Henry (unit) inductor: where "V"("t") denotes the resulting voltage across the circuit, "I"("t") is the current through the circuit, and "L" is the inductance of the circuit. The henry is a derived unit based on four of the seven base units of the International System of Units: kilogram (kg), metre (m), second (s), and ampere (A). Expressed in combinations of SI units, the henry is: in which the following additional derived units occur: coulomb (C), farad (F), joule (J), weber (Wb), tesla (T), volt (V), hertz (Hz), and ohm (Ω). The International System of Units (SI) specifies to write the symbol of
Auxiliary power unit Auxiliary power unit An auxiliary power unit (APU) is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft and naval ships as well as some large land vehicles. Aircraft APUs generally produce 115 V alternating current (AC) at 400 Hz (rather than 50/60 Hz in mains supply), to run the electrical systems of the aircraft; others can produce 28 V direct current (DC). APUs can provide power through single- or three-phase systems. During World War I, the British Coastal class blimps, one of several types of airship operated by