A flicked switch bounces several times before it settles, and organists love it
Press a light switch and it seems to snap cleanly from off to on. Inside, the springy metal contacts actually slam together and rebound a few times, sending a stutter of pulses before settling. Engineers spend real effort hiding this chatter, yet on the Hammond organ it produced a percussive click that composers deliberately exploited.
Most switches are simple: two metal contacts that touch to complete a circuit and part to break it. A switch can be flipped by a person, as with a keyboard key, or tripped by the world itself, as when a thermostat responds to temperature or a sensor confirms a garage door is fully open. One circuit operating a switch in another is called a relay. Contact metals are chosen to resist the insulating oxides most metals grow, and are sometimes plated with noble metals or designed to wipe each other clean.
Jargon describes the wiring. Poles count how many separate circuits one lever controls; throws count how many paths each pole can select. The humble on-off switch is single-pole, single-throw, while a single-pole, double-throw part routes one terminal to either of two others. Changeover designs may connect the new circuit before breaking the old or the reverse, and British and American electricians confusingly use two way and three way to mean different things.
The perfect switch would drop no voltage, handle unlimited current and flip instantly without bouncing. Real ones fall short. In low-power signal circuits, bounce is usually handled by debouncing: filtering the voltage, sampling a key repeatedly until it holds steady, or feeding it through a latch. Mercury-wetted contacts once eliminated bounce mechanically, but mercury's hazards made them rare, and a non-toxic liquid metal alloy has been proposed instead.
At high power, bounce becomes violent. Each rebound can spark an arc while inrush current reaches thousands of amperes, and contact resistance leaps from around 0.2 milliohms to over 30, concentrating heat enough to melt the metal and weld the contacts shut. Repeated arcing gradually pits and erodes contact faces, and in a short circuit, magnetic forces can shove copper contacts apart at around 10 kiloamperes.
Source: Switch
