvania Power & Light Co. formed an interconnection to sell each other power.

The Pennsylvania-New Jersey-Maryland Interconnection - later shortened to PJM - took on its modern form in 1956 with the addition of Baltimore Gas & Electric Co.

The nation's power pools gained more authority after blackouts in 1965 and 1967 darkened New York and New England because utilities had failed to correct an imbalance in the transmission system, which caused a cascade of failures.

PJM procedures now call for systematic "load-shedding" if consumption exceeds the power pool's ability to generate power or its ability to buy it from other areas, such as the Midwest, where the shrinking steel industry has left utilities with excess capacity.

The procedure calls for utilities to first call for conservation and then to curtail service to large customers who have agreed to have their power interrupted in exchange for a lower rate. By cutting interruptible customers, the system can reduce demand by about 1,500 megawatts, Balmat said.

The utilities also can reduce voltage by up to 5 percent, which would reduce demand by about 700 megawatts.