Page 443 - Instrumentation Reference Book 3E

426 Electrical measurements

If, however, the currents in the coils of the fixed (Steele et nl., 1975). Submillimeter wave measure-
pair are in opposite directions then the suspended ments and standards are given by Stone etal.
coil experiences an upward or downward force. (1 975).
The force, F,, is counterbalanced by the weight of These electrical standards are similar to stand-
a known mass, m, and thus ards held by other national laboratories; for
example, the National Institute for Standards
and Testing (NIST) in the USA and Physikalisch-
Technische Bundesanstalt (PTB) in West Germany
where g is the acceleration due to gravity. and others elsewhere.
Ican be determined absolutely, Le., in terms of
the mechanical base units of mass, length, and 20.1.3.1 Standard cells
time, if dMldx is known; dMldx can be calculated
from dimensional measurements made on the The UK primary standard of voltage is provided
suspended and fixed coils. Changes in current by a bank of some thirty Weston saturated mer-
direction and averaging the masses required to cury cadmium cells, the construction of a single cell
restore the balance condition enable the effects being shown in Figure 20.3. The electrodes of the
of interactive forces between opposite sides of the cell are mercury and an amalgam of cadmium and
balance and of external magnetic fields to be mercury. The electrolyte of cadmium sulphate is
eliminated. kept in a saturated condition over its operating
Typically the accuracy of realization of the temperature range by the presence of cadmium
ampere using the current balance has a probable sulphate crystals. The pH of the electrolyte has a
error of several parts in lo6. One of the major considerable effect on the stability of the emf of the
causes of this inaccuracy is the relative magnitude cell and has an optimal value of 1.4 + 0.2 (Froelich
of the force generated by the coils when com- 1974). The mercurous sulphate paste over the
pared with the mass of the suspended coils. Alter- anode acts as a depolarizer. For details concerning
native techniques for the absolute determination the construction, maintenance, and characteristics
of the ampere have been suggested. These include of such cells and their use the reader is directed to
the use of the proton gyromagnetic ratio, "iP, in the NBS monograph listed in the References.
conjunction with weak and strong magnetic field The nominal value of the emf generated by the
measurements (Dix and Bailey 1975; Vigoureux saturated Weston cell is 1.01865V at 20°C. Cells
1971) and the measurement of the force on a constructed from the same materials at the same
coil in a magnetic field together with the time will have emfs differing by only a few pV.
measurement of the potential induced when the Cells produced at different times will have emfs
coil moves in the same magnetic field (Kibble differing by between 10 and 20 pV. The stability
etal. 1983). of such cells can be of order a few parts in lo7 per
year. They can be intercompared by back-to-back
measurements to 1 part in lo8. The internal resist-
20.1.3 National primary standards
ance of the cell is approximately 750 s1.
Because the accuracy of realization of the SI The variation of the cell emf with temperature
ampere by the current balance is significantly can be described by the equation
poorer than the precision of intercomparison of
standard cells and resistors, and also because of v, =vzo - 4.06 10-77- - 20)
the difficulty of storing the realized value of the - 9.07 x 10-7(T - 20)* + 6.6 x 10-9(T - 20)3
ampere, most National Standards Laboratories - 1.5 x ~o-~O(T - 20)~
use standard cell banks and resistors as their
maintained primary standards. Intercomparison where VT is the emf of the cell at a temperature
of these national standards is regularly made T"C and V20 is its emf at 20°C. For small tem-
through the International Bureau of Weights perature variations about 20°C the cell has a
and Measures (BIPM) in Sevres France. Figure temperature coefficient of -40.6 pV/K.
20.2 (taken from Dix and Bailey 1975) shows the To produce a source of emf with high stability it
UK primary standards which are maintained by is necessary to maintain the cells in a thermostatic-
the National Physical Laboratory (NPL). This ally controlled enclosure. At NPL the standard cell
figure also shows the relationships of the primary enclosure contains up to 54 cells housed in groups
standards to the absolute reference standards; to of nine in separate copper containers in an air
the national low-frequency a.c. standards; and to enclosure which has a temperature stability of bet-
the primary standards of other countries. Table ter than 1 mK/h and a maximum temperature dif-
20.1 lists the UK national d.c. and low-frequency ference between any two points in the enclosure of
standards apparatus. Radio frequency and micro- less than 5pK. Measurement of the emfs of the
wave standards at NPL are listed in Table 20.2 cells is effected under computer control.