. Railway and locomotive engineering : a practical journal of railway motive power and rolling stock . the ability of theoil to circulate freely. In order that theoil may have access to all parts of thetransformer, passages are left betweenthe coils and through the iron core. Usu-ally the core is subdivided by oil passages,or ducts, as they are called, so that no former case and cooled, and is forced inagain at the bottom by a pump. Pure oil is a very high grade takes approximately 10,000 volts topuncture of an in. of pure thousand volts will puncture or breakdown 0.
. Railway and locomotive engineering : a practical journal of railway motive power and rolling stock . the ability of theoil to circulate freely. In order that theoil may have access to all parts of thetransformer, passages are left betweenthe coils and through the iron core. Usu-ally the core is subdivided by oil passages,or ducts, as they are called, so that no former case and cooled, and is forced inagain at the bottom by a pump. Pure oil is a very high grade takes approximately 10,000 volts topuncture of an in. of pure thousand volts will puncture or breakdown of an in. of air gap under thesame conditions. This comparison il-lustrates the value of oil as an are not built to operate oncircuits where the voltage is 33,000 orhigher, unless oil is employed to increasethe strength of their insulation. A transformer usually fails because ofits insulation burning out. The insula-tion is destroyed, a hole being formed,through which the current passes, produc-ing a short circuit. The insulation ofa transformer resists the voltage in the. MOUNT SHASTA, FEET HIGH. SOUTHERN PACIFIC part of the iron is more than one inchfrom the oil. These ducts are usuallyquarter-inch openings in the core withlarger ones between the coils. They aremost efficient when they run in a verticaldirection. Care must be taken that theydo not become clogged and so lose theirf In transformers of 500 kw. and over,oil alone is not sufficient for cooling, be-cause the exposed surface of the case can-not radiate the heat fast enough (500kw. arc equivalent to about 660 h. p.)Two methods are employed to overcomethis difficulty. In the first, a coil of cop-per pipe is placed in the oil at the topof the case and cold water is pumpedthrough the coil to absorb the excessof heat. In the second the warm oil iswithdrawn from the fop of the trans- same way that a boiler shell resists thesteam pressure. In a transformer, the in-sulation
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