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From Cecil's Mailbag by the Straight Dope Science Advisory Board

How do automated teller machines work?

Dear Straight Dope:

I was walking around the T at Government Center and I saw an ATM machine in the middle of the floor. How does this lonely little bugger hand out money when it's not attached to a bank? How does it hand out money when it IS attached to a bank? Is there a cash box in the middle? Or does the money flow from a large room full of cash? And who gets to fill these machines? --Tom Andresen

SDSTAFF Tech replies:

Well, Tom, never let anyone tell you that you're in danger of developing any great technological advances.

First things first. It's not an ATM machine. It's an Automated Teller Machine, or merely ATM (or, if you must, an AT machine). The English language is delicate. Please use acronyms wisely.

Onward.

Whether an ATM is in a bank, at a bank drive-thru, in the wall of your shopping mall, or standing out in the middle of nowhere, the process is pretty simple. A real live human being goes to the machine and opens it, then opens the safe-locked door inside, and fills trays in the machine with a set amount of money. If it's a machine that only gives out portraits of Andrew Jackson, then there's either only one tray, or multiple trays that are all filled with twenties. It also dispenses tens (or, rarely, fives also), the multiple trays each holding a separate denomination.

When you pop your plastic in the slot, punch in your PIN, and request funds - assuming you're not a deadbeat who's about to see "We're sorry, but you do not have sufficient funds in your account to complete this transaction" - the computer in the machine triggers mechanisms that feed the appropriate number of bills of the appropriate denomination from the tray(s). Then, they're fed to the slot your cash pops out of, right into your greedy little fist.

If the ATM is in a bank, the door to the ATM is usually behind the machine, where you can't get to it, and a bank employee fills the trays. ATMs located elsewhere are generally filled by men in uniforms, who drive large armored vehicles, carry sidearms, and whose employers carry a lot of insurance.

You didn't ask, but in case you were wondering, when you make a deposit at an ATM, it's merely fed into a bin within the safe. The bank employee or armed guard retrieves the envelopes when they fill the machine, and your deposit is taken to the tellers, who process it just like any other deposit, only they key it in as an ATM deposit so that you don't get credited twice. If you were to try to rip off the bank by not really depositing any funds, at the end of the day, the bank's computer system would say to itself, "Self, this deposit wasn't keyed in by a teller ... this guy's trying to rip us off." At that point, the computer would subtract your deposit back out, and you'd probably be in deep spit.

--SDSTAFF Tech
Straight Dope Science Advisory Board