Electricity that is flowing from one place to another is called current. The volume of electrical current is measured in amperes or amps.
Locate the two joists nearest the new fixture location using a stud finder or tap on the wallboard until the sound indicates the general location of the joists. Use a drill and an eighth inch bit to drill through the plaster where the joists are estimated to be. Drill holes at 1" intervals until you meet resistance and wood from the joist is ejected. Mark the location of each joist with a pencil mark.
Decide on the location for the new electrical fixture and where the source will be. Determine inside of which walls and ceilings the cable must be run. For example, with a new ceiling fixture using a source from an existing wall receptacle, the cable will have to be run inside the wall cavity, through the top plate, into the ceiling cavity, and on to the new fixture.
Where no means exists within the snap switch enclosure for connecting to the equipment grounding conductor or where the wiring method does not include or provide an equipment grounding conductor, a snap switch without a connection to an equipment grounding conductor shall be permitted for replacement purposes only.