Humboldt State University ® Department of Chemistry

Robert A. Paselk Scientific Instrument Museum

From: Duff, A. Wilmer, A Text-Book of Physics, 5th ed., P. Blakiston's Son & Co., Philadelphia (1921) pp. 399-400.
© Copyright 1998 R. Paselk

449. Resistance Standards. - It follows from the definition of the ohm that the "absolute" measurement of the resistance of a conductor consists in determining the ratio of the e.m.f. (volts) and the corresponding current (amperes) in the conductor. To make such a measurement with high accuracy is not a simple process. But to get the ratio of two resistances is, as we shall see later (SS456), a relatively simple measurement and one that can be made easily with very high accuracy. Hence the ordinary process of determining the resistance of a conductor is one of comparing its resistance with a "standard resistance."
Standard resistances are of two classes, (1) the prime standard, a mercury resistance and (2) secondary standards in the form of coils of wire either (a) single coils, or (b) groups of coils mounted in boxes or cases, called resistance boxes.
The prime standard is defined so that it can be reproduced from the specifications of materials and dimensions only. At an International Congress of Electricians held at Chicago in 1893, in which all civilized nations were represented, it was recommended that "the international ohm be the resistance offered to an unvarying electric current by a column of mercury at the temperature of melting ice, 14.4521 grams in mass, of a constant cross-sectional area and of the length of 106.3 centimeters." The cross-sectional area of such a column of mercury is 1 square millimeter. This has been adopted by all nations as the legal ohm. The ohm as thus defined by law was as near the absolute ohm as measurements could fix it at the time.
Resistances in the form of wire coils are the most convenient working standards. First we have single coils made in a form shown in Fig. 318. They are made so that they can be immersed in an oil bath of constant temperature, and are provided with large copper terminals to dip in mercury cups. Resistances of this kind are used primarily for calibrating the working resistance
boxes. They should be supplied with certificates of calibration from one of the national calibrating laboratories.
For general laboratory purposes resistance coils are mounted in boxes as shown in Fig. 319. On the ebonite top there are a series of heavy brass blocks and the ends of the coils are joined
to these blocks, so that the current entering at one terminal passes from block to block through each resistance coil in turn. Any coil can be cut out of the circuit by bridging the brass blocks with a metal plug (Fig. 320). Instead of plugs, a lever with sliding contacts is used successfully in some recent resistance boxes. Most of the high-grade resistance boxes are now
wound with manganin wire. A resistance coil is always wound inductionless (Fig. 321), that is, the coil is wound back on itself so as to avoid magnetic effects and self-induction.

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