<div class="htmlview paragraph"><b>THE word “ceramic” is derived from the Greek word “keramos” meaning “burned stuff,” thus indicating its inorganic character, Mr. Riddle explains. Although the word is commonly associated with the substance known as clay, in modern usage it includes a large number of minerals and rocks and a correspondingly large number of mineral products, such as the many clay products, glass in its many forms, enamels, and the cements. The chief actors on the stage of ceramic insulators, he indicates, are the silicates, especially the silicates of aluminum.</b></div> <div class="htmlview paragraph"><b>Taking up clays first in his discussion of ceramics, he shows that clays differ enormously in their physical properties chiefly because of differences in mineral structure, particle size, and the impurities associated with them because of their geological history. Quartz, feldspar, porcelain and spark-plug porcelain also are treated in his presentation. He shows that spark-plug glaze has a surprising effect on the physical properties of the insulator.</b></div> <div class="htmlview paragraph"><b>Mr. Riddle then lists and discusses some of the properties which are involved in the performance of spark plugs: density, porosity, and refractoriness; mechanical strength; thermal expansion; electrical resistance and other electrical properties; thermal conductivity; resistance to heat shock; and resistance to chemical agencies.</b></div>