Note on a simple form of Lippmann's capillary electrometer useful to physiologists / by Professor John G. M'Kendrick, M.D.

  • McKendrick, John G. (John Gray), 1841-1926.
Date:
[1883?]
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Licence: Public Domain Mark

Credit: Note on a simple form of Lippmann's capillary electrometer useful to physiologists / by Professor John G. M'Kendrick, M.D. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

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    mercury and in the capillary tube, are due to a change in the surface tension produced by the electrical polarisation of the surface of the mercury. Lippmann’s form of the electrometer consists of a tube of ordi- nary glass, 1 metre long and 7 millimetres in diameter, open at both ends, and kept in the vertical position by a stout support. The lower end is drawn into a capillary point, until the diameter of the capillary is *005 of a millimetre. The tube is filled with mercury, and the capillary point is immersed in dilute sulphuric acid (1 to 6 of water in volume), and in the bottom of the vessel containing the acid there is a little more mercury. A platinum wire is put into connection wfith the mercury in each tube, and, finally, arrangements are made by which the capillary point can be seen with a microscope magnifying 250 diameters. Such an instrument is very sensitive; and Lippmann states that it is possible to determine a difference of potential so small as that of the X50 8 0 a Daniell- It is thus a very delicate means of observing and (as it can be graduated by a compensation-method) of measuring minute electro-motive forces. As the sensitiveness of the instrument depends essentially on the smallness of the bore of the capillary, and on the constant electro-motive force between the mercury and the sulphuric acid (termed by Lippmann the difference electrique), it is important to have the tube as fine as possible, and also to prevent variations in the strength of the sulphuric acid by evaporation. These con- siderations led to improvements in the construction of the instru- ment, thus described by Burdon Sanderson :1—“ Two important improvements have been recently made in the construction of this instrument by Professor Loven,2 of Stockholm. The first of these consists in constructing the tube which contains the dilute sul- phuric acid of extremely thin glass, so that the capillary tube which is in contact with its internal surface can be observed with 1 Burdon Sanderson “ On the Electro-motive Properties of the Leaf of Diomea in the Excited and Unexcited States,” Phil. Trans., parti., 1882. See also his lecture “ On the Excitability of Plants,” at the Royal Institution of Great Britain, 9th June 1882. In stating his opinion that “it is much better to measure the electro-motive value of each excursion with the aid of the compensator, than to deduce it, as has been done by Fleischl, from the ‘compensation pressure,’” Dr. Burdon Sanderson refers to Fleischl’s paper, Archiv f. Anat. v. Physiol., 1879, p. 269. 2 Loven, Nordiskt Medic. Arkiv., vol. xi. No. 14.