Measurements of the electrical conductivity of Wood's alloy and other low melting point alloys
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7071 computing voltmeter) enabling measurement of the ~otential drop between the two connecting points in the dloy. The same multimeter was used to measure the poential drop across the known resistor. The central well AMRITA VERMA, formerly Graduate Student Research Assistant, ~epartment of Materials Science and Mineral Engineering, University f California, is Engineer, Applied Materials, Santa Clara, CA 95054. .W. EVANS, Professor of Metallurgy, is with the Department of laterials Science and Mineral Engineering, University of California, ;erkeley, CA 94720. Manuscript submiued November 1, 1993. IETALLURGICAL AND MATERIALS TRANSACTIONS B
of the plastic block contained a precision mercury-inglass thermometer, and the block was immersed in Syltherm 800 (to within approximately 7 mm of its top) in a covered constant temperature bath. Wood's alloy (50 wt pct Bi, 25 pct Pb, 12.5 pct Sn, 12.5 pct Cd, mp 343 K) was prepared by melting weighed samples of reagent-grade constituents under argon and quickly pouring the alloy into the plastic block. Other compositions used were 50 wt pct Bi, 26.7 pct Pb, 13.3 pct Sn, 10 pct Cd (which is the composition of the alloy "Cerrobend," mp 343 K, available from Cerro Metal Products Company, Paramount, CA) and 50 wt pct Bi, 25 pct Sn, 25 pct Cd (mp 368 K, known as "fusible alloy"). These other compositions were prepared in the same way. Preliminary experiments indicated that the temperature reached steady state after approximately 120 minutes, at which point the current was turned on and voltage measurements were commenced immediately. It was found that the resistance of the alloy increased slowly with time, at a rate that increased with current, presumably because of an electrical heating of the alloy. This increase was negligible over the time period of the measurements at 0.45 Amps, a current sufficient to give voltage measurements across the alloy and known resistor with required precision. After completing a set of measurements at each temperature, a caliper gage was used to measure the thermal expansion of the plastic to enable a correction for slight change in geometry due to this effect. The resistance of the alloy (between the potential measuring points) is obtained from measured alloy resistance potential drop across alloy = known resistor • potential drop across resistor
[1]
and Figure 2 depicts typical results for Wood's alloy indicating the reproducibility of the results between two separate runs. Finally, a calibration of the apparatus was carried out at room temperature using mercury (99.9998 pct pure). The resistance of the mercury column measured in this way was 0.002142 I~, which compares favorably with a value (0.002104 f~) obtained by approximating the V-shaped column as a cylinder of the same diameter. The former number was used in the equation for calculating conductivity: alloy conductivity mercury conductivity • mercury resistance measured alloy resistance •
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block thickness at temperature
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[2]
The
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