Double-pole refers to a circuit that uses two 120-volt poles to create one 240-volt circuit. A dimmer switch has a special resistor that allows a varying amount of current through in order to provide a variable wattage to the output. It is called a dimmer switch because it is often used in lighting circuits to allow the lights to be set to different levels of brightness. Only use 240-volt rated lights and switches when installing a double pole dimmer switch.
Switch off the main circuit to the house. Two hundred and forty volts can seriously injure you; do not work on a hot circuit. Unscrew the cover plate over the electrical junction box where you are going to install the dimmer switch. Use only a double pole dimmer switch that is rated to handle the wattage required by your light fixture. Add up the wattage of all the bulbs in the fixture to determine the maximum wattage required.
Locate the two white wires (common wires), the two black wires (hot wires), and the two ground wires (either bare wire or green insulated). Locate the two chrome screws on the switch (they correspond to the white wires), the two brass screws (correspond to the black wires), and the two green screws (they correspond to the ground wires).
Screw the wires to the corresponding terminals. If necessary, strip off the insulation on the last 25 mm (inch) of the wire and bend it over in a hook shape that can be hooked around the screw shaft. Tighten all the screws. Screw the dimmer switch to the mounting points in the junction box.
Screw the new face plate that came with the dimmer switch to the outside of the junction box. Restore the power to the house.