base line. A line used as a datum, or reference line, for measuring.
base metal (metallurgy). Metal to which alloying agents are added.
base metal (welding). The metal to be welded, soldered, or brazed.
BASIC (computer language). A commonly used high-level language for digital computers. The commands used in BASIC are similar to commands used in the English language.
The acronym BASIC comes from Beginners All-purpose Symbolic Instruction Code.
basic operating weight. See BOW.
bastard file. A double-cut, metalworking file with the next to the coarsest teeth. There are five grades of double-cut files based on their teeth. From the coarsest to the finest, these are: coarse-cut, bastard-cut, second-cut, smooth-cut, and dead-smooth-cut.
bathtub capacitor (electrical component). A type of capacitor sealed in a metal container. The bathtub capacitor takes its name from its shape, which is similar to that of a bathtub.
battery (electrical component). An electrochemical device, normally made of a number of individual cells. Electrical energy is stored in a battery by changing it into chemical energy.
Chemical energy in the battery causes a potential difference between the negative and the positive terminals, and when a conductor joins the two terminals, electrons flow from the negative terminal to the positive terminal.

battery analyzer. A special transformer-rectifier unit used to charge nickel-cadmium batteries. In addition
to a source of direct current, the analyzer has a built-in load bank, timers, indicators, and controls for deep-cycling and recharging these batteries.
battery bus. A tie point in an aircraft electrical system to which the battery is connected. Electrical loads connected to the battery bus get their power from the bus.
battery charger. A transformer-rectifier unit that changes alternating current into direct current of the proper voltage to charge storage batteries.
The battery charger is connected to the battery with the positive terminal of the charger connected to the positive terminal of the battery, and the battery is charged until its voltage is the same as that of the charger.
baud (computer operation). A unit of speed for binary data transmission equal to one bit per second.
bauxite. A type of clay or rock, the source of aluminum. To extract the aluminum, the bauxite is changed into alumina (aluminum oxide). Then, the alumina is reduced to metallic aluminum by an electrolytic process.
bay (structural component). A compartment in the structure of an aircraft. A bay is the portion of the structure between adjacent bulkheads, frames, or struts.
bayonet exhaust stack (aircraft reciprocating engine component). The elongated and flattened end of the exhaust stack used on an aircraft reciprocating engine. The gases leave the stack through a slot parallel to its length.
Bayonet stacks decrease both exhaust back pressure and noise and prevent cold air from flowing into the exhaust stack during such maneuvers as slips. This cold air could cause the valves to warp.