Definition
A multi-position rotary electrical switch built from one or more thin insulating discs (wafers), each carrying a set of fixed contacts arranged in a circle around a central shaft. As the shaft is rotated, a movable contact on each wafer connects selected fixed contacts, allowing one input to be routed to any one of several outputs (or vice versa). Multiple wafers can be stacked on the same shaft so that several circuits are switched simultaneously by a single knob.
Plain English
A rotary knob switch made of stacked thin discs. Turning the knob lines up contacts on each disc to send electricity to whichever circuit you've selected. Stacking more discs lets one knob switch several circuits at once.
Context Anchor
Seen in aircraft electrical and avionics maintenance descriptions, especially when discussing rotary switches used to select equipment, modes, or circuit paths.
Derivation
Wafer here refers to a thin, flat disc, the same sense used for thin baked wafers or silicon wafers. Each disc is a separate switching layer, so the name describes the physical construction.
Why Pilots Care
Wafer-type switches are common failure points in older aircraft. Worn or dirty wafer contacts can cause intermittent operation of radios, lights, or fuel gauging, and recognizing the switch type helps when troubleshooting or describing a problem to a technician.
Analogy
Think of it like a rotary knob that lines up an internal pointer with one chosen contact at a time. Each click places the electrical connection on a different path.
Intuition Check
Do not think of “wafer” as a fragile food item here. In this term, it means a thin insulating layer inside the switch that carries electrical contact points.
Example Sentence 1
The intermittent radio fault was traced to a worn wafer-type selector switch behind the audio panel.
Example Sentence 2
During the panel inspection the mechanic checked the wafer-type selector switch contacts for pitting caused by arcing.