Clark normal element
The Clark normal element is a special galvanic cell which supplies a stable DC voltage . It served as a reference voltage source in the early days of electrical engineering, especially from 1893 to 1908 . It was invented by the British engineer Josiah Latimer Clark , who made it popular in 1872 after four years of development.
construction
In the Clark normal element, the anode consists of an amalgam of zinc and mercury , the so-called zinc amalgam. In the figure shown on the right, the zinc amalgam stick is attached to the left side of the cell and connected to terminal K 1 . At the bottom of the cell is the cathode made of liquid mercury, which is connected to terminal K 2 by the electrolyte with an insulating glass rod . The electrolyte consists of an aqueous and saturated zinc sulfate solution . Mercury (I) sulfate (Hg 2 SO 4 ) is used as the depolarizer .
The cell structure of the original Clark cell can be compared with the cell scheme
- (-) Zn (s) | ZnSO 4 · 7H 2 O (c) | ZnSO 4 (sat. Aq) | ZnSO 4 · 7H 2 O (c) | Hg 2 SO 4 (s) | Hg (l) (+)
can be described with s = solid, c = crystalline, l = liquid and sat.aq = saturated aqueous solution. This is abbreviated as
- Zn | ZnSO 4 | Hg 2 SO 4 | Hg.
The amalgamation of zinc was proposed by Lord Rayleigh and H. Sidgwick in 1884 ; it improved the cell considerably.
Properties and use
The Clark cell having a at a temperature of 15 ° C open circuit voltage from 1.4328 V . The electrical current drawn should be as small as possible.
The Clark normal element has some disadvantages such as a comparatively large temperature coefficient of −1.15 mV / ° C and corrosion problems in the area of the connecting wires with the zinc amalgam. The Clark normal element was subsequently replaced by the more temperature -stable Weston normal element . At the international conference on electrical units and standards in London in October 1908, the Clark cell was officially replaced by the Weston element when the standard element was established.
literature
- WE Ayrton and T. Mather: Practical Electricity . Cassell and Company, London 1911, p. 198 to 203 .
Individual evidence
- ↑ a b c d Walter J. Hamer: Standard Cells . Their Construction, Maintenance, and Characteristics (= US National Bureau of Standards [Ed.]: National Bureau of Standards Monograph . No. 84 ). Jan 15, 1965, 3. Early Standard Cells, 4, The Clark Cell, p. 11–15 (English, online on the website of the National Institute of Standards and Technology (NIST) [PDF; 3.9 MB ; accessed on October 19, 2016]): “The Clark cell has three advantages ... it ... supplies its own buffering action ... the emf ... is less dependent ... on the composition of the amalgam .. . ”
- ^ Josiah Latimer Clark: On a Voltaic Standard of Electromotive Force . Communicated by Prof. Sir William Thomson. In: Royal Society (Ed.): Proceedings of the Royal Society of London . tape 20 , May 30, 1872, pp. 444–448 , JSTOR : 113174 (English, online in the Internet Archive [accessed October 19, 2016]).
