Weston Standard Cell
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The Weston standard cell is a wet-chemical cell that produces a highly stable
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
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 know ...
of
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. Ana ...
s. Invented by
Edward Weston Edward Henry Weston (March 24, 1886 – January 1, 1958) was a 20th-century American photographer. He has been called "one of the most innovative and influential American photographers..." and "one of the masters of 20th century photography." ...
in 1893, it was adopted as the International Standard for EMF from 1911 until superseded by the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 is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
is an
amalgam Amalgam most commonly refers to: * Amalgam (chemistry), mercury alloy * Amalgam (dentistry), material of silver tooth fillings ** Bonded amalgam, used in dentistry Amalgam may also refer to: * Amalgam Comics, a publisher * Amalgam Digital ...
of
cadmium Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...
with
mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...
with a
cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
of pure mercury over which a paste of mercurous sulfate and mercury is placed. The
electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
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 solubil ...
of
cadmium sulfate Cadmium sulfate is the name of a series of related inorganic compounds with the formula CdSO4·H2O. The most common form is the monohydrate CdSO4·H2O, but two other forms are known CdSO4·H2O and the anhydrous salt (CdSO4). All salts are colourl ...
, and the
depolarizer A depolarizer or depolariser, in electrochemistry, according to an IUPAC definition, is a synonym of electroactive substance, i.e., a substance which changes its oxidation state, or partakes in a formation or breaking of chemical bonds, in a ch ...
is a paste of
mercurous sulfate Mercury(I) sulfate, commonly called mercurous sulphate ( UK) or mercurous sulfate ( US) is the chemical compound Hg2SO4. Mercury(I) sulfate is a metallic compound that is a white, pale yellow or beige powder. It is a metallic salt of sulfuric acid ...
. 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 with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Platinu ...
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 The Clark cell, invented by English engineer Josiah Latimer Clark in 1873, is a wet-chemical cell (colloquially: ''battery'') that produces a highly stable voltage. In 1893, the output of the Clark cell at 15 °C was defined by the International ...
. 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.


See also

*
List of battery types This list is a summary of notable electric battery types composed of one or more electrochemical cells. Three lists are provided in the table. The primary (non-rechargeable) and secondary (rechargeable) cell lists are lists of battery chemistry ...


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





{{Galvanic cells Battery types Mercury (element)