METALLURGY UK VJAST
in the selection of the round form of cross section, and this mainly on the score of greatest uniformity in physical structure, the corners of the square bar introducing* elements which become troublesome. It is fully realized that the work of testing- bars, especially transversely, is made more difficult by the adoption of the round bar; but, after all, this should only mean the taking of proper precautions in measuring the actual net deflection — that is, deducting the upper and lower indentations in the bar by the knife edges, as ascertained by micrometer measurement, from the deflection record.
There is still a further point of interest in the preparation of test bars, and that is the making of coupons from which the quality of the casting to which they are attached is to be judged. This method is used extensively in government work and in the making of cylinder castings. The idea of obtaining material from the same pour in the same mould as part of the casting itself is good enough in theory. Unfortunately, however, this direct connection introduces elements of segregation and temperature changes in the cast iron which make this test less valuable than is generally supposed. At best, the iron which has passed through the different parts of a mould before entering the space for the coupon will not be representative of the whole body, but rather one portion of it only. We therefore recommend the method shown later on in Fig. 149. The metal can be poured from crane or hand ladle clean and speedy, and possesses the temperature of the average iron in the casting more nearly than the coupon method now practiced.
Your committee, while giving specifications for theatment for the information wauled iu daily practice, in addition,