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- From: Duff, A. Wilmer, A Text-Book of Physics, 5th
ed., P. Blakiston's Son & Co., Philadelphia (1921) pp. 399-400.
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- © Copyright 1998 R. Paselk
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- 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."
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- 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.
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- 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.
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- 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
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- boxes. They should be supplied with certificates
of calibration from one of the national calibrating laboratories.
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- 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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- © R. Paselk
- Last modified 22 July 2000
