(1 + c)/(r + e + c). The money multiplier (also called the credit multiplier or the deposit multiplier) is a measure of the extent to which the creation of money in the banking system causes the growth in the money supply to exceed growth in the monetary base.
A buzzing light switch could mean the fixture is wearing out, causing an arc of electricity behind the wall plate. If this is the case, the potential for an electrical fire may exist.
The black wire in the box holding the light brings power to the fixture. The purpose of the switch is to interrupt that power supply when you don't want the light on. To do that, you create a switch loop, which carries the power supply from the box holding light fixture to the switch box and back up to the fixture.