LESSON 3
PRECISION VOLTAGE MEASUREMENT OF THE STANDARD CELL
TASK
Describe and identify theory, terminology, and
procedures for precision
voltage measurement of the standard cell.
CONDITIONS
Given information and illustrations about theory, terminology,
and
procedures for precision voltage measurement of the standard cell.
STANDARDS
Demonstrate competency of the task skills and knowledge by correctly
responding to 80 percent of the multiple-choice test covering precision
voltage measurement of the standard cell.
REFERENCES
National Bureau of Standards Monograph 84, January 15, 1965
TB 43-177
Learning Event 1:
DESCRIBE THE CONSTRUCTION AND MAINTENANCE OF THE STANDARD CELL
1.
Voltaic cells.
Since the early days of measurements, voltage
references have consisted of voltaic cells of various types.
The cells
were, after a period of time, produced to a high degree of excellence.
Techniques have been perfected so that a group of the precision type can be
produced with individual differences of only a few parts per million. In
addition, the cells have a long life, so they will serve satisfactorally as
a standard.
2.
Electromotive force (EMF). A working standard of electromotive force
was recognized early as a pressing need in electrical measurements, so
experimenters naturally turned to the idea of a voltaic cell to provide a
reference. It was known that the EMF of a cell depended on the electrolyte
and the electrode materials.
Hence, there was a natural inference that a
given combination of materials can be relied upon to give a definite value.
3.
Cell properties.
There are several properties that a cell
should possess to be considered a good standard of EMF:
long life,
small temperature coefficient, and reproducibility.
Edward Weston
produced a cadmium cell in 1892 which had most of these properties.
It was adopted in 1908 by the London International Electric Congress
with the value of 1.0184 international volts.
The international volt
as a working standard was changed in 1910 to 1.0183 volts as the EMF of
the Weston normal (saturated) cell at 20C.
The Weston cell has been
the working (or reference) standard of EMF since that time and,
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