Transformations to open market operations

the long version

By buying or selling bonds, bills and other financial instruments in the open market, a central bank can expand or contract the amount of reserves in the banking system and can ultimately influence the country's money supply. When the central bank sells such instruments it absorbs money from the system. Conversely, when it buys it injects money into the system. This method of trading in the market to control the money supply is called open market operations.