. Methods of making electrolysis surveys. . t Tc rs Fie. 8.—Contactpoint with in-sulating handle Fig. 9.—Rail spanner for making con-tacts on a short length of rail are known so accurately that a calculation based on this data issufficient for all purposes of electrolysis surveys. For example,taking the resistivity of copper at ohms per mil-foot, if aI 000 000 circular mil-cable has a drop of 6 millivolts on a 5-footlength the current will be given by the equation ^ E I 000 000 / = 7S = • X = 112 amperes. R 5 ^ This value would generally be correct within a few per cent, sincea
. Methods of making electrolysis surveys. . t Tc rs Fie. 8.—Contactpoint with in-sulating handle Fig. 9.—Rail spanner for making con-tacts on a short length of rail are known so accurately that a calculation based on this data issufficient for all purposes of electrolysis surveys. For example,taking the resistivity of copper at ohms per mil-foot, if aI 000 000 circular mil-cable has a drop of 6 millivolts on a 5-footlength the current will be given by the equation ^ E I 000 000 / = 7S = • X = 112 amperes. R 5 ^ This value would generally be correct within a few per cent, sinceall the values entering into the calculation are known or can bemeasured with an acciuracy of at least i per cent. If the cablecarries sufficient load to make it perceptibly hot and a high 30 Technologic Papers of the Bureau of Standards accuracy is desired, an allowance must be made for temperaturecoefficient of copper. When rails are the conductors, neither thecross section nor the resistivity are so accurately known as in thecase o
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