Insulation

—1. The condition of a conducting body when insulated by the interposition of non-conductors to prevent the escape of electricity. The substances that will not conduct the electrical current, but interfere with its transmi ion are: oils, porcelain, wool, silk, resin, gutta perch a, shellac, ebonite, paraffin, gla . On account of this property they are extensively used in all the branches of electrical industry where it is desirable to confine the current to definite limits. They are thus called ‘‘insulators’’ (Latin, insula, an island; insulatus, made into an island.) The insulation of the electric current is, of course, not absolute: rather is it correct to say that the resistance of insulators is so great that the leakage through them is practically negligible. Thus, while certain poor conductors offer a high resistance to current, gla , at 20° Centigrade, offers a resistance of 91,000,000 megohms (millions of ohms) per cubic centimeter; the gutta perch a, 450,000,000 megohms; hard rubber, or ebonite, 28,000,000,000 megohms, and paraffin, 34,000,- 000,000 megohms.