DIELECTRIC

or INSULATOR, according to Faraday, is the name given to those substances through which electrostatic induction takes place. When one body attracts another, (viz.), a molecule of dust, the action which takes place is called induction, and the intervening medium in which the action occurs is called the D. There are two kinds of D., the one is gaseous and includes air and all gases, the other is solid and includes. such substances as gla , silk, vulcanite, resin, mica, and gutta-perch a. Bra and all metals are good conductors of electricity, while porcelain, marble, slate, and stoneware are such bad conductors that they are used for insulating telegraph and telephone supports, or the bases of switchboards or stands, etc. In the same way the softer substances, silk, gutta perch a, and india-rubber, are used for insulating telegraph and telephone wires, or coils and wires in other electrical apparatus. The inductive effect of a given charge depends upon the D. surrounding the charged body, and the result obtained is called the specific inductive capacity. When hot, the power of a body is always diminished. Although air is a D., should an electric spark pa through it, its insulating power at once breaks down, and it ceases to act as a-D. In the same way solid D’s. lose their solidity and become cracked, if touched by an electric spark.