Reactance (XL / XC)
Inductive or capacitive reactance at frequency.
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The engineering
Reactance is opposition without dissipation — energy borrowed and returned each cycle. Inductors grow with frequency, capacitors shrink, and where a pair crosses is resonance.
Bypass-cap intuition: 100 nF is 16 Ω at 100 kHz but 0.16 Ω at 10 MHz — until its own lead inductance takes over.
Where this math comes from
The j-notation arithmetic that makes reactance a number you can add came from Charles Steinmetz's 1893 phasor method — before it, every AC problem was a differential equation; after it, algebra. Heaviside had supplied the concepts (inductance as a named quantity is his coinage).
The word 'reactance' itself was adopted by the AIEE in the 1890s as the field professionalized around AC power.
- 1886Oliver HeavisideNames and formalizes inductance; reactive behavior theorized.
- 1893Charles SteinmetzComplex phasor method — reactance becomes arithmetic.
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