A former is a structural member of an aircraft

Aircraft

An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

 fuselage

Fuselage

The fuselage is an aircraft's main body section that holds crew and passengers or cargo. In single-engine aircraft it will usually contain an engine, although in some amphibious aircraft the single engine is mounted on a pylon attached to the fuselage which in turn is used as a floating hull...

, of which a typical fuselage has a series from the nose to the empennage

Empennage

The empennage , also known as the tail or tail assembly, of most aircraft gives stability to the aircraft, in a similar way to the feathers on an arrow...

, typically perpendicular to the longitudinal axis of the aircraft. The primary purpose of formers is to establish the shape of the fuselage and reduce the column length of stringers to prevent instability. Formers are typically attached to longeron

Longeron

In aircraft construction, a longeron or stringer or stiffener is a thin strip of wood, metal or carbon fiber, to which the skin of the aircraft is fastened. In the fuselage, longerons are attached to formers and run the longitudinal direction of the aircraft...

s, which support the skin of the aircraft.

The Former-and-Longeron technique was adopted from boat construction (also called stations and stringers), and was typical of light aircraft built until the advent of structural skins such as fiberglass

Fiberglass

Glass fiber is a material consisting of numerous extremely fine fibers of glass.Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the invention of finer machine tooling...

 and other composite materials. Many of today's light aircraft, and homebuilt aircraft

Homebuilt aircraft

Also known as amateur-built aircraft or kit planes, homebuilt aircraft are constructed by persons for whom this is not a professional activity. These aircraft may be constructed from "scratch," from plans, or from assembly kits.-Overview:...

 in particular, are still designed in this way.

They are also found in the cores of potentiometer

Potentiometer

A potentiometer , informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used , it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on...

s in which the resistive material is wrapped round.