The purchase of a $40,000 Lockheed Mac 16 mini-computer has enabled the Coolidge Bank and Trust Company to offer a 24-hour banking service.

The "Cool-O-Mat" machine, as the computer is called, stands outside the Coolidge office on Mount Auburn St. and performs all the services normally done by tellers. It is part of Coolidge's "Cool Cash" system, a free checking account service. Approximately 2300 new accounts could be directly attributed to the "Cool Cash" system, Cathy Romeo, a customer service representative, said yesterday.

The "Cool Cash" system limits each account to withdrawals of not more than $100 per day. "Our security system is foolproof." David Meehan, assistant vice president of Coolidge Trust, said. "Everyone using the 'Cool-O-Mat' machine is issued a 'Cool Cash' card whose number is known only to the card's owner, the bank, and the machine."

Every transaction for the day is recorded on the card's magnetic strip which eliminates any chance of getting cash from other "Cool-O-Mats" if the limit is exceeded, Romeo said.

Meehan said that despite the labor saving potential of the "Cool-O-Mat", the system will not decrease the need for tellers. "We need tellers to prove the machine's transactions," he said.

Other Cambridge banks have yet to copy the "Cool Cash" system, although most seem to be leaning in that direction.

"There are gimmicks and there are gimmicks," James P. Dwinnel III, a vice president of the Cambridge Trust, said. "We have discussed this innovation at great length and if a vote were taken today, I would be in favor of it."

"As savings banks become more credit oriented, 24-hours banking services might be very possible," Lynn Chase, assistant treasurer of Cambridge Savings said.