How long has it been since you had to replace a light switch? Buy a new stove or refrigerator due to significant failure? Had a blowout? Had your power or water off for more than a few minutes? Had a computer hard drive fail? Had your car simply not start?
Technological things in our modern society are pretty reliable, and getting moreso. The technology behind most items in wide use is pretty mature. Sure, there are still failures. Compared to even a couple of decades ago, though, we're far better off.
Take light switches. Anyone who has manually touched wires together to make a circuit knows that with any significant voltage or amperage you get burns on the wires at the contact points. So how do you design a light switch to handle hundreds of thousands of such makings and breakings of contact during its lifespan?
The stereotypical knife switch worked well enough for a factory or laboratory in the early days. The way the blades slide into the spring-loaded slots tends to wipe away any damage. (Though they are prone to arcing when opened if used for higher voltages.) However, for a home or office these simply have too much exposed metal carrying current.
Multiple different switch designs were tried in the early years of electrical utilities. For a long time all electrical switching in the home or office was done at the individual device; wall switches were seen as wasteful, since you had to run more wiring for them. Also, this followed the pattern of the gas light or heat fixture. Unless you had one of the rare remote ignition systems, you had to turn the gas on at the fixture and light it immediately. So, there were individual appliance switches. These were mostly of the turnkey type, which is still seen in the rotary knob on many floor and desk lamps. However, they also used the same basic concept of a spring-loaded switch making and breaking the contact very quickly.
Soon, though, the convenience of having a switch on the wall turn on one or more lights or other appliances eventually overcame these inclinations.
The shape and function of wall switches went through the rotary, turn key design, then the push-button style before the familiar toggle switch became almost universal. (There was also a round switch similar to the rotary, except you pulled or pushed on the knob instead of turning it.) However, other designs remain available. The pull-chain switch is still used in many applications, especially for ceiling-mounted fans and light fixtures in unfinished rooms. You can buy modern versions of the push-button switches. Rocker switches of various types are also popular, and in some parts of the world are considered much more modern than toggle switches. The mechanical operation of typical rocker switches is different from that of typical toggle switches, but the basic idea is still to make and break contact as quickly as possible.
Beyond the style, however - and frequently driving it - is the requirement for safe and reliable operation. To minimize the effects of making and breaking the contact, electrical switches had to operate quickly, to minimize arcing. This led to the development of several methods of "snap action" or "quick break" switches. The first such switch was patented in 1884, and the basic principles involved are still found in every household or business mechanical light switch today. The toggle switch application of this principal came along in 1917 but needed time to become dominant. Subsequent changes in method of operation in more recent decades has been slow, simply because the quick-break, spring loaded switch works so well and flipping a toggle works so well with it. Turning power on and off is today a mature technology. Even the rotary dimmer knob has been around for over four decades.
A more thorough description of what an electrical switch goes through will help emphasize just how difficult its job is. When turned on - that is, the circuit is closed - the contacts in a switch effectively go from infinite resistance to zero resistance (actually, very low resistance). Meanwhile, heating goes from zero through a sharp peak then back down to a low level. If the heating is too extreme, the contacts can be damaged or even welded together. So, the more quickly the process of making contact and going from zero to full load occurs the better. Corrosion makes the process even more difficult, since it increases resistivity and the time required to physically make full contact. Contacts are therefore made from or plated with a conductor which does not corrode easily.
The reverse of this process happens when a circuit is opened, except that the potential arcing interval is increased. (The situation is even worse when you take inductance into effect, but let's consider that outside the scope of this article. Did I just hear a collective sigh of relief? It's actually not that complicated. For now, just think of inductance as the inertia of an electrical current.)
Internal improvements for switches continue. Early on - and still today for some parts - porcelain was used for insulation. One of the first significant uses of polymers in consumer products was Bakelite for an electrical insulator. Better alloys and methods of construction further improve the durability of the contacts and the reliability and lifespan of the switch. Springs retain their spring and polymers remain structurally sound for longer today.
Mercury switches were popular for several years before the Seventies. Again, they were designed to make and break contact quickly. However, because of their method of operation they were silent, as opposed to the snap of the mechanical quick-break switches. These are no longer made, but millions remain in use. They eventually wear out, but if the capsule containing the mercury remains intact they aren't dangerous. However, they should be disposed of as you would any device containing mercury.
Modern, electronically operated switches have no internal mechanical parts. There is no gross making or breaking of physical contacts. They are as silent as mercury switches and faster than the quick-break style.
Still,
the old-fashioned, mechanical method of closing connections works so
well it will probably be with us for many further decades. Even much
heavy-duty electrical switching uses muscles or air pressure for
making or breaking connections as quickly as possible. Just watch out
for the impedance backlash. This document is Copyright 2019 Rodford Edmiston Smith. Anyone wishing to repost it must have permission from the author, who can be reached at: stickmaker@usa.net