The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is electrical conductance, measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallels with mechanical friction. The SI unit of electrical resistance is the ohm, while electrical conductance is measured in siemens (S).
Running current through a material with resistance creates heat, in a phenomenon called Joule heating. In this picture, a cartridge heater, warmed by Joule heating, is glowing red hot.
Insulator (electricity)
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An electrical insulator is a material in which electric current does not flow freely. The atoms of the insulator have tightly bound electrons which cannot readily move. Other materials—semiconductors and conductors—conduct electric current more easily. The property that distinguishes an insulator is its resistivity; insulators have higher resistivity than semiconductors or conductors. The most common examples are non-metals.
Ceramic insulator used on an electrified railway
Three-core copper wire power cable, each core with an individual colour-coded insulating sheath, all contained within an outer protective sheath
PVC-sheathed mineral-insulated copper-clad cable with two conducting cores
Pin-type glass insulator for long-distance open-wire transmission for telephone communication, manufactured for AT&T in the period from c. 1890 to WW-I; It is secured to its support structure with a screw-like metal or wood pin matching the threading in the hollow internal space. The transmission wire is tied into the groove around the insulator just below the dome.