Switches of the type you use in your house have existed since around the same time as the invention of the light bulb. A simple wall switch for an individual light, called a single pole switch, controls only one circuit. A double pole switch, on the other hand, controls two. Its most common application involves controlling 240-volt circuits, such as the one powering your furnace, which have two separate 120-volt lines.
Thomas Edison usually receives credit for the invention of the light bulb, but Joseph Swan actually simultaneously invented a light bulb in England in 1878. Edison unsuccessfully sued Swan for patent infringement, but they later formed a partnership called the Edison-Swan Electric Company. As Newcastle became one of the first cities lit by electricity, the need for a method to quickly disconnect the new light bulbs became apparent. J. H. Holmes of Newcastle filled this need by inventing the first quick disconnect switch in 1874. His company, the Newcastle Electric Works, manufactured the switches.
The number of connections, or throws, switches can make and the number of wires, or poles, they can control determine their classification. The common single pole, single throw (SPST) switch has only one position besides “Off” and has only one pair of wires. An “On/Off” switch for a 240-volt circuit, on the other hand, is a double pole, single throw (DPST) switch because it controls the two 120-volt hot wires in the circuit simultaneously. You would need a single pole, double throw (SPDT) switch to control two lights independently of each other.
An SPST switch has two hot terminals to which you connect the wires coming from the circuit and going to the fixture you want to control. In the most common form, a lever, easily controllable with your finger, toggles the rocker arm inside to the switch to make contact between the terminals when in the “On” position. A DPST switch functions like two of these joined together. It has four terminals, two for each circuit, and connects them simultaneously when you flip the lever to the “On” position.
Double pole switches basically represent larger versions of single pole ones, making it difficult to tell them apart. The best way to identify a double pole switch is to note that it has four brass terminals instead of two. To make the identification of double pole switches foolproof, manufacturers use green plastic to encase them, instead of the white plastic they use for single pole switches. Except for these two differences, the switches look almost identical. They do not differ significantly in size, and each has a green ground screw on the bottom.
To install a switch in a 240-volt circuit, you need a double pole switch. Possible applications in your house include the furnace, air conditioner or a hot tub. You can also use a double pole switch to control two different light circuits simultaneously and save yourself the inconvenience of installing two separate switches. A double pole switch confers an advantage if you have multiple lights or outlets to control in a room but limited space for electrical boxes. Disadvantages include the fact that both lights or fixtures connected to the switch will always have to come on at the same time.