That 10% reserve number is interesting. I learned about the 10% reserve ratio in macroeconomics class, where I also learned that there's this thing called the money multiplier. See, if banks are required to hold a fraction r (10%) of their deposits in reserve, then obviously they'll lend out the rest, which will in turn be held or spent by the borrower, and one way or another it'll end up back in a bank. So a (1-r) fraction of the actual cash and Fed-issued money ends up lent out and redeposited. But the banks can loan out a (1-r) fraction of /that/, ad infinitum. So the total amount of deposited money is 1 + (1-r) + (1-r)^2 + ... = 1/r of whatever money (cash and things actually deposited at the Fed) the Fed created in the first place. Since r = 10%, the multiplier is 10.

The official page AFAICT is here:

http://www.federalreserve.gov/monetarypolicy/reservereq.htm

The effective ratio is actually under 10% these days.

Edit: Fixed an inconsequential typo.