Editing Switch (section)

==Special types==
[[File:Float switch on.jpg|thumb|Opened [[float switch]] of a dirty-water pump]]

Switches can be designed to respond to any type of mechanical stimulus: for example, vibration (the trembler switch), tilt, air pressure, fluid level (a [[float switch]]), the turning of a key ([[key switch]]), linear or rotary movement (a [[limit switch]] or [[microswitch]]), or presence of a magnetic field (the [[reed switch]]). Many switches are operated automatically by changes in some environmental condition or by motion of machinery. A [[limit switch]] is used, for example, in machine tools to interlock operation with the proper position of tools. In heating or cooling systems a [[sail switch]] ensures that air flow is adequate in a duct. [[Pressure switch]]es respond to fluid pressure.

===Mercury tilt switch===
{{main|Mercury switch}}

The mercury switch consists of a drop of [[mercury (element)|mercury]] inside a [[glass]] bulb with two or more contacts. The two contacts pass through the glass, and are connected by the mercury when the bulb is tilted to make the mercury roll on to them.

This type of switch performs much better than the ball tilt switch, as the [[liquid metal]] connection is unaffected by dirt, debris and oxidation, it wets the contacts ensuring a very low resistance bounce-free connection, and movement and vibration do not produce a poor contact. These types can be used for precision works.

It can also be used where arcing is dangerous (such as in the presence of explosive vapour) as the entire unit is sealed.

===Knife switch===
{{main|Knife switch}}
[[File:Sectionneur.JPG|thumb|upright|A high-voltage disconnect switch used in an electrical substation. Such switches are used mostly to isolate circuits, and usually cannot break load current. High-voltage switches are available for the highest transmission voltages, up to 1&nbsp;million volts. This switch is gang-operated so that all three phases are interrupted at the same time.]]

Knife switches consist of a flat metal blade, hinged at one end, with an insulating handle for operation, and a fixed contact. When the switch is closed, current flows through the hinged pivot and blade and through the fixed contact. Such switches are usually not enclosed. The knife and contacts are typically formed of [[copper]], [[steel]], or [[brass]], depending on the application. Fixed contacts may be backed up with a spring. Several parallel blades can be operated at the same time by one handle. The parts may be mounted on an insulating base with terminals for wiring, or may be directly bolted to an insulated switch board in a large assembly. Since the electrical contacts are exposed, the switch is used only where people cannot accidentally come in contact with the switch or where the voltage is so low as to not present a hazard.

Knife switches are made in many sizes from miniature switches to large devices used to carry thousands of amperes. In electrical transmission and distribution, gang-operated switches are used in circuits up to the highest voltages.

The disadvantages of the knife switch are the slow opening speed and the proximity of the operator to exposed live parts. Metal-enclosed safety [[disconnect switch]]es are used for isolation of circuits in industrial power distribution. Sometimes spring-loaded auxiliary blades are fitted which momentarily carry the full current during opening, then quickly part to rapidly extinguish the arc.

===Reversing switch===
A DPDT switch has six connections, but since polarity reversal is a very common usage of DPDT switches, some variations of the DPDT switch are internally wired specifically for polarity reversal. These crossover switches only have four terminals rather than six. Two of the terminals are inputs and two are outputs. When connected to a battery or other DC source, the 4-way switch selects from either normal or reversed polarity. Such switches can also be used as intermediate switches in a [[multiway switching]] system for control of lamps by more than two switches. {{See also|Commutator (electric)|Auto polarity (digital multimeter)|H-bridge}}