Capillary electrometer

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The capillary electrometer built by Gabriel Lippmann from 1872

A capillary electrometer is a historical device for measuring electrical charge or electrical voltage . It is a special type of electrometer that uses the charge dependency of the surface tension as a measuring principle. The measuring range of the device is small: it is only for voltages with an amount from 0 to 0.9  V suitable. In the 19th century, however, it was considered a very sensitive measuring device , as voltages in the millivolt range or less could be measured. B. used in 1882 for the first electrocardiogram .

Structure and functionality

The capillary electrometer consists of a mercury column in a capillary tube , which with dilute sulfuric acid is covered. In order to measure a voltage (or more precisely the electrical potential difference between two points), the two points are connected to the two electrodes of the capillary electrometer: one electrode is in contact with the acid, the other with the mercury. The contact surface between mercury and sulfuric acid is an interface whose surface charge is changed by an electric current. The device now uses the property that when the surface charge changes, the surface tension of the mercury tip also changes, and that this then shifts in a narrow capillary. A displacement of the meniscus is observed , i.e. H. the curvature of the surface of the mercury column. Small voltage changes from 25 µV can be measured.

Gabriel Lippmann had built various capillary electrometers. One of them consisted of a 1 m high, 7 mm wide, vertical glass tube, which was drawn out at the bottom into a 10 mm long, upwardly bent capillary tube; the latter dipped into a glass vessel with an open top, which contains dilute sulfuric acid and mercury underneath.

In order to improve the reading of the measurement result, the meniscus was followed with suitable optical methods. Lippmann used a cathetometer or a microscope with 220x magnification. A microscope was also used for an electrometer built in 1896. Alternatively, the enlarged shadow of the meniscus was projected onto film and the profile was then analyzed.

Quantitative description

According to the Lippmann equation, also called the Helmholtz-Lippmann equation, the following relationship applies between the surface tension γ, the potential φ and the surface charge density σ (σ = Q / A = charge / area):

A small change in the potential accordingly leads to a small change in the surface tension , the magnitude of which is greater, the greater the surface charge density σ of the surface.

history

Development of the capillary electrometer

The capillary electrometer was developed by Gabriel Lippmann in Heidelberg in 1872 . Lippmann's interest in the surface tension of mercury was aroused by the physiologist Wilhelm Kühne , who had demonstrated the “ beating mercury heart ” to him in Heidelberg in 1871 . In 1875 Lippmann received his doctorate for the work he carried out in Gustav Robert Kirchhoff's laboratory on the relationship between electricity and capillary forces.

Use in cardiography

In 1876, the French physiologist Étienne-Jules Marey used the device to record the electrical activity of the heart. This was an important milestone in the history of electrocardiography . In 1887 the physiologist Augustus Desiré Waller was able to record heart currents for the first time with the help of a capillary electrometer.

Individual evidence

  1. ↑ a b c Pierandrea Malfi: Elettrometro di Lippmann. In: Museo di Fisica "Antonio Maria Traversi"> Museo virtuale di Fisica> Elettroscopi ed elettrometri> Elettrometri. Retrieved April 25, 2019 .
  2. ^ Entry in Meyers Konversations-Lexikon, 1888, see web links
  3. ↑ a b c d Gabriel Lippmann: Relationships between the capillary and electrical phenomena . In: Johann Christian Poggendorff (Ed.): Annals of Physics and Chemistry . 225 (Pogg. Ann. 149), no. 8 . Johann Ambrosius Barth, 1873, ISSN  1521-3889 , p. 546–561 , doi : 10.1002 / andp.18732250807 ( online at Gallica , Bibliothèque nationale de France).
  4. ↑ electrocapillarity. In: Lexicon of Physics. Spectrum der Wissenschaft Verlagsgesellschaft mbH, accessed on April 25, 2019 .
  5. ↑ Gerd Wedler: Textbook of Physical Chemistry . 5th edition. Wiley-VCH, Weinheim 2004, ISBN 3-527-31066-5 , 2.7.8 Die Elektrokapillarität, p. 442 .
  6. ^ Gabriel Lippmann: Relation entre les phénomènes électriques et capillaires . Physique. In: Académie des sciences / Bachelier (ed.): Comptes rendus hebdomadaires des séances de l'Académie des sciences . tape 76 . Gauthier-Villars, Paris January 1873, p. 1407–1408 (French, online at Gallica , Bibliothèque nationale de France [accessed on April 25, 2019] brief summary of its results): «La surface d'une goutte de mercure [...] baignée par de L'acide sulfurique [ ...] »
  7. ↑ Andrea Sella: Lippmann's electrometer. In: Opinion> Andrea Sella. Chemistry World, The Royal Society of Chemistry, August 28, 2015, accessed April 25, 2019 .
  8. ^ Gabriel Lippmann: Relations entre les phénomènes électriques et capillaires . Thèse présentée à la faculté des sciences de Paris. Gauthier-Villars, Paris 1875, OCLC 490702474 (French, online at the University of Pierre and Marie Curie UPMC (since 2018: Sorbonne Université) [accessed on April 25, 2019]): "mesure des forces électromotoriques"
  9. ^ History of electrocardiography ( Memento from June 11, 2009 in the Internet Archive ) Supplementary material for the lecture by private lecturer JM Davis, University of Munich

Web links