Porcelain

—In electric practice, porcelain is employed for high voltage insulation because of its high insulating properties. Porcelain is composed of flint, feldspar, kaolin or china clay, and ball clay; chemically considered it is a double silicate of sodium or pota ium and aluminum. It is homogeneous, giving uniform distribution to electrical strains, 1s practically waterproof, has great mechanical strength, can be molded into various shapes, does not deteriorate and will stand sudden and extreme temperature changes. Porcelain is well suited for spark-plug insulation, since it po e es a very high resistance both to heat and to the electric current. The brittlene of porcelain is the worst objection to its use. Lower qualities of porcelain are, of course, much more easily broken, and thereby produce short-circuiting under ordinary conditions of temperature and electrical tension. Many plugs using porcelain insulation have the porcelain in two or more parts, so as to avoid the troubles arising from yneven temperatures. Heat is liable to break a single long porcelain.