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Impedance (Z) is the sum of resistance (the real part) and reactance (the imaginary part). The reader is invited to search Helium for 'complex numbers' for further explanation of the real and imaginary terms. The reciprocal of impedance is admittance (Y).

Resistance (R) is the reciprocal in conductance (M); or how well something conducts electricity. Reactance (X) is dependant on frequency and the sign of the reactance; inductors (coils or lines) have positive reactance which increases with frequency, while capacitors (condensers or plates) have negative reactance and this decreases with increasing frequency. The reciprocal of reactance is susceptance.

A coil and a capacitor joined together will resonate at a frequency at which their reactive parts of impedance are the same. This simple, elegant feature is the basis of all radio communication, from crystal sets to cellphones. Every conductor has resistance, capacitance and inductance, but most are designed such that one of these quantities dominates, to form a resistor, capacitor or inductor.

The unit of impedance is itself a complex number, of the form R+jX ohms or R-jX ohms. The units of reactance and resistance are also ohms. The unit of admittance is the siemens (S). Capacitance is measured in farads, and inductance in henries. All these units are named after scientists and thinkers who worked on the quantities concerned, in the nineteenth century.

A practical illustration of impedance is in the case of loudspeaker coils. They are rated in ohms as an impedance, and an 8-ohm coil will have a lower direct-current resistance if measured with a simple multimeter. The 'remainder' of the impedance making up the full 8 ohms is the inductive reactance, or the coils reactance to the alternating current of the audio system's output.

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