Editing Metal contact switch (section)

==Design principles==
===Negative and positive action===
<!-- If someone can find better terminology for this, especially real industry terminology, please rename this section! -->
At rest (when the switch is open) the contacts are held apart. As the switch is closed, the contacts are brought together. The contacts can either close of their own accord, or they can be pressed together under operator action. The majority of contact designs isolate the actions of the operator from the contacts: operating the switch removes whatever object was holding the contacts apart ("negative action"), and they close under their own power. Negative action was chosen by [[Cherry]], [[MEI]], [[SMK]] and various other manufacturers. Seldom mentioned, this approach is specifically referenced by [[MEI]] in their [[:File:MEI T-5C and T-5L specification.pdf|catalogue description of the T-5 series]] (name and date of publication lost):

:Switching action is accomplished by movement of one gold plated bar against another at right angles (classic cross bar switching). The bars strike with a velocity determined only by spring forces and not subject to the speed at which the switch plunger is struck. This type of design provides a much more constant and lower contact bounce than is found in mechanical contact switches of more conventional design. The effects of operator differences are substantially eliminated.

Likewise, [[Hi-Tek Dovetail Series]] switches "{{sq|incorporate}} cantilevered contacts which are isolated from the switch dynamics, thus assuring that the contact pressure is independent of switch operations."

Some switch types, such as Alps [[Alps SKCC series|SKCC series]] and [[Alps SKCL/SKCM series|SKCL/SKCM series]], and switches inspired by them such as [[Omron B3G-S series]], use the slider to press the contacts closed ("positive action"). A heavy-handed operator is going to be causing the switch contacts to close with more energy than an operator with a light touch.

Operator force is not the only suggested reason for the design. The 1973, 1974 and 1979 [[Cherry catalogues]] note the following in relation to [[Cherry gold crosspoint]]:

:Contacts are normally held apart for greatest shock resistance.<br>No microphonics or bounce during turn-off or at rest.

The patent for the [[PED keyswitch]] notes that the contacts are held apart to prevent inadvertent actuations due to impact. Unlike the [[Hi-Tek High Profile]] switch that may have inspired them, the contacts don't actually close under their own power and are pressed together via a funnel-shaped section of the slider. This switch combines aspects of both approaches: the contacts are both held apart at rest and closed by direct operator action.

===Contact design===
[[Cherry miniature open]] "mousetrap" switches have flat contacts with a large surface area. This permits a high level of current to pass through them. When these switches were adapted for use in keyboards, the flat contacts were replaced with [[gold crosspoint]] contacts suitable for low-energy circuits. Subsequently, many keyboard switch types used some form of high-pressure contact arrangement where only a very small part of each contact touches the opposing contact. These can take the form of "crossbar" or "crosspoint" contacts where a pair of cylinders or prisms are arranged at right angles, such as in [[Cherry MX]] and [[MEI T-5 series]] switches. They can also take the form of curved and raised surfaces, such as found in many [[four-tab clone]] switches.

Contact form affects bounce time. Non-illuminated [[RAFI RS 76 M]] switches have a 5 ms bounce time using gold crosspoint contacts, while the illuminated version uses flat metal contacts that, while still made of gold alloy, have double that bounce time at 10 ms.

===Contact material===
Gold is frequently chosen as the contact material. One would assume that this is to prevent switch failure through oxidisation, as it would only take a tiny amount of oxidisation to disrupt the low current that passes through switches. Typically, the gold is applied as a plating, as noted in the specifications for [[GRI KBM]], [[MEI T-5 series]] and [[Datanetics DC-50 series]] switches.

[[Hi-Tek Dovetail Series]] switches use contacts inlaid with gold alloy, while [[Datanetics DC-60 series]] switch contacts are inlaid with gold (which in reality is likely to also be gold alloy).

Switch contacts can also be solid gold alloy. Cherry noted in 1979 that [[Cherry gold crosspoint]] switches used a solid prism of 69% gold, 25% silver and 6% platinum for the contacts. [[Cherry M8]] offered a choice of contact materials from AuAg26Ni3 (gold, silver and nickel), AgPd30 (silver and palladium, without gold) and AuAg10 (gold and silver).

It has been noted that the use of gold for contacts reduces bounce. [[MEI Sabrecoil]] used silver contacts instead of gold, and this was noted as increasing the maximum bounce time. 'More expensive keyboards often use gold contacts to reduce bounce. Sabrecoil uses a specially coated silver plate contact that is "almost as good as gold" but allows a reduction in cost.' 

===Contact cleaning===
Some switch designs feature contacts that slide against each other during actuation, allowing the contacts to wipe themselves clean of dirt and oxidisation, albeit only with use. This is cited as a feature of [[GRI KBM-LP]] switches. Infrequently-used key positions will not benefit from this property.