Metallography Volume 1

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1920 Excerpt: ...it until the couple glows uniformly throughout its entire length. This operation is intended to eliminate parasite currents and make it immaterial which portion of the wire comes at the maximum temperature fall along the wire. The platinum, or noble metal couples, are used in metallographic laboratories as pyrometric standards. These are the socalled "normal couples" which have been accurately calibrated either in the private laboratory or at one of the national laboratories, such as the Bureau of Standards at Washington or the National-Physical Laboratory at Teddington. The elements are usually one meter long and 0.6 mm. in diameter. These couples are supplied with certificates which give the values of the electromotive force in millivolts for each 100C. from 300C. to 1600C. The Bureau of Standards certifies the Le Chatelier couple for use at 1200C. with an accuracy of + 2C. The cold junction temperature is usually made 0C. but the Physikalisch-Technische Reichsanstalt, Charlottenburg, standardizes couples at 20C, which is the average room temperature. The calibration curve is usually made from the data supplied by plotting the e.m.f.'s against the corresponding temperatures on rectangular coordinate paper. A smooth curve drawn through the points will give the correct temperatures for all values of the electromotive force. Out of the demand for a couple which is less expensive than the Le Chatelier, and which has a higher thermoelectric force,1 has come the development of the base metal couples using elements of nickel, iron, copper, and various alloys such as constantan, nichrome, etc. The high thermoelectric force of the base metal couple permits the use of millivolt meters of more solid construction, which in many cases is a particular advanta...