» Lippmann Electrometer (0157)

Immagini

DESCRIPTION

Exhibit 157 is a very sensitive electrometer capable of revealing very small voltage differences of the order of 0.0001 volt. The mechanism employed is based on electrocapillarity, a phenomenon for which a difference of potential between two electrodes in liquids (mercury and diluted sulfuric acid) determine a variation of the surface tension at the contact surface between the two liquids which do not mix. The instrument consists of:

Two glass containers connected by a capillary tube, one containing mercury, the other sulfuric acid (soluzione di $Formula: [\rm H_2 SO_4]$ ) in which the two liquids are in contact at the point W inside the capillary (see design 1)
Two electrodes fused in the glass containers in contact with the liquids and connected to an electric circuit constituted by a battery in series with a variable resistance and a potentiometer and including a galvanometer.
A microscope and a mirror for reading the level at which the point W reaches equilibrium.

The variation of the surface tension is measured by the displacement of the point W in the capillary tube; The law of Borelli-Jurin determines in fact a direct proportionality relation between the height of the liquid in a capillary and the surface tension. By measuring the displacement of the point W one measures indirectly the difference of potential applied to the electrodes. Upon varying the potential difference between the electrodes using the potentiometer, one observes an equivalent surface tension rise (rise of W), pass through a maximum and then decrease (see design 2). That behavior is explained by the competition between the electrostatic repulsion of the electric charge in the two liquids that tends to increase the surface tension and the surface tension that tends to reduce that surface. The surface tension measured has diminished, and it is maximum when the potential difference between water and mercury is zero, before inverting.

Phenomenologically one considers the separation between the two liquids as a two plate condenser, the energy of which is thus equal to $Formula: [\rm \frac{1}{2}CV^2]$ where C is the capacity of the condenser and V the difference of potential between the plates. It follows that the surface energy S is diminished by a value equal to the energy stored in the condenser, that is

Equazione: [\rm S=S_0 - \frac{1}{2}CV^2]

where $Formula: [\rm S_0]$ is the surface energy at zero potential difference. Given that surface tension is an energy per unit area, there is an analogous parabolic behavior with potential difference at the electrodes found in the measurement effected.

ISCRIZIONI

Incisione sul microscopio: "Fritz Kohler, Universitatsmechanic, Leipzig"

BIBLIOGRAPHY

[1] Castagnoli Carlo, "Fisica Generale: meccanica - termodinamica", Levrotto & Bella , Torino. Pagina 467
[2] Perucca Eligio, "Fisica Generale", UTET, Torino, 1963. Pag, 677
[3] Worsnop & Flint, "Advanced Practical Physics For Students",Asian edition,1961, pag.585.
[4] P. Fleury, J.P.Mathieu, "Elettrostatica Corrente continua Magnetismo", Zanichelli Bologna, seconda edizione, 1970. §20-15,Pagina 489.

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