The Weston cell or Weston standard cell is a wet-chemical cell that produces a highly stable

voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

suitable as a laboratory standard for

calibration In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known ...

voltmeter A voltmeter is an instrument used for measuring electric potential difference between two points in an electric circuit. It is connected in parallel. It usually has a high resistance so that it takes negligible current from the circuit. A ...

s. Invented by

Edward Weston Edward Henry Weston (March 24, 1886 – January 1, 1958) was an American photographer. He has been called "one of the most innovative and influential American photographers" and "one of the masters of 20th century photography." Over the course ...

in 1893, it was adopted as the International Standard for EMF from 1911 until superseded by the

Josephson voltage standard A Josephson voltage standard is a complex system that uses a superconducting integrated circuit chip operating at a temperature of 4 K to generate stable voltages that depend only on an applied frequency and fundamental constants. It is an int ...

in 1990.

Chemistry

The

anode An anode usually is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, which is usually an electrode of the device through which conventional current leaves the devic ...

is an amalgam of

cadmium Cadmium is a chemical element; it has chemical symbol, symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12 element, group 12, zinc and mercury (element), mercury. Like z ...

with mercury with a

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device such as a lead-acid battery. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. Conventional curren ...

of pure mercury over which a paste of mercurous sulfate and mercury is placed. The

electrolyte An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...

is a

saturated solution In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...

of cadmium sulfate, and the depolarizer is a paste of mercurous sulfate. As shown in the illustration, the cell is set up in an H-shaped glass vessel with the cadmium amalgam in one leg and the pure mercury in the other. Electrical connections to the cadmium amalgam and the mercury are made by

platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...

wires fused through the lower ends of the legs. ;Anode reaction: Cd(s) → Cd²⁺(aq) + 2e⁻ ;Cathode reaction: (Hg⁺)₂(s) + 2e⁻ → 2Hg(l) + (aq) Reference cells must be applied in such a way that no current is drawn from them.

Characteristics

The original design was a saturated cadmium cell producing a reference and had the advantage of having a lower

temperature coefficient A temperature coefficient describes the relative change of a physical property that is associated with a given change in temperature. For a property ''R'' that changes when the temperature changes by ''dT'', the temperature coefficient α is def ...

than the previously used Clark cell. One of the great advantages of the Weston normal cell is its small change of electromotive force with change of temperature. At any temperature between and , :. This temperature formula was adopted by the London conference of 1908. The temperature coefficient can be reduced by shifting to an unsaturated design, the predominant type today. However, an unsaturated cell's output decreases by some 80 microvolts per year, which is compensated by periodic calibration against a saturated cell.

References

Literature

* ''Practical Electricity'' by W. E. Ayrton and T. Mather, published by Cassell and Company, London, 1911, pp 198–203 * , "''Voltaic cell''" *
Standard Cells, Their Construction, Maintenance, and Characteristics
' by Walter J. Hamer, National Bureau of Standards Monograph 84, January 15, 1965.

External links

{{Authority control Battery types Mercury (element)

Chemistry

Characteristics

See also

References

Literature

External links