Insulators, as used in present day practice, are a product of many years of experimenting and numerous unfavorable experiences. The art of manufacturing has been so perfected that a rate of depreciation of less than 1/50th of one per cent per annum had been reached. The insu'ating materials in present use are of two classes, hydro-carbons and silicates. The latter, which includes glass and porcelain are used for outdoor service, but glass because of its characteristics is little used. Porcelain used in American practice is made up of clay and potash feldspar, which acts as a binder. Great deal of attention must be paid to proper firing adequate vitirification, purity and fineness of raw materials, factory organization, etc. in order to produce a uniform grade of porcelain of high quality. The manufactured porcelain insulators are all given an electrical potential test to weed out the defective ones. The test usually consists of applying high frequency peak voltage many times over flashover value at normal frequency. There are now three types of porcelain suspension insulator used extensively interlink, tension and cap-and-pin types. The last named type as used in overwhelming numbers because of its superiority both mechanically and electrically over the other two. It is now generally believed that with the increase in the voltage of transmission lines, this type will be more extensively used. In many systems, grading of insulators is resor_??_ed to. Theoretically, grading will equalize the voltage duty among the several units of the string, but in service, it has been borne out that grading takes care of only one condition, namely with the insulators dry and clear. Additional objections in adopting this method are increase in first cost and in the maintenance cost. The most common practice today is to make the insulator nearest the conductor large enough to withstand the stress and make the rest of the same size. This will be cheaper from manufacturing standpoint as well as from the standpoint of keeping spare units. There are two factors which governs the selection of number of units per string, namely the distance from conductor to cross arm and rate of depreciation of insulators. The first item is well understood but the last factor could be only approximated by the use of law of probability. Austin and Klanber's formula appearing in the Transaction of the American Institute of Electrical Engineers seems to give pretty close results. After the insulators are put in service, at certain intervals, insulation tests are sometimes carried out to determine their condition. Three methods of testing are in common use, megger, “buzz stick” and spark coil. The megger method is most widely used and is quite efficient, but this and the spark coil mothod require the line to be taken out of service. At the present time, there seems to be no satisfactory method of testing pin type insulators. Visual inspection and the use of telephone receiver in the case of wood poles are the only methods at all effective. The use of ground wire is a much debated question today. Many new lines installed recently did not put in the ground wire on the ground that better results will be secured by putting its cost into additional insulation. The practicability of using wood poles with suspension insulators is also under study. In using wood poles, the rate of decay of wood must be balanced against the element of cost. In a certain instance, a 110, 000 volt line was put in with wood poles but the result of operation has not yet been made public.