Insulators are materials which prevent the flow of heat (thermal insulators) or electrical current (electrical insulators). The opposite of electrical insulators are conductorss and semiconductors, which permit the flow of current (Note: a semiconductor is strictly speaking also an insulator, since it prevents the flow of electrical current at low temperatures, unless it is doped with atoms that release extra charges to carry the current).
A perfect insulator is impossible to achieve due to the 2nd law of thermodynamics.
See also: insulation
Early telegraph, telephone, and electric lines were strung overhead on poles. Rather than an insulating covering, the lines were fastened to the poles with insulators made of glass.
Colors and design details varied widely because the insulators were made by small, local manufacturers. Clear and blue-green insulators are the most common. Most were abandoned when the lines were replaced with multipair cables, generally in the 1960s and 1970s. Some are now in the hands of collectors.
Similar insulators are used on overhead power lines, though most are made of porcelain rather than glass.
External links:
Collectible Insulators