Definition
A capacitor enclosed in a rectangular metal can shaped like a small bathtub, with the capacitor element sealed inside using an insulating compound. The metal case typically serves as one electrical connection, with the other connection brought out through an insulated terminal.
Plain English
An older style of capacitor packaged in a small rectangular metal box that looks a bit like a tiny bathtub. It does the same job as any capacitor — storing a small amount of electrical charge — but the shape and metal housing made it sturdy and easy to mount in older aircraft electrical and radio equipment.
Context Anchor
Seen in aircraft maintenance manuals, wiring diagrams, older radio equipment, and electrical noise-filtering circuits.
Derivation
Named purely for its shape. The rectangular metal case with rounded corners and a flat sealed top resembles an old-fashioned bathtub when viewed from the side, so technicians began calling them bathtub capacitors.
Why Pilots Care
Provides stable electrical performance in radios and instruments despite temperature changes, altitude, and vibration.
Intuition Check
Do not read “bathtub” as describing plumbing or liquid use. Here it describes the shape of the metal case that holds the capacitor.
Example Sentence 1
While inspecting the vintage radio, the technician replaced a leaking bathtub capacitor that had begun seeping its sealing compound.
Example Sentence 2
During bench testing, the old bathtub capacitor failed to hold its rated charge.