Insulators, also called dielectrics, are petticoat shaped pieces of glass or porcelain which is used to insulate telephone lines and electrical power lines.  An insulator has a groove around both sides of it to hold the wire and tie wire.  On the inside of the insulator, it is hollow with threads, to keep it on a wooden pin that holds it onto a telephone pole.

Communication and electric line wires in service must be kept as dry as possible in order to function efficiently, and to cut down on loss of current.  The wires are kept off of the ground by being strung between poles.  Something was needed to keep the wires and, sometimes wet, poles apart.  This "something" was the insulator.  It was developed and improved upon over many years.

An insulator conducts electric current poorly because it's electrons are bound so tightly to their nuclei that they cannot move freely from atom to atom.  Therefore, when an insulator is connected to a battery or some other source of electricity, not enough electrons move through the insulator to produce a current.  In contrast, materials called conductors, which include such metals as aluminum, copper, and silver, have electrons that are weekly bound to their nuclei.  These electrons travel freely, resulting in a flow of electricity.  Materials called semi conductors also have relatively low resistance to current flow.  Semi conductors, such as germanium and silicon, conduct electricity better than insulators, but not as well as conductors.

Insulators were developed over the course of more than 100 years.  The need for the insulator arose when the discovery of electricity, which in turn led to the invention of the light bulb, telegraph, telephone, and other electricity-oriented innovations.  Also, as railroads began crisscrossing the continent, there came the need for signal devices.  Electricity had to be moved economically from one place to another to meet the increasing demands generated by these new marvelous inventions.