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Wikipedia writes, that

Propagation speed is affected by insulation, so that in an unshielded copper conductor ranges 95 to 97% that of the speed of light, while in a typical coaxial cable it is about 66% of the speed of light

Why does insulation affect this speed?

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up vote 2 down vote accepted

That's because adding shield means that there is now distributed capacitance between core & shield in addition to the inductance of the central wire.

Current/voltage transitions gets slower because charging/discharging "capacitor" on each segment through "inductor" of the wire takes time -> propagation speed decreases.

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