Here's my attempt, without talking about apples or anything: Essentially, the 'money supply' is determined by the central bank, since they are the issuers of money. You can read on your US dollars that they are issued by the federal reserve. An excellent measure of the money supply is NGDP, since this is the number of dollars traded every year (that sort of quantifies how many dollars are available to be captured by…
Essentially, the 'money supply' is determined by the central bank, since they are the issuers of money. It's not as simple as that. The amount of money in public circulation is largely determined by banks, because every loan given increases the amount of (electronic) money in circulation. The central bank then adjusts the amount of central bank money (reserves) by open market operations, but it does so defensively, t…
Sorry just noticed this. This is a common misperception. They have been using QE to create more money in public circulation now that interest rates are 0, and there's nothing ineffectual about that.
To see why this is possible, just consider the question, could the Fed create inflation at the 0-lower-bound by saying they will print money until they get it? The answer is clearly yes. Actually the Bank of Switzerland did that recently to set a Euro peg.