Heat treating and inspection of metals Page: 9
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HEAT TREATING AND INSPECTION OF METALS
1. Crucible. 5. Thermocouple.
2. Molten salt bath. 6. Pyrometer.
3. Fire brick case. 7. Master control switch.
4. Heating elements. 8. Automatic temperature control.
FIGURE 4.-Salt bath type heat-treating furnace.
b. In order to obtain accurate results, it is necessary to use a thermo-
electric instrument known as a pyrometer which measures the electrical
effect of a thermocouple, and hence, the temperature of the metal
being heated. The operating principle of this device involves the
difference in electromotive force between pure metals and alloys and
may be explained as follows:
(1) If two dissimilar wires are joined together by twisting or
welding, then heated, an electromotive force or voltage is generated.
This voltage may be measured by connecting the opposite or cold
ends of the wires to a galvanometer or millivoltmeter sensitive enough
to read in thousandths of a volt. The strength of this current is pro-
portional to the difference in temperature between the hot and cold
ends. As the pyrometer is calibrated in degrees of temperature
instead of electrical units, it therefore measures heat. A complete
pyrometer consists of a thermocouple, extension leads and a meter.
(a) Thermocouple.-In general, the lower priced iron, cop-
per, nickel, or chromium combination thermocouples are used for
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United States. War Department. Heat treating and inspection of metals, book, September 10, 1941; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc96657/m1/11/: accessed July 26, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.