Weston normal element
The Weston normal element is a special galvanic cell . It was used from 1911 to calibrate the electrical voltage , which is measured in volts .
This electrochemical cell was invented in 1893 by Edward Weston (1850-1936).
It consists of two different electrodes in a saturated cadmium sulphate solution (CdSO 4 ), the saturation of which is maintained by added cadmium sulphate crystals . Of the two electrodes, one is made of mercury and the other is made of cadmium amalgam , a metallic alloy of cadmium and mercury. The cadmium content in the amalgam electrode is 12 to 13%. When a current passes through, cadmium ions migrate to the positive mercury electrode. A mixture of mercury (I) sulfate (Hg 2 SO 4 ) and cadmium sulfate crystals is located above the mercury electrode so that it does not alloy there and thus both electrodes become more or less the same . When a current passes through the cadmium ions and the mercury (I) sulfate form cadmium sulfate and metallic mercury:
- .
Between the two electrodes is formed at a temperature of 20 ° C is a reference voltage of
- U N = 1.01865 V,
which can be tapped at the connection terminals. The temperature dependence of the voltage is relatively small in this element.
Since 1990, the reference of the electrical voltage for practical measurement is no longer formed by electrochemical normal elements. It is determined by the quantum mechanical Josephson effect , from which the magnitude of the electrical voltage can be derived through a much more accurate frequency measurement.
literature
- Magda Fröhlich: The normal element. Akademische Verlagsgesellschaft, Wiesbaden 1978, ISBN 3-400-00352-2 .
- Adolf Thomälen : Short textbook of electrical engineering . 9th edition. Published by Julius Springer, 1922.
